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data/en.wikipedia.org/wiki/A_series_and_B_series-0.md

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+---
+title: "A series and B series"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/A_series_and_B_series"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:40.580369+00:00"
+instance: "kb-cron"
+---
+
+In metaphysics, the A series and the B series are two different descriptions of the temporal ordering relation among events. The two series differ principally in their use of tense to describe the temporal relation between events and the resulting ontological implications regarding time.
+John McTaggart introduced these terms in 1908, in an argument for the unreality of time. They are now commonly used by contemporary philosophers of time.
+
+
+== History ==
+Metaphysical debate about temporal orderings reaches back to the ancient Greek philosophers Heraclitus and Parmenides. Parmenides thought that reality is timeless and unchanging. Heraclitus, in contrast, believed that the world is a process of ceaseless change, flux and decay. Reality for Heraclitus is dynamic and ephemeral, in a state of constant flux, as in his famous statement that it is impossible to step twice into the same river (since the river is flowing).
+
+
+== McTaggart's series ==
+McTaggart distinguished the ancient conceptions as a set of relations. According to McTaggart, there are two distinct modes in which all events can be ordered in time.
+
+
+=== A series ===
+In the first mode, events are ordered as future, present, and past. Futurity and pastness allow of degrees, while the present does not. When we speak of time in this way, we are speaking in terms of a series of positions which run from the remote past through the recent past to the present, and from the present through the near future all the way to the remote future. The essential characteristic of this descriptive modality is that one must think of the series of temporal positions as being in continual transformation, in the sense that an event is first part of the future, then part of the present, and then part of the past. Moreover, the assertions made according to this modality correspond to the temporal perspective of the person who utters them. This is the A series of temporal events.
+Although originally McTaggart defined tenses as relational qualities, i.e. qualities that events possess by standing in a certain relation to something outside of time (that does not change its position in time), today it is popularly believed that he treated tenses as monadic properties. Later philosophers have independently inferred that McTaggart must have understood tense as monadic because English tenses are normally expressed by the non-relational singular predicates "is past", "is present" and "is future", as noted by R. D. Ingthorsson.
+
+
+=== B series ===
+
+From a second point of view, events can be ordered according to a different series of temporal positions by way of two-term relations that are asymmetric, irreflexive and transitive (forming a strict partial order): "earlier than" (or precedes) and "later than" (or follows).
+An important difference between the two series is that while events continuously change their position in the A series, their position in the B series does not. If an event ever is earlier than some events and later than the rest, it is always earlier than and later than those very events. Furthermore, while events acquire their A series determinations through a relation to something outside of time, their B series determinations hold between the events that constitutes the B series. This is the B series, and the philosophy that says all truths about time can be reduced to B series statements is the B-theory of time.
+
+
+=== Distinctions ===
+The logic and the linguistic expression of the two series are radically different. The A series is tensed and the B series is tenseless. For example, the assertion "it is raining here today" is a tensed assertion because it depends on the temporal perspective—the present—of the person who utters it, while the assertion "It rained here on 5 May 2026" is tenseless because it does not so depend. From the point of view of their truth-values, the two propositions are identical (both true or both false) if the first assertion is made on 5 May 2026. The non-temporal relation of precedence between two events, say "E precedes F", does not change over time
+(excluding from this discussion the issue of the relativity of temporal order of causally disconnected events in the theory of relativity). On the other hand, the character of being "past, present or future" of the events "E" or "F" does change with time. In the image of McTaggart the passage of time consists in the fact that terms ever further in the future pass into the present...or that the present advances toward terms ever farther in the future. If we assume the first point of view, we speak as if the B series slides along a fixed A series. If we assume the second point of view, we speak as if the A series slides along a fixed B series.
+
+
+== Relation to other ideas in the philosophy of time ==
+There are two principal varieties of the A-theory, presentism and the growing block universe. Both assume an objective present, but presentism assumes that only present objects exist, while the growing block universe assumes both present and past objects exist, but not future ones. Views that assume no objective present and are therefore versions of the B-theory include eternalism and four-dimensionalism.
+Vincent Conitzer argues that A-theory is related to Benj Hellie's vertiginous question and Caspar Hare's ideas of egocentric presentism and perspectival realism. He argues that A-theory being true and "now" being metaphysically distinguished from other moments of time implies that the "I" is also metaphysically distinguished from other first-person perspectives.
+
+
+== See also ==
+Endurantism
+New riddle of induction
+Perdurantism
+The Unreality of Time
+
+
+== Notes ==
+
+
+== References ==
+Craig, William Lane, The Tensed Theory of Time, Springer, 2000.
+Craig, William Lane, The Tenseless Theory of Time, Springer, 2010.
+Ingthorsson, R. D., "McTaggart's Paradox", Routledge, 2016.
+McTaggart, J. E., 'The Unreality of Time', Mind, 1908.
+McTaggart, J. E.,The Nature of Existence, vols. 1-2, Cambridge University Press, Cambridge, 1968.
+Bradley, F. H., The Principles of Logic, Oxford University Press, Oxford, 1922.
+
+
+== External links ==
+"Notes on McTaggart, 'The Unreality of Time'". Seminar on Philosophy and Time, Trinity University, 2005.
+Zalta, Edward N. (ed.). "McTaggart's A series and B series". Stanford Encyclopedia of Philosophy. ISSN 1095-5054. OCLC 429049174.

data/en.wikipedia.org/wiki/Absolute_space_and_time-0.md

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+---
+title: "Absolute space and time"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Absolute_space_and_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:41.731550+00:00"
+instance: "kb-cron"
+---
+
+Absolute space and time is a concept in physics and philosophy about the properties of the universe. In physics, absolute space and time may be a preferred frame.
+
+== Early concept ==
+A version of the concept of absolute space (in the sense of a preferred frame) can be seen in Aristotelian physics. Robert S. Westman writes that a "whiff" of absolute space can be observed in Copernicus's De revolutionibus orbium coelestium, where Copernicus uses the concept of an immobile sphere of stars.
+
+== Newton ==
+Originally introduced by Sir Isaac Newton in Philosophiæ Naturalis Principia Mathematica, the concepts of absolute time and space provided a theoretical foundation that facilitated Newtonian mechanics. According to Newton, absolute time and space respectively are independent aspects of objective reality:
+
+Absolute, true and mathematical time, of itself, and from its own nature flows equably without regard to anything external, and by another name is called duration: relative, apparent and common time, is some sensible and external (whether accurate or unequable) measure of duration by the means of motion, which is commonly used instead of true time ...
+According to Newton, absolute time exists independently of any perceiver and progresses at a consistent pace throughout the universe. Unlike relative time, Newton believed absolute time was imperceptible and could only be understood mathematically. According to Newton, humans are only capable of perceiving relative time, which is a measurement of perceivable objects in motion (like the Moon or Sun). From these movements, we infer the passage of time.
+
+Absolute space, in its own nature, without regard to anything external, remains always similar and immovable. Relative space is some movable dimension or measure of the absolute spaces; which our senses determine by its position to bodies: and which is vulgarly taken for immovable space ...
+Absolute motion is the translation of a body from one absolute place into another: and relative motion, the translation from one relative place into another ...
+These notions imply that absolute space and time do not depend upon physical events, but are a backdrop or stage setting within which physical phenomena occur. Thus, every object has an absolute state of motion relative to absolute space, so that an object must be either in a state of absolute rest, or moving at some absolute speed. To support his views, Newton provided some empirical examples: according to Newton, a solitary rotating sphere can be inferred to rotate about its axis relative to absolute space by observing the bulging of its equator, and a solitary pair of spheres tied by a rope can be inferred to be in absolute rotation about their center of gravity (barycenter) by observing the tension in the rope.
+
+== Differing views ==
+
+Historically, there have been differing views on the concept of absolute space and time. Gottfried Leibniz was of the opinion that space made no sense except as the relative location of bodies, and time made no sense except as the relative movement of bodies. George Berkeley suggested that, lacking any point of reference, a sphere in an otherwise empty universe could not be conceived to rotate, and a pair of spheres could be conceived to rotate relative to one another, but not to rotate about their center of gravity, an example later raised by Albert Einstein in his development of general relativity.
+A more recent form of these objections was made by Ernst Mach. Mach's principle proposes that mechanics is entirely about relative motion of bodies and, in particular, mass is an expression of such relative motion. So, for example, a single particle in a universe with no other bodies would have zero mass. According to Mach, Newton's examples simply illustrate relative rotation of spheres and the bulk of the universe.
+
+When, accordingly, we say that a body preserves unchanged its direction and velocity in space, our assertion is nothing more or less than an abbreviated reference to the entire universe.—Ernst Mach
+These views opposing absolute space and time may be seen from a modern stance as an attempt to introduce operational definitions for space and time, a perspective made explicit in the special theory of relativity.
+Even within the context of Newtonian mechanics, the modern view is that absolute space is unnecessary. Instead, the notion of inertial frame of reference has taken precedence, that is, a preferred set of frames of reference that move uniformly with respect to one another. The laws of physics transform from one inertial frame to another according to Galilean relativity, leading to the following objections to absolute space, as outlined by Milutin Blagojević:
+
+The existence of absolute space contradicts the internal logic of classical mechanics since, according to Galilean principle of relativity, none of the inertial frames can be singled out.
+Absolute space does not explain inertial forces since they are related to acceleration with respect to any one of the inertial frames.
+Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon.
+
+Newton himself recognized the role of inertial frames.
+
+The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line.
+As a practical matter, inertial frames often are taken as frames moving uniformly with respect to the fixed stars. See Inertial frame of reference for more discussion on this.

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+---
+title: "Absolute space and time"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Absolute_space_and_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:41.731550+00:00"
+instance: "kb-cron"
+---
+
+== Mathematical definitions ==
+Space, as understood in Newtonian mechanics, is three-dimensional and Euclidean, with a fixed orientation. It is denoted E3. If some point O in E3 is fixed and defined as an origin, the position of any point P in E3 is uniquely determined by its radius vector 
+  
+    
+      
+        
+          r
+        
+        =
+        
+          
+            
+              
+                O
+                P
+              
+              →
+            
+          
+        
+      
+    
+    {\displaystyle \mathbf {r} ={\vec {OP}}}
+  
+ (the origin of this vector coincides with the point O and its end coincides with the point P). The three-dimensional linear vector space R3 is a set of all radius vectors. The space R3 is endowed with a scalar product ⟨ , ⟩.
+Time is a scalar which is the same in all space E3 and is denoted as t. The ordered set { t } is called a time axis.
+Motion (also path or trajectory) is a function r : Δ → R3 that maps a point in the interval Δ from the time axis to a position (radius vector) in R3.
+The above four concepts are the "well-known" objects mentioned by Isaac Newton in his Principia:
+
+I do not define time, space, place and motion, as being well known to all.
+
+== Special relativity ==
+The concepts of space and time were separate in physical theory prior to the advent of special relativity theory, which connected the two and showed both to be dependent upon the reference frame's motion. In Einstein's theories, the ideas of absolute time and space were superseded by the notion of spacetime in special relativity, and curved spacetime in general relativity.
+Absolute simultaneity refers to the concurrence of events in time at different locations in space in a manner agreed upon in all frames of reference. The theory of relativity does not have a concept of absolute time because there is a relativity of simultaneity. An event that is simultaneous with another event in one frame of reference may be in the past or future of that event in a different frame of reference, which negates absolute simultaneity.
+
+== Einstein ==
+
+Quoted below from his later papers, Einstein identified the term aether with "properties of space", a terminology that is not widely used. Einstein stated that in general relativity the "aether" is not absolute anymore, as the geodesic and therefore the structure of spacetime depends on the presence of matter.
+
+To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view. For the mechanical behaviour of a corporeal system hovering freely in empty space depends not only on relative positions (distances) and relative velocities, but also on its state of rotation, which physically may be taken as a characteristic not appertaining to the system in itself. In order to be able to look upon the rotation of the system, at least formally, as something real, Newton objectivises space. Since he classes his absolute space together with real things, for him rotation relative to an absolute space is also something real. Newton might no less well have called his absolute space “Ether”; what is essential is merely that besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real.
+Because it was no longer possible to speak, in any absolute sense, of simultaneous states at different locations in the aether, the aether became, as it were, four-dimensional, since there was no objective way of ordering its states by time alone. According to special relativity too, the aether was absolute, since its influence on inertia and the propagation of light was thought of as being itself independent of physical influence....The theory of relativity resolved this problem by establishing the behaviour of the electrically neutral point-mass by the law of the geodetic line, according to which inertial and gravitational effects are no longer considered as separate. In doing so, it attached characteristics to the aether which vary from point to point, determining the metric and the dynamic behaviour of material points, and determined, in their turn, by physical factors, namely the distribution of mass/energy. Thus the aether of general relativity differs from those of classical mechanics and special relativity in that it is not ‘absolute’ but determined, in its locally variable characteristics, by ponderable matter.
+
+== General relativity ==
+Special relativity eliminates absolute time (although Gödel and others suspect absolute time may be valid for some forms of general relativity) and general relativity further reduces the physical scope of absolute space and time through the concept of geodesics. There appears to be absolute space in relation to the distant stars because the local geodesics eventually channel information from these stars, but it is not necessary to invoke absolute space with respect to any system's physics, as its local geodesics are sufficient to describe its spacetime.
+
+== See also ==
+
+== References and notes ==
+
+== External links ==
+ Media related to Absolute space and time at Wikimedia Commons

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+title: "Aevum"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Aevum"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:42.925412+00:00"
+instance: "kb-cron"
+---
+
+In scholastic philosophy, the aevum (also called aeviternity) is the temporal mode of existence experienced by angels and by the saints in heaven. In some ways, it is a state that logically lies between the eternity (timelessness) of God and the temporal experience of material beings. It is sometimes referred to as "improper eternity" or "participated eternity".
+
+
+== Etymology ==
+The word aevum is Latin, originally signifying "age", "aeon", or "everlasting time". It comes from the Greek term "αἰών" meaning eternity in the atemporal sense. Before the 13th century, the term aevum was used synonymously with eternity. The word aeviternity comes from the Medieval Latin neologism aeviternitas.
+
+
+== History ==
+Both Plato and Aristotle recognized two modes of duration: Time and Eternity. Plato said that the soul must be in time while Aristotle said that the soul must be in eternity. The Neoplatonist philosopher Simplicius of Cilicia, said there must be a third mode to describe celestial bodies and souls which are everlasting but still change.
+In his exegesis of the Book of Genesis, Augustine of Hippo also recognized a mode of duration distinct from time and eternity. In Augustine's view, Heaven and earth, taken to mean formless matter, were created before time, and thus had existence in a participated eternity.
+In the beginning of the 13th century, independently of direct influence from Simplicius, a doctrine of three measures of duration was developing at the University of Paris. These were known as eternity, sempiternity (or perpetuity), and time. In the Summa attributed to Alexander of Hales, this middle measure of duration was used to describe angels, souls, and celestial spheres, all of which have a beginning but no end.
+The concept of the aevum, using the term as distinct from eternity, dates back at least to Albertus Magnus's first systematic study of time, De quattuor coaequaevis. In contrast to the concept of sempiternity, Albertus notes that even if something has no beginning and no end, its mode of duration would be distinct from the eternity of God. To Albertus, eternity is pure actuality while aevum is always actual, but only the actuality of some potency.
+A description of aevum is found in the Summa theologica of Thomas Aquinas. Aquinas identifies the aevum as the measure of the existence of beings that "recede less from permanence of being, forasmuch as their being neither consists in change, nor is the subject of change; nevertheless they have change annexed to them either actually, or potentially". As examples, he cites the heavenly bodies (which, in medieval science, were considered changeless in their nature, though variable in their position.) and the angels, which "have an unchangeable being as regards their nature with changeableness as regards choice".
+At the end of the 13th century, in De mensuris, Theodoric of Freiberg, further developed the philosophy of time. Theodoric distinguishes five measures of duration: 
+
+Superaetenitas - the measure of divine being; beyond eternity
+Aeternitas - the measure of the being of divine intelligences "if such beings exist"
+Aevum - the measure of angelic beings, which have a beginning, an everlasting existence, with no change in their substance but change in their accidents
+Aeviternitas - the measure of celestial bodies, which have a beginning and are everlasting but inferior in perfection to angels
+Time - the measure of beings with limited period of existence
+In the 14th century, both Berthold of Moosburg and William of Ockham rejected the concept of aevum. Berthold held that there were only two measures of duration: eternity and time. William did not even believe eternity could be properly called a measure and held that time was the only measure of duration.
+
+
+== Contemporary philosophy ==
+Frank Sheed, in his book Theology and Sanity, said that the aevum is also the measure of existence for the saints in heaven: "Aeviternity is the proper sphere of every created spirit, and therefore of the human soul... At death, [the body's] distracting relation to matter's time ceases to affect the soul, so that it can experience its proper aeviternity."
+
+
+== See also ==
+Philosophy of space and time
+Temporal finitism
+Physics (Aristotle)
+Proclus
+
+
+== References ==

data/en.wikipedia.org/wiki/An_Experiment_with_Time-0.md

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+---
+title: "An Experiment with Time"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/An_Experiment_with_Time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:02.376353+00:00"
+instance: "kb-cron"
+---
+
+An Experiment with Time is a book by the British soldier, aeronautical engineer and philosopher J. W. Dunne (1875–1949) about his precognitive dreams and a theory of time which he later called "Serialism". First published in March 1927, the book was widely read. Although never accepted by mainstream scientists or philosophers, it has influenced imaginative literature ever since. Dunne published four sequels: The Serial Universe (1934), The New Immortality (1938), Nothing Dies (1940) and Intrusions? (1955).
+
+== Description ==
+
+=== Overview ===
+An Experiment with Time discusses two main topics.
+The first half of the book describes a number of precognitive dreams, most of which Dunne himself had experienced. His key conclusion was that such precognitive visions foresee future personal experiences by the dreamer and not mere general events.
+The second half develops a theory to try to explain them. Dunne's starting point is the observation that the moment of "now" is not described by science. Contemporary science described physical time as a fourth dimension and Dunne's argument led to an endless sequence of higher dimensions of time to measure our passage through the dimension below. Accompanying each level was a higher level of consciousness. At the end of the chain was a supreme ultimate observer.
+According to Dunne, our wakeful attention prevents us from seeing beyond the present moment, whilst when dreaming that attention fades and we gain the ability to recall more of our timeline. This allows fragments of our future to appear in pre-cognitive dreams, mixed in with fragments or memories of our past. Other consequences include the phenomenon known as deja vu and the existence of life after death.
+
+=== Dreams and the experiment ===
+Following a discussion of brain function in which Dunne expounds mind-brain parallelism and highlights the problem of subjective experience, he gives anecdotal accounts of precognitive dreams which, for the most part, he himself had experienced.
+The first he records occurred in 1898, in which he dreamed of his watch stopping at an exact time before waking up and finding that it had in fact done so. Later dreams appeared to foretell several major disasters; a volcanic eruption in Martinique, a factory fire in Paris, and the derailing of the Flying Scotsman express train from the embankment approaching the Forth Railway Bridge in Scotland.
+Dunne tells how he sought to make sense of these dreams, coming slowly to the conclusion that they foresaw events from his own future, such as reading a newspaper account of a disaster rather than foreseeing the disaster itself. In order to try and prove this to his satisfaction, he developed the experiment which gives the book its title. He kept a notepad by his bedside and wrote down details of any dreams immediately on waking, then later went back and compared them to subsequent events in his life. He also persuaded some friends to try the same experiment, as well as experimenting on himself with waking reveries approaching a hypnagogic state.
+Based on the results, he claimed that they demonstrated that such precognitive fragments were common in dreams, even that they were mixed up in equal occurrence with past memories, and therefore they were difficult to identify until after the event they foresaw. He believed that the dreaming mind was not drawn wholly to the present, as it was during wakefulness, but was able to perceive events in its past and future with equal facility.
+
+=== The theory of Serialism ===
+Having presented Dunne's evidence for precognition, the book moves on to a possible theory in explanation which he called Serialism.
+The theory harks back to an experience with his nurse when he was nine years old. Already thinking about the problem, the boy asked her if Time was the moments like yesterday, today and tomorrow, or was it the travelling between them that we experience as the present moment? Any answer was beyond her, but the observation formed the basis of Serialism.
+Within the fixed spacetime landscape described by the recently published theory of general relativity, an observer travels along a timeline running in the direction of physical time, t1. Quantum mechanics was also a newly emerging science, though in a less-developed state. Neither relativity nor quantum mechanics offered any explanation of the observer's place in spacetime, but both required it in order to develop the physical theory around it. The philosophical problems raised by this lack of rigorous foundation were already beginning to be recognised.
+The theory resolves the issue by proposing a higher dimension of Time, t2, in which our consciousness experiences its travelling along the timeline in t1. The physical brain itself inhabits only t1, requiring a second level of mind to inhabit t2 and it is at this level that the observer experiences consciousness.
+However, Dunne found that his logic led to a similar difficulty with t2 in that the passage between successive events in t2 was not included in the model. This led to an even higher t3 in which a third-level observer could experience not just the mass of events in t2 but the passage of those experiences in t2, and so on in the infinite regress of time dimensions and observers which gives the theory its name.
+Dunne suggested that when we die, it is only our physical selves in t1 who die and that our higher selves are outside of mundane time. Our conscious selves therefore have no mechanism to die in the same kind of way and are effectively immortal. At the end of the chain he proposed a "superlative general observer, the fount of all ... consciousness".

data/en.wikipedia.org/wiki/An_Experiment_with_Time-1.md

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+---
+title: "An Experiment with Time"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/An_Experiment_with_Time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:02.376353+00:00"
+instance: "kb-cron"
+---
+
+== Publishing history ==
+An Experiment with Time was first published by A & C Black in March 1927. Dunne continued to update it and many new editions and impressions were published over his lifetime. Black brought out a 1929 second edition, prefaced with editorial notes and an extract from a 1928 letter from Arthur Eddington. Dunne then changed publisher to Faber & Faber, with whom he would remain. The third edition incorporated major new material and was published by Faber's in 1932; this and subsequent editions were published in the US by Macmillan. The final version which he had a hand in was published as a "reprint" in 1948.
+Faber continued printing paperback editions until at least 1973, and others have appeared since.
+
+== Reception ==
+
+=== Academic ===
+Initial reactions from the scientific and scientifically-minded community were broadly positive. Nature carried a review by the philosopher and mathematician Hyman Levy. They accepted that Dunne was a sober and rational investigator who was doing his best to take a scientific approach. They acknowledged that if his ideas about time and consciousness were true then his book would be truly revolutionary. However opinions differed over the existence of dream precognition, while his infinite regress was almost universally judged to be logically flawed and incorrect.
+Philosophers who criticised An Experiment with Time on much the same basis included J. A. Gunn, C. D. Broad and M. F. Cleugh.
+The physicist and parapsychologist G. N. M. Tyrrell explained:
+
+Mr. J. W. Dunne, in his book, An Experiment with Time, introduces a multidimensional scheme in an attempt to explain precognition and he has further developed this scheme in later publications. But, as Professor Broad has shown, these unlimited dimensions are unnecessary, ... and the true problem of time—the problem of becoming, or the passage of events from future through present to past, is not explained by them but is still left on the author's hands at the end.
+Later editions continued to receive attention. In 1981 a new impression of the 1934 (third) edition was published with an introduction by the writer and broadcaster Brian Inglis. The last (1948) edition was reprinted in 1981 with an introduction by the physicist and parapsychologist Russell Targ. A review of it in New Scientist described it as a "definitive classic".
+Mainstream scientific opinion remains that, while Dunne was an entertaining writer, there is no scientific evidence for either dream precognition or more than one time dimension and his arguments do not convince.
+
+=== Popular ===
+An Experiment with Time became well known and was widely discussed. Not to have read him became a "mark of singularity" in society. Critical essays on Serialism — some positive, some negative — appeared in popular works. Among others, H. G. Wells wrote an essay, "New Light on Mental Life: Mr. J.W. Dunne’s Experiments with Dreaming" in 1927, Jorge Luis Borges wrote a short essay, "El Tiempo y J. W. Dunne" (Time and J. W. Dunne) in 1940. and J. B. Priestley gave an accessible account in his study Man and Time (1964). Interest remains today, with for example Gary Lachman discussing Dunne's Serialism in 2022.
+
+== Sequels ==
+Besides issuing new editions of An Experiment with Time, Dunne published sequels exploring different aspects of Serialism. The Serial Universe (1934) examined its relation to contemporary physics in relativity and quantum mechanics. The New Immortality (1938) and Nothing Dies (1940) explored the metaphysical aspect of Serialism, especially in relation to immortality. Intrusions? (1955) contained autobiographical accounts of the angelic visions and voices which had accompanied many of his precognitive dreams. It was incomplete at the time of his death in 1949; it was completed with the help of his family and finally published some years later. It revealed that he believed himself to be a spiritual medium. He had deliberately chosen to leave this material out of An Experiment with Time as he judged that it would have affected the scientific reception of his theory.
+
+== Literary influence ==
+The popularity of An Experiment with Time was reflected in the many authors who subsequently referenced him and his ideas in literary works of fiction. He "undoubtedly helped to form something of the imaginative climate of those [interwar] years". One of the first and most significant writers was J. B. Priestley, who used Dunne's ideas in three of his "Time plays": Time and the Conways, Dangerous Corner, and An Inspector Calls.
+Dunne's theory strongly influenced the unfinished novels The Notion Club Papers by J. R. R. Tolkien and The Dark Tower by C. S. Lewis. Tolkien and Lewis were both members of the Inklings literary circle. Tolkien used Dunne's ideas about parallel time dimensions in developing the differing natures of time in The Lord of the Rings between "Lórien time" and time in the rest of Middle-earth. Lewis used the imagery of serialism in the afterlife he depicted at the end of The Last Battle, the closing tale in the Chronicles of Narnia.
+Other important contemporary writers who used his ideas, whether as a narrative or literary device, included John Buchan (The Gap in the Curtain), James Hilton (Random Harvest), his old friend H. G. Wells (The Queer Story of Brownlow’s Newspaper and The Shape of Things to Come), Graham Greene (The Bear Fell Free) and Rumer Godden (A Fugue in Time). Literary figures less overtly influenced included T. S. Eliot, James Joyce and Flann O'Brien.
+Following Dunne's death in 1949, the popularity of his themes continued. Philippa Pearce's 1958 childhood fantasy Tom's Midnight Garden makes use of Dunne's theory of time and won the British literary Carnegie Medal. The writer Vladimir Nabokov undertook his own dream experiment in 1964, following Dunne's instructions, and it strongly influenced his subsequent novels, especially Ada or Ardor: A Family Chronicle.
+
+== See also ==
+Dreamtime, an Australian aboriginal merging of past, present and future.
+C. H. Hinton, an early proponent of time as the fourth dimension who influenced Dunne.
+P. D. Ouspensky, who proposed an alternative theory of cyclic time.
+
+== References ==
+
+=== Bibliography ===
+Flieger, Verlyn (2001). A Question of Time: J.R.R. Tolkien's Road to Faërie. Kent State University Press. pp. 38–47. ISBN 978-0-87338-699-9.
+Jones, Darryl (2020). "J. W. Dunne: The Time Traveller". In Ferguson, T. (ed.). Literature and Modern Time. Palgrave Macmillan. pp. 209–231. doi:10.1007/978-3-030-29278-2_9. ISBN 978-3-030-29277-5.
+Priestley, J. B. (1989) [1964 (Aldus)]. Man and Time. Bloomsbury. ISBN 978-1870630672. OCLC 796254114.
+Stewart, Victoria (Autumn 2008). "J. W. Dunne and literary culture in the 1930s and 1940s". Literature and History. 17 (2): 62–81. doi:10.7227/LH.17.2.5. S2CID 192883327.
+
+== Further reading ==
+Ernest Nagel. (1927). An Experiment with Time. The Journal of Philosophy 24 (25): 690-692.
+Samuel Soal. (1927). Review: An Experiment with Time. Journal of the Society for Psychical Research 24: 119-123.
+
+== External links ==
+An Experiment with Time at Faded Page (Canada)
+An Experiment with Time at Internet Archive
+An Experiment with Time at HathiTrust

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+title: "Arrow of time"
+chunk: 1/4
+source: "https://en.wikipedia.org/wiki/Arrow_of_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:44.126821+00:00"
+instance: "kb-cron"
+---
+
+The arrow of time, also called time's arrow, is the concept positing the "one-way direction" or "asymmetry" of time. It was developed in 1927 by the British astrophysicist Arthur Eddington, and is an unsolved general physics question. This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies, and might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper").
+The arrow of time paradox was originally recognized in the 1800s for gases (and other substances) as a discrepancy between microscopic and macroscopic description of thermodynamics / statistical physics. At the microscopic level physical processes are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) of time.
+
+== Overview ==
+The symmetry of time (T-symmetry) can be understood simply as the following: if time were perfectly symmetrical, a video of real events would seem realistic whether played forwards or backwards. Gravity, for example, is a time-reversible force. A ball that is tossed up, slows to a stop, and falls is a case where recordings would look equally realistic forwards and backwards. The system is T-symmetrical. However, the process of the ball bouncing and eventually coming to a stop is not time-reversible. While going forward, kinetic energy is dissipated and entropy is increased. Entropy may be one of the few processes that is not time-reversible. According to the statistical notion of increasing entropy, the "arrow" of time is identified with a decrease of free energy.
+In his book The Big Picture, physicist Sean M. Carroll compares the asymmetry of time to the asymmetry of space: While physical laws are in general isotropic, near Earth there is an obvious distinction between "up" and "down", due to proximity to this huge body, which breaks the symmetry of space. Similarly, physical laws are in general symmetric to the flipping of time direction, but near the Big Bang (i.e., in the first many trillions of years following it), there is an obvious distinction between "forward" and "backward" in time, due to relative proximity to this special event, which breaks the symmetry of time. Under this view, all the arrows of time are a result of our relative proximity in time to the Big Bang and the special circumstances that existed then. (Strictly speaking, the weak interactions are asymmetric to both spatial reflection and to flipping of the time direction. However, they do obey a more complicated symmetry that includes both.)
+
+== Conception by Eddington ==
+In the 1928 book The Nature of the Physical World, which helped to popularize the concept, Eddington stated:
+
+Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase 'time's arrow' to express this one-way property of time which has no analogue in space.
+Eddington then gives three points to note about this arrow:
+
+It is vividly recognized by consciousness.
+It is equally insisted on by our reasoning faculty, which tells us that a reversal of the arrow would render the external world nonsensical.
+It makes no appearance in physical science except in the study of organization of a number of individuals. (In other words, it is only observed in entropy, a statistical mechanics phenomenon arising from a system.)
+
+== Arrows ==
+
+=== Psychological/perceptual arrow of time ===

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+title: "Arrow of time"
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+category: "reference"
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+date_saved: "2026-05-05T11:13:44.126821+00:00"
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+---
+
+A related mental arrow arises because one has the sense that one's perception is a continuous movement from the known past to the unknown future. This phenomenon has two aspects: memory (we remember the past but not the future) and volition (we feel we can influence the future but not the past). The two aspects are a consequence of the causal arrow of time: past events (but not future events) are the cause of our present memories, as more and more correlations are formed between the outer world and our brain (see correlations and the arrow of time); and our present volitions and actions are causes of future events. This is because the increase of entropy is thought to be related to increase of both correlations between a system and its surroundings and of the overall complexity, under an appropriate definition; thus all increase together with time.
+Past and future are also psychologically associated with additional notions. English, along with other languages, tends to associate the past with "behind" and the future with "ahead", with expressions such as "to look forward to welcoming you", "to look back to the good old times", or "to be years ahead". However, this association of "behind ⇔ past" and "ahead ⇔ future" is culturally determined. For example, the Aymara language associates "ahead ⇔ past" and "behind ⇔ future" both in terms of terminology and gestures, corresponding to the past being observed and the future being unobserved. Similarly, the Chinese term for "the day after tomorrow" 後天 ("hòutiān") literally means "after (or behind) day", whereas "the day before yesterday" 前天 ("qiántiān") is literally "preceding (or in front) day", and Chinese speakers spontaneously gesture in front for the past and behind for the future, although there are conflicting findings on whether they perceive the ego to be in front of or behind the past. There are no languages that place the past and future on a left–right axis (e.g., there is no expression in English such as *the meeting was moved to the left), although at least English speakers associate the past with the left and the future with the right, which seems to have its origin in the left-to-right writing system.
+The words "yesterday" and "tomorrow" both translate to the same word in Hindi: कल ("kal"), meaning "[one] day remote from today." The ambiguity is resolved by verb tense. परसों ("parson") is used for both "day before yesterday" and "day after tomorrow", or "two days from today".
+The other side of the psychological passage of time is in the realm of volition and action. We plan and often execute actions intended to affect the course of events in the future. From the Rubaiyat:
+
+— Omar Khayyam (translation by Edward Fitzgerald).
+
+In June 2022, researchers reported in Physical Review Letters finding that salamanders were demonstrating counter-intuitive responses to the arrow of time in how their eyes perceived different stimuli.
+
+=== Thermodynamic arrow of time ===
+
+The arrow of time is the "one-way direction" or "asymmetry" of time. The thermodynamic arrow of time is provided by the second law of thermodynamics, which says that in an isolated system, entropy tends to increase with time. Entropy can be thought of as a measure of microscopic disorder; thus the second law implies that time is asymmetrical with respect to the amount of order in an isolated system: as a system advances through time, it becomes more statistically disordered. This asymmetry can be used empirically to distinguish between future and past, though measuring entropy does not accurately measure time. Also, in an open system, entropy can decrease with time. An interesting thought experiment would be to ask: "if entropy was increased in an open system, would the arrow of time flip in polarity and point towards the past."
+British physicist Sir Alfred Brian Pippard wrote: "There is thus no justification for the view, often glibly repeated, that the Second Law of Thermodynamics is only statistically true, in the sense that microscopic violations repeatedly occur, but never violations of any serious magnitude. On the contrary, no evidence has ever been presented that the Second Law breaks down under any circumstances." However, there are a number of paradoxes regarding violation of the second law of thermodynamics, one of them due to the Poincaré recurrence theorem.
+This arrow of time seems to be related to all other arrows of time and arguably underlies some of them, with the exception of the weak arrow of time.
+Harold Blum's 1951 book Time's Arrow and Evolution discusses "the relationship between time's arrow (the second law of thermodynamics) and organic evolution." This influential text explores "irreversibility and direction in evolution and order, negentropy, and evolution." Blum argues that evolution followed specific patterns predetermined by the inorganic nature of the earth and its thermodynamic processes.
+
+=== Cosmological arrow of time ===
+
+The cosmological arrow of time points in the direction of the universe's expansion. It may be linked to the thermodynamic arrow, with the universe heading towards a heat death (Big Chill) as the amount of Thermodynamic free energy becomes negligible. Alternatively, it may be an artifact of our place in the universe's evolution (see the Anthropic bias), with this arrow reversing as gravity pulls everything back into a Big Crunch.
+If this arrow of time is related to the other arrows of time, then the future is by definition the direction towards which the universe becomes bigger. Thus, the universe expands—rather than shrinks—by definition.
+The thermodynamic arrow of time and the second law of thermodynamics are thought to be a consequence of the initial conditions in the early universe. Therefore, they ultimately result from the cosmological set-up.

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+title: "Arrow of time"
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+date_saved: "2026-05-05T11:13:44.126821+00:00"
+instance: "kb-cron"
+---
+
+=== Radiative arrow of time ===
+Waves, from radio waves to sound waves to those on a pond from throwing a stone, expand outward from their source, even though the wave equations accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves, so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it, making the latter contradictory to the second law of thermodynamics in usual circumstances.
+
+=== Causal arrow of time ===
+A cause precedes its effect: the causal event occurs before the event it causes or affects. Birth, for example, follows a successful conception and not vice versa. Thus causality is intimately bound up with time's arrow.
+An epistemological problem with using causality as an arrow of time is that, as David Hume maintained, the causal relation per se cannot be perceived; one only perceives sequences of events. Furthermore, it is surprisingly difficult to provide a clear explanation of what the terms cause and effect really mean, or to define the events to which they refer. However, it does seem evident that dropping a cup of water is a cause while the cup subsequently shattering and spilling the water is the effect.
+Physically speaking, correlations between a system and its surrounding are thought to increase with entropy, and have been shown to be equivalent to it in a simplified case of a finite system interacting with the environment. The assumption of low initial entropy is indeed equivalent to assuming no initial correlations in the system; thus correlations can only be created as we move forward in time, not backwards. Controlling the future, or causing something to happen, creates correlations between the doer and the effect, and therefore the relation between cause and effect is a result of the thermodynamic arrow of time, a consequence of the second law of thermodynamics. Indeed, in the above example of the cup dropping, the initial conditions have high order and low entropy, while the final state has high correlations between relatively distant parts of the system – the shattered pieces of the cup, as well as the spilled water, and the object that caused the cup to drop.
+
+=== Quantum arrow of time ===
+Quantum evolution is governed by equations of motions that are time-symmetric (such as the Schrödinger equation in the non-relativistic approximation), and by wave function collapse, which is a time-irreversible process, and is only physically real in explicit collapse interpretations of quantum theory, such as the Diósi–Penrose model, the Ghirardi–Rimini–Weber theory, or the Transactional interpretation, which uses the direct-action or "absorber" theory of fields. 
+The conventional approach is to assume that quantum decoherence explains irreversibility and the second law of thermodynamics, thus claiming to derive the quantum arrow of time from the thermodynamic arrow of time; however this is a matter of some debate, since the underlying dynamics is assumed to be unitary and thus reversible.
+ 
+A conventional account of decoherence is to say that following any particle scattering or interaction between two larger systems, the relative phases of the two systems are at first orderly related, but subsequent interactions (with additional particles or systems) make them less so, so that the two systems become decoherent. Thus decoherence is a form of increase in microscopic disorder –  in short, decoherence increases entropy. Two decoherent systems can no longer interact via quantum superposition, unless they become coherent again, which is normally impossible, by the second law of thermodynamics. In the language of relational quantum mechanics, the observer becomes entangled with the measured state, where this entanglement increases entropy. As stated by Seth Lloyd, "the arrow of time is an arrow of increasing correlations".
+However, under special circumstances, one can prepare initial conditions that will cause a decrease in decoherence and in entropy. This has been shown experimentally in 2019, when a team of Russian scientists reported the reversal of the quantum arrow of time on an IBM quantum computer, in an experiment supporting the understanding of the quantum arrow of time as emerging from the thermodynamic one. By observing the state of the quantum computer made of two and later three superconducting qubits, they found that in 85% of the cases, the two-qubit computer returned to the initial state. The state's reversal was made by a special program, similarly to the random microwave background fluctuation in the case of the electron. However, according to the estimations, throughout the age of the universe (around 14 billion years) such a reversal of the electron's state would only happen once, for 0.06 nanoseconds. The scientists' experiment led to the possibility of a quantum algorithm that reverses a given quantum state through complex conjugation of the state.
+Note that quantum decoherence merely allows the appearance of quantum wave collapse (based on the vanishing of diagonal elements of the density matrix); it is a matter of dispute whether the collapse itself actually takes place or is redundant and apparent only. While the theory of quantum decoherence is widely accepted and has been supported experimentally at the level of the applicable density matrix, the conventional theory's inability to predict actual measurement outcomes via non-unitary collapse remains. That is, the density matrix obtained from standard unitary-only decoherence (without actual collapse) is an improper mixture that cannot be interpreted as reflecting a determinate measurement outcome. Thus the arrow of time question continues to be addressed by way of explicit collapse approaches.
+
+=== Particle physics (weak) arrow of time ===

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+title: "Arrow of time"
+chunk: 4/4
+source: "https://en.wikipedia.org/wiki/Arrow_of_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:44.126821+00:00"
+instance: "kb-cron"
+---
+
+Certain subatomic interactions involving the weak nuclear force violate the conservation of both parity and charge conjugation, but only very rarely. An example is the kaon decay. According to the CPT theorem, this means they should also be time-irreversible, and so establish an arrow of time. Such processes should be responsible for matter creation in the early universe.
+That the combination of parity and charge conjugation is broken so rarely means that this arrow only "barely" points in one direction, setting it apart from the other arrows whose direction is much more obvious. This arrow had not been linked to any large-scale temporal behaviour until the work of Joan Vaccaro, who showed that T violation could be responsible for conservation laws and dynamics.
+
+== See also ==
+A Brief History of Time
+Anthropic principle
+Ilya Prigogine
+Loschmidt's paradox
+Maxwell's demon
+Quantum Zeno effect
+Royal Institution Christmas Lectures 1999
+Samayā
+Time evolution
+Time flies like an arrow
+Time reversal signal processing
+Wheeler–Feynman absorber theory
+
+== References ==
+
+== Further reading ==
+Lebowitz, Joel L. (2008). "Time's arrow and Boltzmann's entropy". Scholarpedia. 3 (4): 3448. Bibcode:2008SchpJ...3.3448L. doi:10.4249/scholarpedia.3448.
+Boltzmann, Ludwig (1964). Lectures On Gas Theory. University Of California Press. Translated from the original German by Stephen G. Brush. Originally published 1896/1898.
+Carroll, Sean (2010). From Eternity to Here: The Quest for the Ultimate Theory of Time. Dutton. Website.
+Coveney, Peter; Highfield, Roger (1990), The Arrow of Time: A voyage through science to solve time's greatest mystery, London: W. H. Allen, Bibcode:1990atvt.book.....C, ISBN 978-1-85227-197-8.
+Feynman, Richard (1965). The Character of Physical Law. BBC Publications. Chapter 5.
+Halliwell, J. J.; et al. (1994). Physical Origins of Time Asymmetry. Cambridge. ISBN 978-0-521-56837-1. (technical).
+Mersini-Houghton, L., Vaas, R. (eds.) (2012) The Arrows of Time. A Debate in Cosmology. Springer. 2012-06-22. ISBN 978-3-642-23258-9. (partly technical).
+Münster, Gernot (2026). "What is Time? - Thoughts of a Physicist". In Blass, Heribert; Bleger, Leopoldo; Picard, Joëlle (eds.). Time and the Experience of Time. London and New York: Routledge. pp. 41–53. ISBN 978-1-041-11405-5.{{cite book}}:  CS1 maint: multiple names: editors list (link)
+Peierls, R (1979). Surprises in Theoretical Physics. Princeton. Bibcode:1979stp..book.....P. Section 3.8.
+Penrose, Roger (1989). The Emperor's New Mind. Oxford University Press. ISBN 978-0-19-851973-7. Chapter 7.
+Penrose, Roger (2004). The Road to Reality. Jonathan Cape. ISBN 978-0-224-04447-9. Chapter 27.
+Price, Huw (1996). Time's Arrow and Archimedes' Point. Oxford University Press. ISBN 978-0-19-510095-2. Website.
+Zeh, H. D (2010). The Physical Basis of The Direction of Time. Springer. ISBN 978-3-540-42081-1. Official website for the book.
+"BaBar Experiment Confirms Time Asymmetry".
+
+== External links ==
+Fieser, James; Dowden, Bradley (eds.). "The Arrow of Time". Internet Encyclopedia of Philosophy. ISSN 2161-0002. OCLC 37741658.
+The Ritz-Einstein Agreement to Disagree, a review of historical perspectives of the subject, prior to the evolvement of quantum field theory
+The Thermodynamic Arrow: Puzzles and Pseudo-Puzzles Huw Price on Time's Arrow
+Arrow of time in a discrete toy model
+The Arrow of Time
+Why Does Time Run Only Forwards, by Adam Becker, bbc.com
+Carroll, Sean M. (14 January 2011). Cosmology and the arrow of time: Sean Carroll at TEDxCaltech (Video). New York; Vancouver, British Columbia: TED Conferences LLC. Archived from the original on 20 December 2019. Retrieved 20 January 2020.

data/en.wikipedia.org/wiki/Astronomical_year_numbering-0.md

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+---
+title: "Astronomical year numbering"
+chunk: 1/1
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+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:37.692854+00:00"
+instance: "kb-cron"
+---
+
+Astronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0; the years before that are designated with negative numbers and the years after that are designated with positive numbers. Astronomers use the Julian calendar for years before 1582, including the year 0, and the Gregorian calendar for years after 1582, as exemplified by Jacques Cassini (1740), Simon Newcomb (1898) and Fred Espenak (2007).
+The prefix AD and the suffixes CE, BC or BCE (Common Era, Before Christ or Before Common Era) are dropped. The year 1 BC/BCE is numbered 0, the year 2 BC is numbered −1, and in general the year n BC/BCE is numbered "−(n − 1)" (a negative number equal to 1 − n). The numbers of AD/CE years are not changed and are written with either no sign or a positive sign; thus in general n AD/CE is simply n or +n. For normal calculation a number zero is often needed, here most notably when calculating the number of years in a period that spans the epoch; the end years need only be subtracted from each other.
+The system is so named due to its use in astronomy. Few other disciplines outside history deal with the time before year 1, some exceptions being dendrochronology, archaeology and geology, the latter two of which use 'years before the present'. Although the absolute numerical values of astronomical and historical years only differ by one before year 1, this difference is critical when calculating astronomical events like eclipses or planetary conjunctions to determine when historical events which mention them occurred.
+
+
+== Usage of the year zero ==
+
+In his Rudolphine Tables (1627), Johannes Kepler used a prototype of year zero which he labeled Christi (Christ's) between years labeled Ante Christum (Before Christ) and Post Christum (After Christ) on the mean motion tables for the Sun, Moon, Saturn, Jupiter, Mars, Venus and Mercury. In 1702, the French astronomer Philippe de la Hire used a year he labeled Christum 0 at the end of years labeled ante Christum (BC), and immediately before years labeled post Christum (AD) on the mean motion pages in his Tabulæ Astronomicæ, thus adding the designation 0 to Kepler's Christi. Finally, in 1740 the French astronomer Jacques Cassini (Cassini II), who is traditionally credited with the invention of year zero, completed the transition in his Tables astronomiques, simply labeling this year 0, which he placed at the end of Julian years labeled avant Jesus-Christ (before Jesus Christ or BC), and immediately before Julian years labeled après Jesus-Christ (after Jesus Christ or AD).
+Cassini gave the following reasons for using a year 0: 
+
+The year 0 is that in which one supposes that Jesus Christ was born, which several chronologists mark 1 before the birth of Jesus Christ and which we marked 0, so that the sum of the years before and after Jesus Christ gives the interval which is between these years, and where numbers divisible by 4 mark the leap years as so many before or after Jesus Christ.
+Fred Espenak of NASA lists 50 phases of the Moon within year 0, showing that it is a full year, not an instant in time. Jean Meeus gives the following explanation:
+
+There is a disagreement between astronomers and historians about how to count the years preceding year 1. In [Astronomical Algorithms], the 'B.C.' years are counted astronomically. Thus, the year before the year +1 is the year zero, and the year preceding the latter is the year −1. The year which historians call 585 B.C. is actually the year −584.
+The astronomical counting of the negative years is the only one suitable for arithmetical purpose. For example, in the historical practice of counting, the rule of divisibility by 4 revealing Julian leap-years no longer exists; these years are, indeed, 1, 5, 9, 13, ... B.C. In the astronomical sequence, however, these leap-years are called 0, −4, −8, −12, ..., and the rule of divisibility by 4 subsists.
+
+
+== Signed years without the year zero ==
+Although he used the usual French terms "avant J.-C." (before Jesus Christ) and "après J.-C." (after Jesus Christ) to label years elsewhere in his book, the Byzantinist historian Venance Grumel (1890–1967) used negative years (identified by a minus sign, −) to label BC years and unsigned positive years to label AD years in a table. He may have done so to save space and he put no year 0 between them.
+Version 1.0 of the XML Schema language, often used to describe data interchanged between computers in XML, includes built-in primitive datatypes date and dateTime. Although these are defined in terms of ISO 8601 which uses the proleptic Gregorian calendar and therefore should include a year 0, the XML Schema specification states that there is no year zero. Version 1.1 of the defining recommendation realigned the specification with ISO 8601 by including a year zero, despite the problems arising from the lack of backward compatibility.
+
+
+== See also ==
+Julian day, another calendar commonly used by astronomers
+Astronomical chronology
+Holocene calendar
+ISO 8601
+
+
+== References ==

data/en.wikipedia.org/wiki/Atom_(time)-0.md

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+---
+title: "Atom (time)"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Atom_(time)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:45.286592+00:00"
+instance: "kb-cron"
+---
+
+An atom of time is the smallest possible unit of time.
+
+
+== History ==
+One of the earliest occurrences of the word "atom" to mean the smallest possible unit of measuring time is found in the Greek text of the New Testament in Paul's 1 Corinthians 15:52. The text compares the length of time of the "atom" to the time needed for "the twinkling of an eye." The text reads: "ἐν ἀτόμῳ, ἐν ῥιπῇ ὀφθαλμοῦ" – "en atomo, en ripe ophthamou" – the word "atom" is usually translated "a moment" – "In a moment, in the twinkling of an eye". 
+It was later referred to in medieval philosophical writings in this sense as the smallest possible division of time. The earliest known occurrence in English is in Byrhtferth's Enchiridion of 1010–1012, where it was defined as 1/564 of a momentum (1½ minutes), and thus equal to almost 160 milliseconds. It was used in the computus, the calculation used to determine the calendar date of Easter.
+
+
+== See also ==
+Planck time
+
+
+== References ==

data/en.wikipedia.org/wiki/B-theory_of_time-0.md

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+---
+title: "B-theory of time"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/B-theory_of_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:46.457538+00:00"
+instance: "kb-cron"
+---
+
+The B-theory of time, also called the "tenseless theory of time", is one of two positions regarding the temporal ordering of events in the philosophy of time. B-theorists argue that the flow of time is only a subjective illusion of human consciousness, that the past, present, and future are equally real, and that time is tenseless: temporal becoming is not an objective feature of reality. Therefore, there is nothing privileged about the present, ontologically speaking.
+The B-theory is derived from a distinction drawn by J. M. E. McTaggart between A series and B series. The B-theory is often drawn upon in theoretical physics and is seen in theories such as eternalism.
+
+== Origin of terms ==
+The terms A-theory and B-theory, first coined by Richard M. Gale in 1966, derive from Cambridge philosopher J. M. E. McTaggart's analysis of time and change in "The Unreality of Time" (1908), in which events are ordered via a tensed A-series or a tenseless B-series. It is popularly assumed that the A-theory represents time like an A-series, while the B-theory represents time like a B-series.
+Events (or "times"), McTaggart observed, may be characterized in two distinct but related ways. On the one hand they can be characterized as past, present or future, normally indicated in natural languages such as English by the verbal inflection of tenses or auxiliary adverbial modifiers. Alternatively, events may be described as earlier than, simultaneous with, or later than others. Philosophers are divided as to whether the tensed or tenseless mode of expressing temporal fact is fundamental. Some philosophers have criticised hybrid theories, where one holds a tenseless view of time but asserts that the present has special properties, as falling foul of McTaggart's paradox. For a thorough discussion of McTaggart's paradox, see R. D. Ingthorsson (2016).
+The debate between A-theorists and B-theorists is a continuation of a metaphysical dispute reaching back to the ancient Greek philosophers Heraclitus and Parmenides. Parmenides thought that reality is timeless and unchanging. Heraclitus, in contrast, believed that the world is a process of ceaseless change or flux. Reality for Heraclitus is dynamic and ephemeral. Indeed, the world is so fleeting, according to Heraclitus, that it is impossible to step twice into the same river. The metaphysical issues that continue to divide A-theorists and B-theorists concern the reality of the past, the reality of the future, and the ontological status of the present.
+
+== B-theory in metaphysics ==
+The difference between A-theorists and B-theorists is often described as a dispute about temporal passage or 'becoming' and 'progressing'. B-theorists argue that this notion is purely psychological. Many A-theorists argue that in rejecting temporal 'becoming', B-theorists reject time's most vital and distinctive characteristic. It is common (though not universal) to identify A-theorists' views with belief in temporal passage. Another way to characterise the distinction revolves around what is known as the principle of temporal parity, the thesis that contrary to what appears to be the case, all times really exist in parity. A-theory (and especially presentism) denies that all times exist in parity, while B-theory insists all times exist in parity.
+B-theorists such as D. H. Mellor and J. J. C. Smart wish to eliminate all talk of past, present and future in favour of a tenseless ordering of events, believing the past, present, and future to be equally real, opposing the idea that they are irreducible foundations of temporality. B-theorists also argue that the past, present, and future feature very differently in deliberation and reflection. For example, we remember the past and anticipate the future, but not vice versa. B-theorists maintain that the fact that we know much less about the future simply reflects an epistemological difference between the future and the past: the future is no less real than the past; we just know less about it.
+
+== Opposition ==

data/en.wikipedia.org/wiki/B-theory_of_time-1.md

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+---
+title: "B-theory of time"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/B-theory_of_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:46.457538+00:00"
+instance: "kb-cron"
+---
+
+=== Irreducibility of tense ===
+Earlier B-theorists argued that one could paraphrase tensed sentences (such as "the sun is now shining", uttered on September 28) into tenseless sentences (such as "on September 28, the sun shines") without loss of meaning. Later B-theorists argued that tenseless sentences could give the truth conditions of tensed sentences or their tokens. Quentin Smith argues that "now" cannot be reduced to descriptions of dates and times, because all date and time descriptions, and therefore truth conditionals, are relative to certain events. Tensed sentences, on the other hand, do not have such truth conditionals. The B-theorist could argue that "now" is reducible to a token-reflexive phrase such as "simultaneous with this utterance", yet Smith states that even such an argument fails to eliminate tense. One can think the statement "I am not uttering anything now", and such a statement would be true. The statement "I am not uttering anything simultaneous with this utterance" is self-contradictory, and cannot be true even when one thinks the statement. Finally, while tensed statements can express token-independent truth values, no token-reflexive statement can do so (by definition of the term "token-reflexive"). Smith claims that proponents of the B-theory argue that the inability to translate tensed sentences into tenseless sentences does not prove A-theory.
+Logician and philosopher Arthur Prior has also drawn a distinction between what he calls A-facts and B-facts. The latter are facts about tenseless relations, such as the fact that the year 2025 is 25 years later than the year 2000. The former are tensed facts, such as that the Jurassic age is in the past, or that the end of the universe is in the future. Prior asks the reader to imagine having a headache, and after the headache subsides, saying "thank goodness that's over." Prior argues that the B-theory cannot make sense of this sentence. It seems bizarre to be thankful that a headache is earlier than one's utterance, anymore than being thankful that the headache is later than one's utterance. Indeed, most people who say "thank goodness that's over" are not even thinking of their own utterance. Therefore, when people say "thank goodness that's over," they are thankful for an A-fact, and not a B-fact. Yet, A-facts are only possible on the A-theory of time. (See also: Further facts.)
+
+=== Endurantism and perdurantism ===
+Opponents also charge the B-theory with being unable to explain persistence of objects. The two leading explanations for this phenomenon are endurantism and perdurantism. According to the former, an object is wholly present at every moment of its existence. According to the latter, objects are extended in time and therefore have temporal parts. Hales and Johnson explain endurantism as follows: "something is an enduring object only if it is wholly present at each time in which it exists. An object is wholly present at a time if all of its parts co-exist at that time." Under endurantism, all objects must exist as wholes at each point in time, but an object such as a rotting fruit will have the property of being not rotten one day and being rotten on another. On eternalism, and hence the B-theory, it seems that one is committed to two conflicting states for the same object. The spacetime (Minkowskian) interpretation of relativity adds an additional problem for endurantism under B-theory. On the spacetime interpretation, an object may appear as a whole at its rest frame, but on an inertial frame, it will have proper parts at different positions, and therefore different parts at different times. Hence it will not exist as a whole at any time, contradicting endurantism.
+Opponents will then charge perdurantism with numerous difficulties of its own. First, it is controversial whether perdurantism can be formulated coherently. An object is defined as a collection of spatiotemporal parts, defined as pieces of a perduring object. If objects have temporal parts, this leads to difficulties. For example, the rotating discs argument asks the reader to imagine a world containing nothing more than a homogeneous spinning disk. Under endurantism, the same disc endures despite its rotations. The perdurantist supposedly has a difficult time explaining what it means for such a disc to have a determinate state of rotation. Temporal parts also seem to act unlike physical parts. A piece of chalk can be broken into two physical halves, but it seems nonsensical to talk about breaking it into two temporal halves. American epistemologist Roderick Chisholm argued that someone who hears the bird call "Bob White" knows "that his experience of hearing 'Bob' and his experience of hearing 'White' were not also had by two other things, each distinct from himself and from each other. The endurantist can explain the experience as "There exists an x such that x hears 'Bob' and then x hears 'White'" but the perdurantist cannot give such an account. Peter van Inwagen asks the reader to consider Descartes as a four-dimensional object that extends from 1596 to 1650. If Descartes had lived a much shorter life, he would have had a radically different set of temporal parts. This diminished Descartes, he argues, could not have been the same person on perdurantism, since their temporal extents and parts are so different.
+
+=== First-person perspectives ===
+Vincent Conitzer has argued against B-theory due to the existence of first-person perspectives and Benj Hellie's vertiginous question. He argues that arguments in favor of the A-theory of time are more effective as arguments for the combined position that A-theory is true and the "I" is metaphysically privileged from other perspectives. Caspar Hare has discussed similar ideas with the theories of egocentric presentism and perspectival realism.
+
+== Notes ==
+
+== References ==
+
+== External links ==
+Markosian, Ned, 2002, "Time", Stanford Encyclopedia of Philosophy
+Arthur Prior, Stanford Encyclopedia of Philosophy

data/en.wikipedia.org/wiki/Centered_world-0.md

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+---
+title: "Centered world"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Centered_world"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:48.889143+00:00"
+instance: "kb-cron"
+---
+
+A centered world, according to David Kellogg Lewis, consists of (1) a possible world, (2) an agent in that world, and (3) a time in that world.  The concept of centered worlds has epistemic as well as metaphysical uses; for the latter, the three components of a centered world have connections to theories such as actualism, solipsism (especially egocentric presentism and perspectival realism), and presentism, respectively.
+
+
+== References ==

data/en.wikipedia.org/wiki/Chronosophy-0.md

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+---
+title: "Chronosophy"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Chronosophy"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:50.070367+00:00"
+instance: "kb-cron"
+---
+
+Chronosophy is the neologistic designation given by scholar Julius Thomas (J.T.) Fraser to "the interdisciplinary and normative study of time sui generis."
+
+== Overview ==
+
+=== Etymology ===
+Fraser derived the term from the Ancient Greek: χρόνος, chronos, "time" and σοφία, sophia, "wisdom". Chronosophia is thus defined as the "specific human skill or knowledge . . . pertaining to time . . . [which] all men seem to possess to some degree . . .".
+
+=== Purpose ===
+Fraser outlined the purpose of the discipline of chronosophy in five intentions, as follows:
+
+to encourage the search for new knowledge related to time;
+to set up and apply criteria regarding which fields of knowledge contribute to an understanding of time, and what they may contribute;
+to assist in epistemological studies, especially in those related to the structure of knowledge;
+to provoke communication between the humanities and the sciences using time as the common theme; and
+to help us learn more about the nature of time by providing channels for the direct confrontation of a multitude of views.
+
+=== Assumptions ===
+According to Fraser, any pursuit of chronosophical knowledge necessarily makes two assumptions:
+
+When specialists speak of 'time', they speak of various aspects of the same entity.
+Said entity is amenable to study by the methods of the sciences, can be made a meaningful subject of contemplation by the reflective mind, and can be used as proper material for intuitive interpretation by the creative artist.
+Fraser labeled these two assumptions the unity of time. Together they amount to the proposition that all of us, working separately, are nevertheless headed toward the same central idea (i.e., chronosophia).
+In contradistinction to the aforementioned, Fraser posits the diversity of time: the existence of time's myriad manifestations, which "hardly needs proof; it is all too apparent."
+The continued qualitative and quantitative mediation of the unresolvable conflict between the unity and diversity of time would thus be the sole methodological criterion for measuring chronosophical progress. This conflict manifests itself not so much as between the humanities and the sciences (although this interpretation is cogent and apt), but rather between knowledge felt (i.e., passion) and knowledge understood (i.e., knowledge proper). Fraser envisions the total creativity of a society as being dependent on the effectiveness of "a harmonious dialogue between the two great branches of knowledge." He observes (paraphrasing Giordano Bruno): "The creative activity of the mind consists of the search for the one in the many, for simplicity in variety. There is no better and more fundamental problem than the problem of time in respect to which such [a] search may be conducted. It is always present and always tantalizing, it is the basic material of man's rational and emotive inquiries." Just as a mature individual can reconcile within themselves the unity and diversity of day-to-day noetic existence, so too could a mature social conception of time mediate the difference between—and perhaps ultimately reconcile—the unity and diversity of time.
+
+=== Organization ===
+Chronosophy defies systematic organization, for—like philosophy—it is a kind of ur-discipline, subsuming all other disciplines through a proposed unifying characteristic: temporality. (Hence, the possibility of producing a branch of knowledge lacking temporal import, e.g. [arguably] ontology and/or metaphysics, remains; however, "atemporality" is still, by definition, a temporal category: a regress ensues.)
+Fraser wrote that a successful study of time would "encourage communication across the traditional boundaries of systems of knowledge and seek a framework which . . . may permit interaction of experience and theorizing related to time without regard to the sources of experience and theory." Thus, the only methodological commitment that a chronosopher need make is to interdisciplinarity.
+While Fraser neglects to develop a systematic chronosophical methodology in The Voices of Time, he does proffer a selection of idiomatically interdisciplinary categories to spur the research of future scholars:
+
+surveys of historical and current ideas of time in the sciences and in the humanities;
+studies of the relation of time to ideas of conceptual extremities such as a) to motion and rest, b) to atomicity and continuity, c) to the spatially very large and very small, and d) to the quantities of singular and many;
+comparative analysis of those properties of time that various fields of learning and intuitive expressions designate unproblematically as "the nature of time";
+inquiries into the processes and methods whereby man learns to perceive, proceeds to measure, and proposes to reason about time;
+exploration of the role of time in the communication of thought and emotion;
+search for an understanding of the relation of time to personal identity and death;
+research concerning time and organic evolution, time and the psychological development of man, and the role f time in the growth of civilizations; and
+determination of the status of chronosophy vis-à-vis the traditional systems of knowledge.
+The nature of the above categories would require that chronosophy be regarded as an independent system of experiential, experimental, and theoretical knowledge about time.
+
+=== General characteristics ===
+In general, chronosophical pursuits are characterized by

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+---
+title: "Chronosophy"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Chronosophy"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:50.070367+00:00"
+instance: "kb-cron"
+---
+
+expansion beyond or abandonment of [traditional areas of] specialization, and
+the espousal of interdisciplinary or pan-disciplinary methodologies;
+(1) is a weak criterion, while (2) is a strong one.
+Neither of the above criteria make reference to time or temporality; for while the ontological possibly of timeless knowledge must always remain, admission of this possibility begs the question (petitio principii): e.g., what form does timeless knowledge take? how would it come to us? how could we ever be separated from such knowledge to begin with? et cetera ad nauseam. The admission is therefore a paradox (akin to Wittgenstein's seventh proposition in Tractatus Logico-Philosophicus: "Whereof one cannot speak, thereof one must be silent."). We must conclude that the proposal of the possibility of timeless knowledge is both necessary and senseless, a conceptual counterpart to the tautologous nature of the concept of time. Should we come to possess knowledge of that which is "beneath" or "behind" time (or, alternatively, conclude we could never have lost possession of it), there would be no discernible need for further chronosophical inquiry—in the face of such eternal truth, it would instead be chronosophy as currently conceived that would appear both necessary and senseless.
+Hence: all disciplines are necessarily chronosophical (until proven otherwise).
+Caveat: for the sake of logicality, future manifestations of chronosophy may resemble more closely some methods of knowing than others; however, due to the character of the "problem" of time no chronosophical endeavor could ever be thoroughly purged of its interdisciplinary perspectives: a satisfactory theory of time must necessarily satisfy a wide variety of specifications (i.e., by definition a satisfactory or sufficient chronosophy would accommodate every office of human knowledge as pertains to the subject, time).
+
+=== Envoi ===
+Why should we afford time this privileged status among our speculative and empirical undertakings?
+A Fraserian chronosopher would argue that mediation of the problem(s) of time is essential to the creation and retention of individual and social identity. Hence, as long as we—as individuals and as social groups—continue to partake in the process of clarifying and defining our individual and collective identities over and against those of the world (in whole or in part) around us, the necessarily contemporaneous clarification and definition of the problem(s) of time must, by extension (mutatis mutandis), be universal and continuous.
+
+== See also ==
+Time
+Temporality
+Julius Thomas Fraser
+Natural Philosophy
+Philosophy of Space and Time
+Horology
+Cosmology
+Eschatology
+Ontology
+Metaphysics
+
+== References ==
+
+== External links ==
+International Society for the Study of Time Homepage

data/en.wikipedia.org/wiki/Coincidence-0.md

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+---
+title: "Coincidence"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Coincidence"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:51.260103+00:00"
+instance: "kb-cron"
+---
+
+A coincidence is a remarkable concurrence of events or circumstances that have no apparent causal connection with one another. The perception of remarkable coincidences may lead to supernatural, occult, or paranormal claims, or it may lead to belief in fatalism, which is a doctrine that events will happen in the exact manner of a predetermined plan. In general, the perception of coincidence, for lack of more sophisticated explanations, can serve as a link to folk psychology and philosophy.
+From a statistical perspective, coincidences are inevitable and often less remarkable than they may appear intuitively. Usually, coincidences are chance events with underestimated probability. An example is the birthday problem, which shows that the probability of two persons having the same birthday already exceeds 50% in a group of only 23 persons. Generalizations of the birthday problem are a key tool used for mathematically modelling coincidences.
+
+
+== Etymology ==
+The first known usage of the word coincidence is from c. 1605 with the meaning "exact correspondence in substance or nature" from the French coincidence, from coincider, from Medieval Latin coincidere. The definition evolved in the 1640s as "occurrence or existence during the same time". The word was introduced to English readers in the 1650s by Sir Thomas Browne, in A Letter to a Friend (circa 1656 pub. 1690) and in his discourse The Garden of Cyrus (1658).
+
+
+== Synchronicity ==
+
+Swiss psychiatrist Carl Jung developed a theory that states that remarkable coincidences occur because of what he called "synchronicity," which he defined as an "acausal connecting principle."
+
+The Jung-Pauli theory of "synchronicity", conceived by a physicist and a psychologist, both eminent in their fields, represents perhaps the most radical departure from the world-view of mechanistic science in our time. Yet they had a precursor, whose ideas had a considerable influence on Jung: the Austrian biologist Paul Kammerer, a wild genius who committed suicide in 1926, at the age of forty-five.
+One of Kammerer's passions was collecting coincidences. He published a book titled Das Gesetz der Serie (The Law of Series), which has not been translated into English. In this book, he recounted 100 or so anecdotes of coincidences that led him to formulate his theory of seriality.
+He postulated that all events are connected by waves of seriality. Kammerer was known to make notes in public parks of how many people were passing by, how many of them carried umbrellas, etc. Albert Einstein called the idea of seriality "interesting and by no means absurd." Carl Jung drew upon Kammerer's work in his book Synchronicity.
+A coincidence lacks an apparent causal connection. A coincidence may be synchronicity — the experience of events that are causally unrelated — and yet their occurrence together has meaning for the person who observes them. To be counted as synchronicity, the events should be unlikely to occur together by chance, but this is questioned because there is usually a chance, no matter how small and in vast numbers of opportunities such coincidences do happen by chance if it is only non-zero (see law of truly large numbers).
+Some skeptics (e.g., Georges Charpak and Henri Broch) argue synchronicity is merely an instance of apophenia. They argue that probability and statistical theory (exemplified, e.g., in Littlewood's law) suffice to explain remarkable coincidences.
+Charles Fort also compiled hundreds of accounts of interesting coincidences and strange phenomena.
+
+
+== Causality ==
+
+Measuring the probability of a series of coincidences is the most common method of distinguishing a coincidence from causally connected events.
+
+The mathematically naive person seems to have a more acute awareness than the specialist of the basic paradox of probability theory, over which philosophers have puzzled ever since Pascal initiated that branch of science [in 1654] .... The paradox consists, loosely speaking, of the fact that probability theory is able to predict with uncanny precision the overall outcome of processes made up of numerous individual happenings, each of which in itself is unpredictable. In other words, we observe many uncertainties producing certainty, and many chance events creating a lawful total outcome.
+To establish cause and effect (i.e., causality) is notoriously difficult, as is expressed by the commonly heard statement that "correlation does not imply causation." In statistics, it is generally accepted that observational studies can give hints but can never establish cause and effect. But, considering the probability paradox (see Koestler's quote above), it appears that the larger the set of coincidences, the more certainty increases, and the more it seems that there is some cause behind a remarkable coincidence.
+
+... it is only the manipulation of uncertainty that interests us. We are not concerned with the matter that is uncertain. Thus we do not study the mechanism of rain; only whether it will rain.
+It is no great wonder if in the long process of time, while fortune takes her course hither and thither, numerous coincidences should spontaneously occur.
+
+
+== See also ==
+
+
+== References ==
+
+
+== Bibliography ==
+David Marks: The Psychology of the Psychic. pp. 227–46
+Joseph Mazur (2016). Fluke: The Maths and Myths of Coincidences, London: Oneworld Publications. ISBN 978-1-78074-899-3
+
+
+== Further reading ==
+Beitman, Bernard (6 September 2022). Meaningful Coincidences How and Why Synchronicity and Serendipity Happen. Inner Traditions Bear. ISBN 9781644115718.
+Beitman, Bernard (7 March 2016). Connecting with Coincidence The New Science for Using Synchronicity and Serendipity in Your Life. Health Communications, Incorporated. ISBN 9780757318849.
+
+
+== External links ==
+
+Collection of Historical Coincidence, nephiliman.com (web.archive.org)
+Unlikely Events and Coincidence, Austin Society to Oppose Pseudoscience
+Why coincidences happen, UnderstandingUncertainty.org
+The Cambridge Coincidences Collection, University of Cambridge Statslab
+The mathematics of coincidental meetings

data/en.wikipedia.org/wiki/Craig_Callender-0.md

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+---
+title: "Craig Callender"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Craig_Callender"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:47.702523+00:00"
+instance: "kb-cron"
+---
+
+Craig Callender (born 1968) is a professor of philosophy at the University of California, San Diego. His main areas of research are philosophy of science, philosophy of physics and metaphysics.
+
+
+== Education and career ==
+Callender obtained his PhD in 1997 from Rutgers University with a thesis entitled Time's Arrow under the supervision of Robert Weingard. From 1996-2000, he worked in the Department of Philosophy, Logic & Scientific Method at the London School of Economics. Currently, he is a professor of philosophy at the University of California, San Diego where he is also the co-director of the Institute for Practical Ethics at the University of California, San Diego. Callender serves on the Committee for Freedom and Responsibility of Science of the International Science Council.
+Callender has written articles for Scientific American on the philosophy of time and participated in the World Science Festival 2013 with Tim Maudlin and Max Tegmark on the same topic.
+
+
+== Selected publications ==
+In reverse chronological order, unless otherwise specified.
+
+
+=== Books ===
+Callender, Craig (2017). What Makes Time Special? Oxford University Press, Oxford, ISBN 978-0-19-879730-2
+Callender, Craig, ed. (2011). The Oxford handbook of philosophy of time. Oxford Handbooks in Philosophy. Oxford University Press. ISBN 978-0-19-929820-4.
+Craig Callender (ed.): Time, Reality and Experience, Cambridge University Press, August 2002, ISBN 978-0-521-52967-9
+Craig Callender, Nick Huggett (eds.): Physics meets philosophy at the Planck scale: contemporary theories in quantum gravity, Cambridge University Press, 2001, ISBN 0-521-66280-X / ISBN 0-521-66445-4
+Craig Callender, Ralph Edney: Introducing time, Totem Books, 1997, ISBN 978-1-84046-263-0
+
+
+=== Articles ===
+Callender, Craig (June 2010). "Is time an illusion?". Scientific American. 302 (6): 40–47. Bibcode:2010SciAm.302f..58C. doi:10.1038/scientificamerican0610-58. PMID 20521481.
+Craig Callender, Robert Weingard: Topology change and the unity of space, Studies in History and Philosophy of Modern Physics, vol. 31, no. 2, pp. 227–246, 2000, full text
+Craig Callender, Robert Weingard: Nonlocality in the expanding infinite well, Foundations of Physics Letters, vol. 11, no. 5, pp. 495–498, 1998, full text
+Robert Weingard, Craig Callender: Trouble in paradise: Problems for Bohm's theory, The Monist, Quantum Mechanics and the Real World, vol. 80, no. 1 January 1997, abstract (in French language)
+Craig Callender, Robert Weingard: Time, Bohm's theory, and quantum cosmology, Philosophy of Science, vol. 63, September 1996, pp. 470–474, abstract
+Craig Callender, Robert Weingard: Bohmian cosmology and the quantum smearing of the initial singularity (communicated by Peter R. Holland), Physics Letters A, Volume 208, Issues 1-2, 20 November 1995, pp. 59–61, abstract
+Craig Callender, Robert Weingard: The Bohmian model of quantum cosmology, Philosophy of Science Association, PSA 1994, Vol. 1, pp. 218–227, abstract
+
+
+== References ==
+
+
+== External links ==
+C. Callendar (USCD)
+C. Callendar, publications list (USCD)
+C. Callendar, publications list (University of California Irvine)
+Discussion on Philosophy TV with Sean Carroll

data/en.wikipedia.org/wiki/Duration_(philosophy)-0.md

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+---
+title: "Duration (philosophy)"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Duration_(philosophy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:52.453466+00:00"
+instance: "kb-cron"
+---
+
+Duration (French: la durée) is a theory of time and consciousness posited by the French philosopher Henri Bergson. Bergson sought to improve upon inadequacies he perceived in the philosophy of Herbert Spencer, due, he believed, to Spencer's lack of comprehension of mechanics, which led Bergson to the conclusion that time eluded mathematics and science. Bergson became aware that the moment one attempted to measure a moment, it would be gone: one measures an immobile, complete line, whereas time is mobile and incomplete. For the individual, time may speed up or slow down, whereas, for science, it would remain the same. Hence Bergson decided to explore the inner life of man, which is a kind of duration, neither a unity nor a quantitative multiplicity. Duration is ineffable and can only be shown indirectly through images that can never reveal a complete picture. It can only be grasped through a simple intuition of the imagination.
+Bergson first introduced his notion of duration in his essay Time and Free Will: An Essay on the Immediate Data of Consciousness. It is used as a defense of free will in a response to Immanuel Kant, who believed free will was only possible outside time and space.
+
+== Responses to Kant and Zeno ==
+
+Zeno of Elea believed reality was an uncreated and indestructible immobile whole. He formulated four paradoxes to present mobility as an impossibility. We can never, he said, move past a single point because each point is infinitely divisible, and it is impossible to cross an infinite space. But to Bergson, the problem only arises when mobility and time, that is, duration, are mistaken for the spatial line that underlies them. Time and mobility are mistakenly treated as things, not progressions.  They are treated retrospectively as a thing's spatial trajectory, which can be divided ad infinitum, whereas they are, in fact, an indivisible whole.
+Bergson's response to Kant is that free will is possible within a duration within which time resides. Free will is not really a problem but merely a common confusion among philosophers caused by the immobile time of science. To measure duration (durée), it must be translated into the immobile, spatial time (temps) of science, a translation of the unextended into the extended. It is through this translation that the problem of free will arises. Since space is a homogeneous, quantitative multiplicity, as opposed to what Bergson calls a heterogenous, qualitative multiplicity, duration becomes juxtaposed and converted into a succession of distinct parts, one coming after the other and therefore "caused" by one another. Nothing within a duration can be the cause of anything else within it. Hence determinism, the belief everything is determined by a prior cause, is an impossibility. One must accept time as it really is through placing oneself within duration where freedom can be identified and experienced as pure mobility.
+
+== Images of duration ==
+The first is of two spools, one unrolling to represent the continuous flow of ageing as one feels oneself moving toward the end of one's life-span, the other rolling up to represent the continuous growth of memory which, for Bergson, equals consciousness. No two successive moments are identical, for the one will always contain the memory left by the other. A person with no memory might experience two identical moments but, Bergson says, that person's consciousness would thus be in a constant state of death and rebirth, which he identifies with unconsciousness. The image of two spools, however, is of a homogeneous and commensurable thread, whereas, according to Bergson, no two moments can be the same, hence duration is heterogeneous.
+Bergson then presents the image of a spectrum of a thousand gradually changing shades with a line of feeling running through them, being both affected by and maintaining each of the shades. Yet even this image is inaccurate and incomplete, for it represents duration as a fixed and complete spectrum with all the shades spatially juxtaposed, whereas duration is incomplete and continuously growing, its states not beginning or ending but intermingling.
+
+Instead, let us imagine an infinitely small piece of elastic, contracted, if that were possible, to a mathematical point. Let us draw it out gradually in such a way as to bring out of the point a line which will grow progressively longer. Let us fix our attention not on the line as line, but on the action which traces it. Let us consider that this action, in spite of its duration, is indivisible if one supposes that it goes on without stopping; that, if we intercalate a stop in it, we make two actions of it instead of one and that each of these actions will then be the indivisible of which we speak; that it is not the moving act itself which is never indivisible, but the motionless line it lays down beneath it like a track in space. Let us take our mind off the space subtending the movement and concentrate solely on the movement itself, on the act of tension or extension, in short, on pure mobility. This time we shall have a more exact image of our development in duration.
+Even this image is incomplete, because the wealth of colouring is forgotten when it is invoked. But as the three images illustrate, it can be stated that duration is qualitative, unextended, multiple yet a unity, mobile and continuously interpenetrating itself. Yet these concepts put side-by-side can never adequately represent duration itself;

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+title: "Duration (philosophy)"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Duration_(philosophy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:52.453466+00:00"
+instance: "kb-cron"
+---
+
+The truth is we change without ceasing...there is no essential difference between passing from one state to another and persisting in the same state. If the state which "remains the same" is more varied than we think, [then] on the other hand the passing of one state to another resembles—more than we imagine—a single state being prolonged: the transition is continuous.  Just because we close our eyes to the unceasing variation of every physical state, we are obliged when the change has become so formidable as to force itself on our attention, to speak as if a new state were placed alongside the previous one. Of this new state we assume that it remains unvarying in its turn and so on endlessly.
+
+Because a qualitative multiplicity is heterogeneous and yet interpenetrating, it cannot be adequately represented by a symbol; indeed, for Bergson, a qualitative multiplicity is inexpressible. Thus, to grasp duration, one must reverse habitual modes of thought and place oneself within duration by intuition.
+
+== Influence on Gilles Deleuze ==
+Gilles Deleuze was profoundly influenced by Bergson's theory of duration, particularly in his work Cinema 1: The Movement Image in which he described cinema as providing people with continuity of movement (duration) rather than still images strewn together.
+
+== Physics and Bergson's ideas ==
+
+Bergson had a correspondence with physicist Albert Einstein in 1922 and a debate over Einstein's theory of relativity and its implications. For Bergson, the primary disagreement was over metaphysical and epistemological claims made by the theory of relativity, rather than a dispute about scientific evidence for or against the theory. Bergson famously stated of the theory that it is "a metaphysics grafted upon science, it is not science".
+
+== See also ==
+Loop quantum gravity
+Problem of time
+Specious present
+Uncertainty principle
+
+== References ==
+
+== External links ==
+1910 English translation of Time and Free Will
+Multiple formats at Internet Archive

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+title: "Egocentric presentism"
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+source: "https://en.wikipedia.org/wiki/Egocentric_presentism"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:53.611819+00:00"
+instance: "kb-cron"
+---
+
+Egocentric presentism is a form of solipsism introduced by Caspar Hare in which other persons can be conscious, but their experiences are simply not present.
+Similarly, in related work, Hare argues for a theory of perspectival realism in which other perspectives do exist, but the present perspective has a defining intrinsic property.
+In one example that Hare uses to illustrate his theory (starting on page 354 of the official version of his paper), you learn that you are one of two people, named A and B, who have just been in a train crash; and that A is about to have incredibly painful surgery. You cannot remember your name. According to Hare, naturally, you hope to be B. The point of the example is that you know everything relevant that there is to know about the objective world; all that is missing is your position in it, that is, whose experiences are present, A's or B's. This example is easily handled by egocentric presentism because under this theory, the case where the present experiences are A's is fundamentally different from the case where the present experiences are B's. Hare points out that similar examples can be given to support theories like presentism in the philosophy of time.
+Several other philosophers have written reviews of Hare's work on this topic. Giovanni Merlo has given a detailed comparison to his own closely related subjectivist theory.
+Vincent Conitzer is another philosopher who has discussed similar ideas. In the paper "The Personalized A-Theory of Time and Perspective", Conitzer makes the case that the metaphysics of the self are connected to the metaphysics of time. He argues that arguments in favor of the A-theory of time are more effective as arguments for the combined position of both A-theory being true and the "I" being metaphysically privileged from other perspectives.
+
+
+== See also ==
+Benj Hellie's vertiginous question
+J. J. Valberg's personal horizon
+Centered world
+Further facts
+Indexicality
+Presentism (historical analysis)
+The Egg (Weir short story)
+
+
+== References ==
+
+
+== External links ==
+Hare, Caspar. Self-Bias, Time-Bias, and the Metaphysics of Self and Time. Preprint of article in The Journal of Philosophy (2007).
+Hare, Caspar. On Myself, and Other, Less Important Subjects. Early draft of book published by Princeton University Press (2009).
+Hare, Caspar. Realism About Tense and Perspective. Preprint of article in Philosophy Compass (2010).

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+title: "Eternalism (philosophy of time)"
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+---
+
+In the philosophy of space and time, eternalism is an ontological view according to which all existence in time is equally real, as opposed to presentism or the growing block universe theory of time, in which at least the future is not the same as any other time. Some forms of eternalism give time a similar ontology to that of space, as a dimension, with different times being as real as different places, and future events are "already there" in the same sense other places are already there, and that there is no objective flow of time.
+It is sometimes referred to as the "block time" or "block universe" theory due to its description of space-time as an unchanging four-dimensional "block", as opposed to the view of the world as a three-dimensional space modulated by the passage of time.
+
+== The present ==
+In classical philosophy, time is divided into three distinct regions: the "past", the "present", and the "future". Using that representational model, the past is generally seen as being immutably fixed, and the future as at least partly undefined. As time passes, the moment that was once the present becomes part of the past, and part of the future, in turn, becomes the new present. In this way time is said to pass, with a distinct present moment moving forward into the future and leaving the past behind. One view of this type, presentism, argues that only the present exists. The present does not travel forward through an environment of time, moving from a real point in the past and toward a real point in the future. Instead, it merely changes. The past and future do not exist and are only concepts used to describe the real, isolated, and changing present. This conventional model presents a number of difficult philosophical problems and may be difficult to reconcile with currently accepted scientific theories such as the theory of relativity.
+
+It can be argued that special relativity eliminates the concept of absolute simultaneity and a universal present: according to the relativity of simultaneity, observers in different frames of reference can have different measurements of whether a given pair of events happened at the same time or at different times, with there being no physical basis for preferring one frame's judgments over those of another. However, there are events that may be non-simultaneous in all frames of reference: when one event is within the light cone of another—its causal past or causal future—then observers in all frames of reference show that one event preceded the other. The causal past and causal future are consistent within all frames of reference, but any other time is "elsewhere", and within it there is no present, past, or future. There is no physical basis for a set of events that represents the present.
+Many philosophers have argued that relativity implies eternalism. Philosopher of science Dean Rickles says that, "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism." Christian Wüthrich argues that supporters of presentism can salvage absolute simultaneity only if they reject either empiricism or relativity. Dean Zimmerman and others argue for a single privileged frame whose judgments about length, time, and simultaneity are the true ones, even if there is no empirical way to distinguish this frame. Hilary Putnam concluded in 1967 that it follows from special relativity that ″any future event X is already real″ and eternalism is the only view compatible with special relativity. The philosopher Mauro Dorato interprets Putnam's arguments differently and asserts that ″the opposition between presentism − only the presently existing
+event exist − and eternalism − past present and future events are equally real − which is
+somewhat presupposed in Putnam 1967, is misguided.″
+
+== The flow of time ==
+
+=== Antiquity ===
+Arguments for and against an independent flow of time have been raised since antiquity, represented by fatalism, reductionism, and Platonism: Classical fatalism argues that every proposition about the future exists, and it is either true or false, hence there is a set of every true proposition about the future, which means these propositions describe the future exactly as it is, and this future is true and unavoidable. Fatalism is challenged by positing that there are propositions that are neither true nor false, for example they may be indeterminate. Reductionism questions whether time can exist independently of the relation between events, and Platonism argues that time is absolute, and it exists independently of the events that occupy it.
+Earlier, pre-Socratic Greek philosopher Parmenides of Elea had posited that existence is timeless and change is impossible (an idea popularized by his disciple Zeno of Elea and his paradoxes about motion).
+
+=== Middle ages ===
+The philosopher Katherin A. Rogers argued that Anselm of Canterbury took an eternalist view of time, although the philosopher Brian Leftow argued against this interpretation, suggesting that Anselm instead advocated a type of presentism. Rogers responded to this paper, defending her original interpretation. Rogers also discusses this issue in her book Anselm on Freedom, using the term "four-dimensionalism" rather than "eternalism" for the view that "the present moment is not ontologically privileged", and commenting that "Boethius and Augustine do sometimes sound rather four-dimensionalist, but Anselm is apparently the first consistently and explicitly to embrace the position." Taneli Kukkonen argues in the Oxford Handbook of Medieval Philosophy that "what Augustine's and Anselm's mix of eternalist and presentist, tenseless and tensed language tells is that medieval philosophers saw no need to choose sides" the way modern philosophers do.
+Augustine of Hippo wrote that God is outside of time—that time exists only within the created universe.  Thomas Aquinas took the same view, and many theologians agree.  On this view, God would perceive something like a block universe, while time might appear differently to the finite beings contained within it.

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+title: "Eternalism (philosophy of time)"
+chunk: 2/2
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+tags: "science, encyclopedia"
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+instance: "kb-cron"
+---
+
+=== Modern period ===
+One of the most famous arguments about the nature of time in modern philosophy is presented in The Unreality of Time by J. M. E. McTaggart. It argues that time is an illusion. McTaggart argued that the description of events as existing in absolute time is self-contradictory, because the events have to have properties about being in the past and in the future, which are incompatible with each other. McTaggart viewed this as a contradiction in the concept of time itself, and concluded that reality is non-temporal. He called this concept the B-theory of time.
+Dirck Vorenkamp, a professor of religious studies, argued in his paper "B-Series Temporal Order in Dogen's Theory of Time" that the Zen Buddhist teacher Dōgen presented views on time that contained all the main elements of McTaggart's B-series view of time (which denies any objective present), although he noted that some of Dōgen's reasoning also contained A-Series notions, which Vorenkamp argued may indicate some inconsistency in Dōgen's thinking.
+Eternalism also encapsulates the theory of world lines, and the concept of linear reality that is - the individual perception of linear time.
+
+=== Quantum physics ===
+Some philosophers appeal to a specific theory that is "timeless" in a more radical sense than the rest of physics, the theory of quantum gravity. This theory is used, for instance, in Julian Barbour's theory of timelessness. On the other hand, George Ellis argues that time is absent in cosmological theories because of the details they leave out.
+Recently, Hrvoje Nikolić has argued that a block time model solves the black hole information paradox.
+
+== Objections ==
+Philosophers such as John Lucas argue that "The Block universe gives a deeply inadequate view of time. It fails to account for the passage of time, the pre-eminence of the present, the directedness of time and the difference between the future and the past." Similarly, Karl Popper argued in his discussion with Albert Einstein against determinism and eternalism from a common-sense standpoint.
+A flow-of-time theory with a strictly deterministic future, which nonetheless does not exist in the same sense as the present, would not satisfy common-sense intuitions about time. Some have argued that common-sense flow-of-time theories can be compatible with eternalism, for example John G. Cramer's transactional interpretation. Kastner (2010) "proposed that in order to preserve the elegance and economy of the interpretation, it may be necessary to consider offer and confirmation waves as propagating in a "higher space" of possibilities.
+In Time Reborn, Lee Smolin argues that time is physically fundamental, in contrast to Einstein's view that time is an illusion. Smolin hypothesizes that the laws of physics are not fixed, but rather evolve over time via a form of cosmological natural selection. In The Singular Universe and the Reality of Time, co-authored with philosopher Roberto Mangabeira Unger, Smolin goes into more detail on his views on the physical passage of time. In contrast to the orthodox block universe view, Smolin argues that what instead exists is a "thick present" in which two events in the present can be causally related to each other. Marina Cortês and Lee Smolin also argue that certain classes of discrete dynamical systems demonstrate time asymmetry and irreversibility, which is inconsistent with the block universe interpretation of time.
+Avshalom Elitzur vehemently rejects the block universe interpretation of time. At the Time in Cosmology conference, held at the Perimeter Institute for Theoretical Physics in 2016, Elitzur said: "I'm sick and tired of this block universe, ... I don't think that next Thursday has the same footing as this Thursday. The future does not exist. It does not! Ontologically, it's not there." Elitzur and Shahar Dolev argue that quantum mechanical experiments such as the Quantum Liar and the evaporation of black holes challenge the mainstream block universe model, and support the existence of an objective passage of time. Elitzur and Dolev believe that an objective passage of time and relativity can be reconciled, and that it would resolve many of the issues with the block universe and the conflict between relativity and quantum mechanics. Additionally, Elitzur and Dolev believe that certain quantum mechanical experiments provide evidence of apparently inconsistent histories, and that spacetime itself may therefore be subject to change affecting entire histories.
+Some philosophers have made objections to eternalism based on the existence of the self and concepts such as Benj Hellie's vertiginous question. Vincent Conitzer argues that arguments in favor of the A-theory of time are more effective as arguments for the combined position of both A-theory being true and the "I" being metaphysically privileged from other perspectives. Similar arguments have been made by Caspar Hare with the theories of egocentric presentism and perspectival realism.
+
+== See also ==
+A series and B series
+Arrow of time
+Centered world
+Eternity of the world
+Imaginary time
+Philosophical presentism
+Philosophy of space and time
+Problem of time
+Steady state
+Steady-state model
+Strata-cut animation
+
+== References ==
+
+== Bibliography ==
+Smart, Jack. "River of Time". In Anthony Kenny. Essays in Conceptual Analysis. pp. 214–215.
+van Inwagen, Peter (2008). "Metaphysics." Stanford Encyclopedia of Philosophy.
+
+== External links ==
+Biswas; Shaw; Modak (1999). "Time in Quantum Gravity". International Journal of Modern Physics D. 10 (4): 595–606. arXiv:gr-qc/9906010. Bibcode:2001IJMPD..10..595B. doi:10.1142/S0218271801001384. S2CID 119472003.
+Davies, Paul (September 2002). "That Mysterious Flow". Scientific American. 287 (3): 40–45. Bibcode:2002SciAm.287c..40D. doi:10.1038/scientificamerican0902-40. PMID 12197100.
+Markosian, Ned (2002). "Time: 8. The 3D/4D Controversy". Stanford Encyclopedia of Philosophy. Retrieved 2006-12-20.
+Nikolic, Hrvoje. "Block time: Why many physicists still don't accept it?" (PDF).
+Slavov, Matias (2024) "Eternalism" The Internet Encyclopedia of Philosophy. ISSN 2161-0002.

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+title: "Eternity"
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+category: "reference"
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+---
+
+Eternity,  also referred to as sempiternity or  forever, is time with no end i.e. infinite. 
+In the context of human life, eternity and death are co-existing realities.
+
+
+== Etymology ==
+Cicero used the word aeternitatis, written at some uncertain time between the years 88 - 81 BC   (work: De Inventione 1, 27, 39. : tempus autem est—id quo nunc utimur, nam ipsum quidem generaliter definire difficile est—pars quaedam aeternitatis cum alicuius annui, menstrui, diurni nocturnive spatii certa significatione.) which is an early or the earliest extant written form from which the English word is derived;  first shown in history in an approx. 1374 translation by Chaucer.  The first usage in French is 1175: eternitez: B. de Ste-Maure, 'Ducs Normandie. 
+
+
+== Philosophy ==
+
+Classical period (8th-7th century BC - 5th-9th century AD) Plato (c. 428–423 BC - 348/347 BC) described time as the moving image of eternity in Timaeus (37 D) using the word: αἰών. Aristotle (384–322 BC) stated οὐρανοῦ was eternal (in Book I of Περὶ οὐρανοῦ) and an eternal world (in Physics).  
+The ancient Greek word for everlastingness was ἀίδιος (aidios) as exists via Plotinus, who also used the word aoin  (eternity), in Ennead III.7.  The thought of Classical period Augustine, as exists in Book XI of the Confessions, and Boethius (c. 480–524 AD), in Book V of the Consolation of Philosophy were adopted as the reality of the subject for later thinkers in the western tradition of philosophy. 
+Thomas Hobbes (1588–1679) and many others in the Age of Enlightenment drew on the classical distinction to put forward metaphysical hypotheses such as "eternity is a permanent now".
+
+
+== Religion ==
+
+Ancient Egyptian eternity terms were neheh, for cyclical time, and djet, for linear.  Rameses III  (c.1187-1156 B.C.E.)  funerary temple  was: 'United-with -Eternity'  
+In Genesis 21:33 of the Old Testament  El-Olam  is God-Eternal. 
+Mythic  Iliadical  ἀθάνατος (athanatos) is the immortal.  
+Eternity as infinite duration is an important concept in many lives and religions. God or gods are often said to endure eternally, or exist for all time, forever, without beginning or end. Religious views of an afterlife may speak of it in terms of eternity or eternal life. Christian theologians may regard immutability, like the eternal Platonic forms, as essential to eternity.
+The ancient greek word for everlasting and, or, eternal exists in the Orphica Hymni. 
+Boethius stated eternity was: interminabilis vitae tota simul et perfecta possessio,  which is translated as "simultaneously full and perfect possession of interminable life". and nunc permanens, which in English is a: permanent now.  Thomas Aquinas (c. 1225 – 1274) believed in an eternal God, without either a beginning or end; the concept of eternity is of divine simplicity, thus incapable of being defined or fully understood by humankind.
+
+
+== Physics ==
+The possibility of eternal universes with reference to General Relativity was a subject of physics since the 21st century. 
+
+
+== Symbolism ==
+
+Eternity is often symbolized by the endless snake, swallowing its own tail, the ouroboros. The circle, band, or ring is also commonly used as a symbol for eternity, as is the mathematical symbol of infinity, 
+  
+    
+      
+        ∞
+      
+    
+    {\displaystyle \infty }
+  
+. Symbolically these are reminders that eternity has no beginning or end.
+
+
+== See also ==
+
+
+== Notes ==
+
+
+== References ==
+
+
+=== Works cited ===
+
+
+== Further reading ==
+Yu, Jiyuan (2003). The Structure of Being in Aristotle's Metaphysics. Springer. pp. 188–. ISBN 9781402015373.
+
+
+== External links ==
+Entry in the Internet Encyclopedia of Philosophy on the relationship between God and Time.

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+title: "Eternity of the world"
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+instance: "kb-cron"
+---
+
+The eternity of the world is the question, in philosophy, of whether the world has a beginning in time or has existed for eternity. It was a concern for ancient philosophers as well as  theologians and philosophers of the 13th century. The problem became a focus of a dispute in the 13th century, when some of the works of Aristotle, who believed in the eternity of the world, were rediscovered in the Latin West. This view conflicted with the view of the Catholic Church that the world had a beginning in time. The Aristotelian view was prohibited in the Condemnations of 1210–1277.
+
+== Aristotle ==
+The ancient Greek philosopher Aristotle argued that the world must have existed from eternity in his Physics as follows.  In Book I, he argues that everything that comes into existence does so from a substratum. Therefore, if the underlying matter of the universe came into existence, it would come into existence from a substratum.  But the nature of matter is precisely to be the substratum from which other things arise.  Consequently, the underlying matter of the universe could have come into existence only from an already existing matter exactly like itself; to assume that the underlying matter of the universe came into existence would require assuming that an underlying matter already existed. As this assumption is self-contradictory, Aristotle argued, matter must be eternal.
+In Book VIII, his argument from motion is that if an absolute beginning of motion should be assumed, the object to undergo the first motion must either:
+
+Option A is self-contradictory because an object cannot move before it comes into existence, and the act of coming into existence is itself a "movement," so that the first movement requires a movement before it, that is, the act of coming into existence. Option B is also unsatisfactory for two reasons:
+
+First, if the world began at a state of rest, the coming into existence of that state of rest would itself have been motion.
+Second, if the world changed from a state of rest to a state of motion, the cause of that change to motion would itself have been a motion.
+He concludes that motion is necessarily eternal.
+Aristotle argued that a "vacuum" (that is, a place where there is no matter) is impossible.  Material objects can come into existence only in place, that is, occupy space.  Were something to come from nothing, "the place to be occupied by what comes into existence would previously have been occupied by a vacuum, inasmuch as no body existed."  But a vacuum is impossible, and matter must be eternal.
+The Greek philosopher Critolaus (c. 200-c. 118 BC) of Phaselis defended Aristotle's doctrine of the eternity of the world, and of the human race in general, against the Stoics.  There is no observed change in the natural order of things; mankind recreates itself in the same manner according to the capacity given by Nature, and the various ills to which it is heir, though fatal to individuals, do not avail to modify the whole. Just as it is absurd to suppose that humans are merely earth-born, so the possibility of their ultimate destruction is inconceivable. The world, as the manifestation of eternal order, must itself be eternal.
+
+== The Neo-Platonists ==
+The Neoplatonist philosopher Proclus (412 – 485 AD) advanced in his De Aeternitate Mundi (On the Eternity of the World) eighteen proofs for the eternity of the world, resting on the divinity of its creator.
+John Philoponus in 529 wrote his critique Against Proclus On the Eternity of the World in which he systematically argued against every proposition put forward for the eternity of the world. The intellectual battle against eternalism became one of Philoponus’ major preoccupations and dominated several of his publications (some now lost) over the following decade.
+Philoponus originated the argument now known as the Traversal of the infinite.  If the existence of something requires that something else exist before it, then the first thing cannot come into existence without the thing before it existing. An infinite number cannot actually exist, nor be counted through or 'traversed,' or be increased. Something cannot come into existence if this requires an infinite number of other things existing before it.  Therefore, the world cannot be infinite.
+The Aristotelian commentator Simplicius of Cilicia and contemporary of Philoponus held that Philoponus’ arguments relied on a fundamental misunderstanding of Aristotelian physics: “To my mind I have demonstrated that when this man objected against these demonstrations he did not comprehend a thing of what Aristotle said.” Simplicius adhered to the Aristotelian doctrine of the eternity of the world and strongly opposed Philoponus, who asserted the beginning of the world through divine creation.
+
+=== Philoponus' arguments ===
+Philoponus' arguments for temporal finitism were severalfold.  Contra Aristotlem has been lost, and is chiefly known through the citations used by Simplicius of Cilicia in his commentaries on Aristotle's Physics and De Caelo.  Philoponus' refutation of Aristotle extended to six books, the first five addressing De Caelo and the sixth addressing Physics, and from comments on Philoponus made by Simplicius can be deduced to have been quite lengthy.
+A full exposition of Philoponus' several arguments, as reported by Simplicius, can be found in Sorabji.  One such argument was based upon Aristotle's own theorem that there were not multiple infinities, and ran as follows:  If time were infinite, then as the universe continued in existence for another hour, the infinity of its age since creation at the end of that hour must be one hour greater than the infinity of its age since creation at the start of that hour.  But since Aristotle holds that such treatments of infinity are impossible and ridiculous, the world cannot have existed for infinite time.
+Philoponus's works were adopted by many; his first argument against an infinite past being the "argument from the impossibility of the existence of an actual infinite", which states:

data/en.wikipedia.org/wiki/Eternity_of_the_world-1.md

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+---
+title: "Eternity of the world"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Eternity_of_the_world"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:59.985243+00:00"
+instance: "kb-cron"
+---
+
+"An actual infinite cannot exist."
+"An infinite temporal regress of events is an actual infinite."
+"Thus an infinite temporal regress of events cannot exist."
+This argument defines event as equal increments of time. Philoponus argues that the second premise is not controversial since the number of events prior to today would be an actual infinite without beginning if the universe is eternal. The first premise is defended by a reductio ad absurdum where Philoponus shows that actual infinites can not exist in the actual world because they would lead to contradictions albeit being a possible mathematical enterprise. Since an actual infinite in reality would create logical contradictions, it can not exist including the actual infinite set of past events. The second argument, the "argument from the impossibility of completing an actual infinite by successive addition", states:
+
+"An actual infinite cannot be completed by successive addition."
+"The temporal series of past events has been completed by successive addition."
+"Thus the temporal series of past events cannot be an actual infinite."
+The first statement states, correctly, that a finite (number) cannot be made into an infinite one by the finite addition of more finite numbers. The second skirts around this; the analogous idea in mathematics, that the (infinite) sequence of negative integers "..-3, -2, -1" may be extended by appending zero, then one, and so forth; is perfectly valid.
+
+== Medieval period ==
+Avicenna argued that prior to a thing's coming into actual existence, its existence must have been 'possible.'  Were its existence necessary, the thing would already have existed, and were its existence impossible, the thing would never exist.  The possibility of the thing must therefore in some sense have its own existence.  Possibility cannot exist in itself, but must reside within a subject.  If an already existent matter must precede everything coming into existence, clearly nothing, including matter, can come into existence ex nihilo, that is, from absolute nothingness.  An absolute beginning of the existence of matter is therefore impossible.
+The Aristotelian commentator Averroes supported Aristotle's view, particularly in his work The Incoherence of the Incoherence (Tahafut al-tahafut), in which he defended Aristotelian philosophy against al-Ghazali's claims in The Incoherence of the Philosophers (Tahafut al-falasifa).
+Averroes' contemporary Maimonides challenged Aristotle's assertion that "everything in existence comes from a substratum," on that basis that his reliance on induction and analogy is a fundamentally flawed means of explaining unobserved phenomenon.  According to Maimonides, to argue that "because I have never observed something coming into existence without coming from a substratum it cannot occur" is equivalent to arguing that "because I cannot empirically observe eternity it does not exist."
+Maimonides himself held that neither creation nor Aristotle's infinite time were provable, or at least that no proof was available. (According to scholars of his work, he didn't make a formal distinction between unprovability and the simple absence of proof.) However, some of Maimonides' Jewish successors, including Gersonides and Crescas, conversely held that the question was decidable, philosophically.
+In the West, the 'Latin Averroists' were a group of philosophers writing in Paris in the middle of the thirteenth century, who included Siger of Brabant, Boethius of Dacia.  They supported Aristotle's doctrine of the eternity of the world against conservative theologians such as John Pecham and Bonaventure. The conservative position is that the world can be logically proved to have begun in time, of which the classic exposition is Bonaventure's argument in the second book of his commentary on Peter Lombard's sentences, where he repeats Philoponus' case against a traversal of the infinite.
+Thomas Aquinas, like Maimonides, argued against both the conservative theologians and the Averroists, claiming that neither the eternity nor the finite nature of the world could be proved by logical argument alone. According to Aquinas the possible eternity of the world and its creation would be contradictory if an efficient cause were to precede its effect in duration or if non-existence precedes existence in duration. But an efficient cause, such as God, which instantaneously produces its effect would not necessarily precede its effect in duration.  God can also be distinguished from a natural cause which produces its effect by motion, for a cause that produces motion must precede its effect. God could be an instantaneous and motionless creator, and could have created the world without preceding it in time. To Aquinas, that the world began was an article of faith.
+The position of the Averroists was condemned by Stephen Tempier in 1277.
+Giordano Bruno, famously, believed in eternity of the world (and this was one of the heretical beliefs for which he was burned at the stake).
+
+== See also ==
+Age of the universe
+Averroism
+Al-Biruni
+Condemnations of 1210–1277
+Eternalism (philosophy of time)
+Big Bang
+Law of conservation of energy
+Abraham Solomon ben Isaac ben Samuel Catalan, author of a treatise on eternity of the world
+
+== References ==
+
+== Bibliography ==
+Richard C. Dales (1990). Medieval Discussions of the Eternity of the World. Leiden: Brill. ISBN 90-04-09215-3.
+
+== External links ==
+"On The Eternity of the World" by Thomas Aquinas from the Internet Medieval Sourcebook

data/en.wikipedia.org/wiki/Event_(philosophy)-0.md

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+---
+title: "Event (philosophy)"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Event_(philosophy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:01.214153+00:00"
+instance: "kb-cron"
+---
+
+In philosophy, events are objects in time or instantiations of properties in objects. On some views, only changes in the form of acquiring or losing a property can constitute events, like the lawn's becoming dry. According to others, there are also events that involve nothing but the retaining of a property, e.g. the lawn's staying wet. Events are usually defined as particulars that, unlike universals, cannot repeat at different times. Processes are complex events constituted by a sequence of events. But even simple events can be conceived as complex entities involving an object, a time and the property exemplified by the object at this time. Traditionally, metaphysicians tended to emphasize static being over dynamic events. This tendency has been opposed by so-called process philosophy or process ontology, which ascribes ontological primacy to events and processes.
+
+== Kim’s property-exemplification ==
+Jaegwon Kim theorized that events are structured.
+They are composed of three things:
+
+object(s) 
+  
+    
+      
+        [
+        x
+        ]
+      
+    
+    {\displaystyle [x]}
+  
+,
+a property 
+  
+    
+      
+        [
+        P
+        ]
+      
+    
+    {\displaystyle [P]}
+  
+ and
+time or a temporal interval 
+  
+    
+      
+        [
+        t
+        ]
+      
+    
+    {\displaystyle [t]}
+  
+.
+Events are defined using the operation 
+  
+    
+      
+        [
+        x
+        ,
+        P
+        ,
+        t
+        ]
+      
+    
+    {\displaystyle [x,P,t]}
+  
+.
+A unique event is defined by two principles:
+
+a) the existence condition and
+b) the identity condition.
+The existence condition states “
+  
+    
+      
+        [
+        x
+        ,
+        P
+        ,
+        t
+        ]
+      
+    
+    {\displaystyle [x,P,t]}
+  
+ exists if and only if object 
+  
+    
+      
+        x
+      
+    
+    {\displaystyle x}
+  
+ exemplifies the 
+  
+    
+      
+        n
+      
+    
+    {\displaystyle n}
+  
+-adic 
+  
+    
+      
+        P
+      
+    
+    {\displaystyle P}
+  
+ at time 
+  
+    
+      
+        t
+      
+    
+    {\displaystyle t}
+  
+.” This means a unique event exists if the above is met. The identity condition states “
+  
+    
+      
+        [
+        x
+        ,
+        P
+        ,
+        t
+        ]
+      
+    
+    {\displaystyle [x,P,t]}
+  
+ is 
+  
+    
+      
+        [
+        y
+        ,
+        Q
+        ,
+        
+          t
+          ′
+        
+        ]
+      
+    
+    {\displaystyle [y,Q,t']}
+  
+ if and only if 
+  
+    
+      
+        x
+        =
+        y
+      
+    
+    {\displaystyle x=y}
+  
+, 
+  
+    
+      
+        P
+        =
+        Q
+      
+    
+    {\displaystyle P=Q}
+  
+ and 
+  
+    
+      
+        t
+        =
+        
+          t
+          ′
+        
+      
+    
+    {\displaystyle t=t'}
+  
+.”
+Kim uses these to define events under five conditions:
+
+One, they are unrepeatable, unchangeable particulars that include changes and the states and conditions of that event.
+Two, they have a semi-temporal location.
+Three, only their constructive property creates distinct events.
+Four, holding a constructive property as a generic event creates a type-token relationship between events, and events are not limited to their three requirements (i.e. 
+  
+    
+      
+        [
+        x
+        ,
+        P
+        ,
+        t
+        ]
+      
+    
+    {\displaystyle [x,P,t]}
+  
+). Critics of this theory such as Myles Brand have suggested that the theory be modified so that an event had a spatiotemporal region; consider the event of a flash of lightning. The idea is that an event must include both the span of time of the flash of lightning and the area in which it occurred.
+
+Other problems exist within Kim's theory, as he never specified what properties were (e.g. universals, tropes, natural classes, etc.). In addition, it is not specified if properties are few or abundant. The following is Kim's response to the above.
+
+. . . [T]he basic generic events may be best picked out relative to a scientific theory, whether the theory is a common-sense theory of the behavior of middle-sized objects or a highly sophisticated physical theory. They are among the important properties, relative to the theory, in terms of which lawful regularities can be discovered, described, and explained. The basic parameters in terms of which the laws of the theory are formulated would, on this view, give us our basic generic events, and the usual logical, mathematical, and perhaps other types of operations on them would yield complex, defined generic events. We commonly recognize such properties as motion, colors, temperatures, weights, pushing, and breaking, as generic events and states, but we must view this against the background of our common-sense explanatory and predictive scheme of the world around us. I think it highly likely that we cannot pick out generic events completely a priori.
+There is also a major debate about the essentiality of a constitutive object. There are two major questions involved in this: If one event occurs, could it have occurred in the same manner if it were another person, and could it occur in the same manner if it would have occurred at a different time? Kim holds that neither are true and that different conditions (i.e. a different person or time) would lead to a separate event. However, some consider it natural to assume the opposite.
+
+== Davidson ==
+
+Donald Davidson and John Lemmon proposed a theory of events that had two major conditions, respectively: a causal criterion and a spatiotemporal criterion.
+The causal criterion defines an event as two events being the same if and only if they have the same cause and effect.
+The spatiotemporal criterion defines an event as two events being the same if and only if they occur in the same space at the same time. Davidson however provided this scenario; if a metal ball becomes warmer during a certain minute, and during the same minute rotates through 35 degrees, must we say that these are the same event? However, one can argue that the warming of the ball and the rotation are possibly temporally separated and are therefore separate events.

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+---
+title: "Event (philosophy)"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Event_(philosophy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:01.214153+00:00"
+instance: "kb-cron"
+---
+
+== Lewis ==
+David Lewis theorized that events are merely spatiotemporal regions and properties (i.e. membership of a class). He defines an event as "e is an event only if it is a class of spatiotemporal regions, both thisworldly (assuming it occurs in the actual world) and otherworldly." The only problem with this definition is it only tells us what an event could be, but does not define a unique event. This theory entails modal realism, which assumes possible worlds exist; worlds are defined as sets containing all objects that exist as a part of that set. However, this theory is controversial. Some philosophers have attempted to remove possible worlds, and reduce them to other entities. They hold that the world we exist in is the only world that actually exists, and that possible worlds are only possibilities.
+Lewis’ theory is composed of four key points. Firstly, the non-duplication principle; it states that x and y are separate events if and only if there is one member of x that is not a member of y (or vice versa). Secondly, there exist regions that are subsets of possible worlds and thirdly, events are not structured by an essential time.
+
+== Deleuze ==
+Gilles Deleuze lectured on the concept of event on March 10, 1987. A sense of the lecture is described by James Williams. Williams also wrote, "From the point of view of the difference between two possible worlds, the event is all important". He also stated, "Every event is revolutionary due to an integration of signs, acts and structures through the whole event. Events are distinguished by the intensity of this revolution, rather than the types of freedom or chance." In 1988 Deleuze published a magazine article "Signes et événements"
+In his book Nietszche and Philosophy, he addresses the question "Which one is beautiful?" In the preface to the English translation he wrote:
+
+The one that ... does not refer to an individual, to a person, but rather to an event, that is, to the forces in their various relationships to a proposition or phenomenon, and the genetic relationship that determines these forces (power).
+
+== Badiou ==
+In Being and Event, Alain Badiou writes that the event (événement) is a multiple which basically does not make sense according to the rules of the "situation," in other words existence.  Hence, the event "is not," and therefore, in order for there to be an event, there must be an "intervention" which changes the rules of the situation in order to allow that particular event to be. ("To be", meaning, to be a multiple which belongs to the multiple of the situation — these terms are drawn from or defined in reference to set theory.) In his view, there is no "one," and everything that is is a "multiple."  "One" happens when the situation "counts," or accounts for, acknowledges, or defines something: it "counts it as one."  For the event to be counted as one by the situation, or counted in the one of the situation, an intervention needs to decide its belonging to the situation.  This is because his definition of the event violates the prohibition against self-belonging (in other words, it is a set-theoretical definition which violates set theory's rules of consistency), thus does not count as extant on its own.
+
+== Kirkeby ==
+The Danish philosopher Ole Fogh Kirkeby deserves mentioning, as he has written a comprehensive trilogy about the event, or in Danish "begivenheden". In the first work of the trilogy "Eventum tantum – begivenhedens ethos" (Eventum tantum - the ethos of the event) he distinguishes between three levels of the event, inspired from Nicholas of Cusa: Eventum tantum as non aliud, the alma-event and the proto-event.
+
+== See also ==
+Free play (Derrida)
+Happening – Type of performance artwork
+Philosophy of space and time
+
+== References ==
+
+== External links ==
+Roberto Casati & Achille Varzi, Events, from Stanford Encyclopedia of Philosophy.
+Susan Schneider, Events, from The Internet Encyclopedia of Philosophy.
+Byron Kaldis, Events, from Encyclopedia of Philosophy and the Social Sciences.

data/en.wikipedia.org/wiki/From_Eternity_to_Here-0.md

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+---
+title: "From Eternity to Here"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/From_Eternity_to_Here"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:03.535877+00:00"
+instance: "kb-cron"
+---
+
+From Eternity to Here: The Quest for the Ultimate Theory of Time is a nonfiction book by American theoretical physicist Sean M. Carroll, published on January 7, 2010, by Dutton.
+
+
+== Background ==
+In the book, Carroll explores the nature of the arrow of time, that goes forward from the past to the future, and posits that the arrow owes its existence to conditions before the Big Bang. However, reasoning about what was there before the Big Bang has traditionally been dismissed as meaningless, for space and time are considered to be created exactly at the Big Bang. Carroll argues that "understanding the arrow of time is a matter of understanding the origin of the universe" and in his explanations relies on the second law of thermodynamics, which states that all systems in the Universe tend to become more and more disorganized (increase in entropy). His proposed explanation for the arrow of time is based on ideas that go back to Ludwig Boltzmann, an Austrian physicist of the 1870s.
+
+
+== Book organization ==
+The book is divided into four parts and 15 chapters and has an appendix for the relevant math. Part one is entitled, "Time, Experience, and the Universe." Part two is named, "Time in Einstein’s Universe." Part three is called, "Entropy and Time’s Arrow." Part four is entitled, "From the Kitchen to the Multiverse."
+
+
+== Reception ==
+Manjit Kumar in his review for the Daily Telegraph called the book "a rewarding read" that was "not for the faint hearted". Writing for The A.V. Club, Donna Bowman commented, "Its appeal lies in Carroll's gift for leading readers through the train of thought that connects black holes, light cones, event horizons, Laplace's demon (or Maxwell’s), dark energy, and entropy with the question of time...  Like all great teachers, he makes his subject irresistible, and makes his students feel smarter." A reviewer of Kirkus Reviews added, "Not for the scientifically disinclined, but determined readers will come away with a rewarding grasp of a complex subject."
+Andreas Albrecht, writing for Physics Today, gave the book a generally positive review, while noting that Carroll's attempts to provide material for both lay and expert readers might at times leave both dissatisfied. In his review for New Scientist, philosopher Craig Callender wrote that "Carroll seems slightly embarrassed by the many leaps of faith he asks of his reader" in explaining his hypothesis for the origin of the arrow of time. Eric Winsberg's evaluation of Carroll's proposal concluded by saying that its conceptual costs "seem high, and the benefits few."
+
+
+== References ==
+
+
+== External links ==
+From Eternity to Here: The Quest for the Ultimate Theory of Time on YouTube

data/en.wikipedia.org/wiki/Further_facts-0.md

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+---
+title: "Further facts"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Further_facts"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:04.749751+00:00"
+instance: "kb-cron"
+---
+
+In philosophy, further facts  are facts that do not logically follow from the physical facts of the world. Reductionists who argue that at bottom there is nothing more than the physical facts thus argue against the existence of further facts. The concept of further facts plays a key role in some of the major works in analytic philosophy of the late 20th century, including in Derek Parfit's Reasons and Persons, and David Chalmers's The Conscious Mind.
+One context in which the existence of further facts is debated is that of personal identity across time: in what sense is Alice today the same person as Alice yesterday, given that across the two days the state of her brain is different and the atoms that constitute her are different? On the one hand, we may believe that at bottom, there is nothing more than the atoms and their arrangement at different points in time; while we may for practical purposes come up with some notion of sameness of a person, this notion does not reflect anything deeper about reality. Under such a view there would be no further facts. On the other hand, we may believe that there is an additional fact of the matter whether Alice yesterday and Alice today really are the same person. For example, if we believe in Cartesian souls, we may believe that Alice yesterday and Alice today are the same person if and only if they correspond to the same soul. Or we may not believe in Cartesian souls, but yet believe that whether Alice yesterday and Alice today are the same person is a question about something other than facts about which atoms constitute them and how they are arranged. These would both be further-fact views.
+
+This debate about further facts concerning personal identity over time is most closely associated with Derek Parfit.  In his Reasons and Persons, he describes the non-reductionist's view that "personal identity is a deep further fact, distinct from physical and psychological continuity". Parfit takes a reductionist stance and argues against this further-fact view. As a result it is not clear whether a person has any reason to be worried about his or her future self in a special way that does not also apply to worrying about others: Parfit argues that it is plausible that "only the [implausible] deep further fact gives me a reason to be specially concerned about my future". Sydney Shoemaker objected that it is not clear how a further fact would give a reason for such special concerns either; Harold Langsam attempted to give a positive account of how a further fact would give such a reason.
+
+David Chalmers lists some other candidates for further facts. One is facts about conscious experience. For example, it is difficult to see how it follows from the physical facts what it is like to experience seeing red; indeed, inverted spectrum scenarios, where we imagine that experiences of colors are swapped without anything else changing, suggest that the experience of red-seeing could have been a different one without the physical facts changing. Another candidate for a further fact is that there is any conscious experience at all, rather than everyone being a philosophical zombie. (Christopher Hill and Brian Mclaughlin argued against the idea that facts about consciousness are further facts, disputing the logical possibility of a world physically identical to ours in which the facts about consciousness are different.)
+Chalmers also considers indexicality. He cites the fact that "I am David Chalmers", noting that its significance seems to go beyond the tautology that David Chalmers is David Chalmers. (See also Caspar Hare's egocentric presentism and Benj Hellie's vertiginous question.) Similarly, in the philosophy of time, what date and time it is now might be considered a candidate for a further fact, in the sense that a being that knows everything about the full four-dimensional block of spacetime would still not know what time it is now. (See also the A-theory and the B-theory of time.)
+Chalmers also considers negative facts. For example, a statement like "there do not exist nonphysical angels." If in fact true, it does not seem that this logically follows from any of the physical facts by themselves; but, he argues, it would follow if one added a "That is all" statement at the end of the list of all the physical facts.
+Vincent Conitzer has devised a number of thought experiments involving the detection of further facts. He imagines a sequence of hypothetical realities where further facts exist, but where the different realities exist along a spectrum of how difficult it is for observers within each reality to conclude that further facts must exist. On one end of the spectrum is a reality where the observer can easily conclude that further facts exist. On the other end is our reality, where the existence of further facts is more ambiguous. Conitzer argues that it is unclear where along the sequence it stops becoming trivial to prove that further facts must exist.
+
+
+== See also ==
+Centered world
+Simulation hypothesis
+
+
+== References ==
+
+
+== External links ==
+Conitzer, Vincent. "A Puzzle about Further Facts." Open access version of article in Erkenntnis.

data/en.wikipedia.org/wiki/Future-0.md

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+---
+title: "Future"
+chunk: 1/4
+source: "https://en.wikipedia.org/wiki/Future"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:05.922691+00:00"
+instance: "kb-cron"
+---
+
+The future is the time after the past and the present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the apparent nature of the reality and the unavoidability of the future, everything that currently exists and will exist can be categorized as either permanent, meaning that it will exist forever, or temporary, meaning that it will end. In the Occidental view, which uses a linear conception of time, the future is the portion of the projected timeline that is anticipated to occur. In special relativity, the future is considered absolute future, or the future light cone.
+In the philosophy of time, presentism is the belief that only the present exists and the future and the past are unreal. Religions consider the future when they address issues such as karma, life after death, and eschatologies that study what the end of time and the end of the world will be. Religious figures such as prophets and diviners have claimed to see into the future.
+Future studies, or futurology, is the science, art, and practice of postulating possible futures. Modern practitioners stress the importance of alternative and plural futures, rather than one monolithic future, and the limitations of prediction and probability, versus the creation of possible and preferable futures. Predeterminism is the belief that the past, present, and future have been already decided.
+The concept of the future has been explored extensively in cultural production, including art movements and genres devoted entirely to its elucidation, such as the 20th-century movement futurism.
+
+== In physics ==
+
+In physics, time is the fourth dimension. Physicists argue that spacetime can be understood as a sort of stretchy fabric that bends due to forces such as gravity. In classical physics the future is just a half of the timeline, which is the same for all observers. In special relativity the flow of time is relative to the observer's frame of reference. The faster an observer is traveling away from a reference object, the slower that object seems to move through time. Hence, the future is not an objective notion anymore. A more modern notion is absolute future, or the future light cone. While a person can move backward or forwards in the three spatial dimensions, many physicists argue one may only move forward in time.
+In theory, time dilation, a consequence of special relativity, would make it possible for passengers in a fast-moving vehicle to advance into the future in a short period of their own time. With sufficiently high speeds, the effect would be dramatic. For example, one year of travel might correspond to ten years on Earth. Indeed, a constant 1 g acceleration would permit humans to travel through the entire known Universe in one human lifetime.
+Some physicists claim that by using a wormhole to connect two regions of spacetime a person could theoretically travel in time. Physicist Michio Kaku points out that to power this hypothetical time machine and "punch a hole into the fabric of space-time" would require the energy of a star. Another theory is that a person could travel in time with cosmic strings.
+
+== In philosophy ==
+
+In the philosophy of time, presentism is the belief that only the present exists, and the future and past are unreal. Past and future "entities" are construed as logical constructions or fictions. The opposite of presentism is 'eternalism', which is the belief that things in the past and things yet to come exist eternally. Another view (not held by many philosophers) is sometimes called the 'growing block' theory of time—which postulates that the past and present exist, but the future does not.
+Presentism is compatible with Galilean relativity, in which time is independent of space, but is probably incompatible with Lorentzian/Albert Einsteinian relativity in conjunction with certain other philosophical theses that many find uncontroversial. Saint Augustine proposed that the present is a knife edge between the past and the future and could not contain any extended period of time.
+
+Contrary to Saint Augustine, some philosophers propose that conscious experience is extended in time. For instance, William James said that time is "...the short duration of which we are immediately and incessantly sensible." Augustine proposed that God is outside of time and present for all times, in eternity. Other early philosophers who were presentists include the Buddhists (in the tradition of Indian Buddhism). A leading scholar from the modern era on Buddhist philosophy is Stcherbatsky, who has written extensively on Buddhist presentism: Everything past is unreal, everything future is unreal, everything imagined, absent, mental... is unreal... Ultimately real is only the present moment of physical efficiency [i.e., causation].

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+---
+title: "Future"
+chunk: 2/4
+source: "https://en.wikipedia.org/wiki/Future"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:05.922691+00:00"
+instance: "kb-cron"
+---
+
+== In psychology ==
+Human behavior is known to encompass anticipation of the future. Anticipatory behavior can be the result of a psychological outlook toward the future, for examples optimism, pessimism, and hope.
+Optimism is an outlook on life such that one maintains a view of the world as a positive place. People would say that optimism is seeing the glass "half full" of water as opposed to half empty. It is the philosophical opposite of pessimism. Optimists generally believe that people and events are inherently good, so that most situations work out in the end for the best. Hope is a belief in a positive outcome related to events and circumstances in one's life. Hope implies a certain amount of despair, wanting, wishing, suffering or perseverance—i.e., believing that a better or positive outcome is possible even when there is some evidence to the contrary. "Hopefulness" is somewhat different from optimism in that hope is an emotional state, whereas some theories point to optimism as a conclusion reached through a deliberate thought pattern that leads to a positive personal attitudes and by extension is linked to more philanthropic behaviours.
+Pessimism as stated before is the opposite of optimism. It is the tendency to see, anticipate, or emphasize only bad or undesirable outcomes, results, or problems. The word originates in Latin from Pessimus meaning worst and Malus meaning bad and has a link to misanthropic belief systems.
+
+== In religion ==
+Religions consider the future when they address issues such as karma, life after death, and eschatologies which consider what the end of time and the end of the world will be like. In religion, major prophets are said to have the power to change the future. Common religious figures have claimed to see into the future, such as minor prophets and diviners.
+The term "afterlife" refers to the continuation of existence of the soul, spirit or mind of a human (or animal) after physical death, typically in a spiritual or ghostlike afterworld. Deceased persons are usually believed to go to a specific region or plane of existence in this afterworld, often depending on the rightness of their actions during life.
+Some believe the afterlife includes some form of preparation for the soul to transfer to another body (reincarnation). The major views on the afterlife derive from religion, esotericism and metaphysics. There are those who are skeptical of the existence of the afterlife, or believe that it is absolutely impossible, such as the materialist-reductionists, who believe that the topic is supernatural, therefore does not really exist or is unknowable. In metaphysical models, theists generally, believe some sort of afterlife awaits people when they die. Atheists generally do not believe in a life after death. Members of some generally non-theistic religions such as Buddhism, tend to believe in an afterlife like reincarnation but without reference to God.
+Agnostics generally hold the position that like the existence of God, the existence of supernatural phenomena, such as souls or life after death, is unverifiable and therefore unknowable. Many religions, whether they believe in the soul's existence in another world like Christianity, Islam and many pagan belief systems, or in reincarnation like many forms of Hinduism and Buddhism, believe that one's status in the afterlife is a reward or punishment for their conduct during life, with the exception of Calvinistic variants of Protestant Christianity, which believe one's status in the afterlife is a gift from God and cannot be earned during life.
+Eschatology is a part of theology and philosophy concerned with the final events in the Human history, or the ultimate destiny of humanity, commonly referred to as the end of the world. While in mysticism the phrase refers metaphorically to the end of ordinary reality and reunion with the Divine, in many traditional religions it is taught as an actual future event prophesied in sacred texts or folklore. More broadly, eschatology may encompass related concepts such as the Messiah or Messianic Age, the end time, and the end of days.
+
+== In grammar ==

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+title: "Future"
+chunk: 3/4
+source: "https://en.wikipedia.org/wiki/Future"
+category: "reference"
+tags: "science, encyclopedia"
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+instance: "kb-cron"
+---
+
+In English grammar, actions are classified according to one of the following twelve verb tenses: past (past, past continuous, past perfect, or past perfect continuous), present (present, present continuous, present perfect, or present perfect continuous), or future (future, future continuous, future perfect, or future perfect continuous). The future tense refers to actions that have not yet happened, but which are due, expected, or may occur in the future. For example, in the sentence, "She will walk home," the verb "will walk" is in the future tense because it refers to an action that is going to, or may, happen at a point in time beyond the present.
+Verbs in the future continuous tense indicate actions that will happen beyond the present and will continue for a period of time. In the sentence, "She will be walking home," the verb phrase "will be walking" is in the future continuous tense because the action described is not happening now, but will happen sometime afterwards and is expected to continue happening for some time. Verbs in the future perfect tense indicate actions that will be completed at a particular point in the future. For example, the verb phrase, "will have walked," in the sentence, "She will have walked home," is in the future perfect tense because it refers to an action that is completed as of a specific time in the future. Finally, verbs in the future perfect continuous tense combine the features of the perfect and continuous tenses, describing the future status of actions that have been happening continually from now, the past or some time in the future through to a particular time (later) in the future. In the sentence, "She will have been walking home," the verb phrase "will have been walking" is in the future perfect continuous tense because it refers to an action that the speaker anticipates will have continued until some time in the future.
+Another way to think of the various future tenses is that actions described by the future tense will be completed at an unspecified time in the future, actions described by the future continuous tense will keep happening in the future, actions described by the future perfect tense will be completed at a specific time in the future, and actions described by the future perfect continuous tense are expected to be continuing as of a specific time in the future.
+
+== Linear and cyclic culture ==
+
+The linear view of time (common in Western thought) draws a stronger distinction between past and future than does the more common cyclic time of cultures such as India, where past and future can coalesce much more readily.
+
+== Futures studies ==
+
+Futures studies or futurology is the science, art, and practice of postulating possible, probable, and preferable futures and the worldviews and myths that underlie them. Futures studies seek to understand what is likely to continue, what is likely to change, and what is novel. Part of the discipline thus seeks a systematic and pattern-based understanding of past and present, and to determine the likelihood of future events and trends. A key part of this process is understanding the potential future impact of decisions made by individuals, organizations, and governments. Leaders use the results of such work to assist in decision-making.
+
+Take hold of the future or the future will take hold of you.
+Futures is an interdisciplinary field, studying yesterday's and today's changes, and aggregating and analyzing both lay and professional strategies, and opinions with respect to tomorrow. It includes analyzing the sources, patterns, and causes of change and stability in the attempt to develop foresight and to map possible futures. Modern practitioners stress the importance of alternative and plural futures, rather than one monolithic future, and the limitations of prediction and probability, versus the creation of possible and preferable futures.
+Three factors usually distinguish futures studies from the research conducted by other disciplines (although all disciplines overlap, to differing degrees). First, futures studies often examines not only possible but also probable, preferable, and "wild card" futures. Second, futures studies typically attempts to gain a holistic or systemic view based on insights from a range of different disciplines. Third, futures studies challenges and unpacks the assumptions behind dominant and contending views of the future. The future thus is not empty but fraught with hidden assumptions.
+Futures studies do not generally include the work of economists who forecast movements of interest rates over the next business cycle, or of managers or investors with short-term time horizons. Most strategic planning, which develops operational plans for preferred futures with time horizons of one to three years, is also not considered futures. But plans and strategies with longer time horizons that specifically attempt to anticipate and be robust to possible future events, are part of a major subdiscipline of futures studies called strategic foresight.
+The futures field also excludes those who make future predictions through professed supernatural means. At the same time, it does seek to understand the model's such groups use and the interpretations they give to these models.
+
+=== Forecasting ===

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+category: "reference"
+tags: "science, encyclopedia"
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+instance: "kb-cron"
+---
+
+Forecasting is the process of estimating outcomes in uncontrolled situations. Forecasting is applied in many areas, such as weather forecasting, earthquake prediction, transport planning, and labour market planning. Due to the element of the unknown, risk and uncertainty are central to forecasting.
+Statistically based forecasting employs time series with cross-sectional or longitudinal data. Econometric forecasting methods use the assumption that it is possible to identify the underlying factors that might influence the variable that is being forecast. If the causes are understood, projections of the influencing variables can be made and used in the forecast. Judgmental forecasting methods incorporate intuitive judgments, opinions, and probability estimates, as in the case of the Delphi method, scenario building, and simulations.
+Prediction is similar to forecasting but is used more generally, for instance, to also include baseless claims on the future. Organized efforts to predict the future began with practices like astrology, haruspicy, and augury. These are all considered to be pseudoscience today, evolving from the human desire to know the future in advance.
+Modern efforts such as futures studies attempt to predict technological and societal trends, while more ancient practices, such as weather forecasting, have benefited from scientific and causal modelling. Despite the development of cognitive instruments for the comprehension of future, the stochastic and chaotic nature of many natural and social processes has made precise forecasting of the future elusive.
+
+== In art and culture ==
+
+=== Futurism ===
+
+Futurism as an art movement originated in Italy at the beginning of the 20th century. It developed largely in Italy and in Russia, although it also had adherents in other countries—in England and Portugal for example. The Futurists explored every medium of art, including painting, sculpture, poetry, theatre, music, architecture, and even gastronomy. Futurists had passionate loathing of ideas from the past, especially political and artistic traditions. They also espoused a love of speed, technology, and violence. Futurists dubbed the love of the past passéisme. The car, the plane, and the industrial town were all legendary for the Futurists because they represented the technological triumph of people over nature. The Futurist Manifesto of 1909 declared: "We will glorify war—the world's only hygiene—militarism, patriotism, the destructive gesture of freedom-bringers, beautiful ideas worth dying for, and scorn for woman." Though it owed much of its character and some of its ideas to radical political movements, it had little involvement in politics until the autumn of 1913.
+Futurism in Classical Music arose during this same time period. Closely identified with the central Italian Futurist movement were brother composers Luigi Russolo (1885–1947) and Antonio Russolo (1877–1942), who used instruments known as intonarumori—essentially sound boxes used to create music out of noise. Luigi Russolo's futurist manifesto, "The Art of Noises", is considered one of the most important and influential texts in 20th-century musical aesthetics. Other examples of futurist music include Arthur Honegger's "Pacific 231" (1923), which imitates the sound of a steam locomotive, Prokofiev's "The Steel Step" (1926), Alexander Mosolov's "Iron Foundry" (1927), and the experiments of Edgard Varèse.
+Literary futurism made its debut with F.T. Marinetti's Manifesto of Futurism (1909). Futurist poetry used unexpected combinations of images and hyper-conciseness (not to be confused with the actual length of the poem). Futurist theater works have scenes a few sentences long, use nonsensical humor, and try to discredit the deep-rooted dramatic traditions with parody. Longer literature forms, such as novels, had no place in the Futurist aesthetic, which had an obsession with speed and compression.
+Futurism expanded to encompass other artistic domains and ultimately included painting, sculpture, ceramics, graphic design, industrial design, interior design, theatre design, textiles, drama, literature, music and architecture. In architecture, it featured a distinctive thrust towards rationalism and modernism through the use of advanced building materials. The ideals of futurism remain as significant components of modern Western culture; the emphasis on youth, speed, power and technology finding expression in much of modern commercial cinema and commercial culture. Futurism has produced several reactions, including the 1980s-era literary genre of cyberpunk—which often treated technology with a critical eye.
+
+=== Science fiction ===
+
+More generally, one can regard science fiction as a broad genre of fiction that often involves speculations based on current or future science or technology. Science fiction is found in books, art, television, films, games, theater, and other media. Science fiction differs from fantasy in that, within the context of the story, its imaginary elements are largely possible within scientifically established or scientifically postulated laws of nature (though some elements in a story might still be pure imaginative speculation). Settings may include the future, or alternative time-lines, and stories may depict new or speculative scientific principles (such as time travel or psionics), or new technology (such as nanotechnology, faster-than-light travel or robots). Exploring the consequences of such differences is the traditional purpose of science fiction, making it a "literature of ideas".
+Some science fiction authors construct a postulated history of the future called a "future history" that provides a common background for their fiction. Sometimes authors publish a timeline of events in their history, while other times the reader can reconstruct the order of the stories from information in the books. Some published works constitute "future history" in a more literal sense—i.e., stories or whole books written in the style of a history book but describing events in the future. Examples include H.G. Wells' The Shape of Things to Come (1933)—written in the form of a history book published in the year 2106 and in the manner of a real history book with numerous footnotes and references to the works of (mostly fictitious) prominent historians of the 20th and 21st centuries.
+
+== See also ==
+
+== References ==

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+---
+title: "Grim Reaper paradox"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Grim_Reaper_paradox"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:07.081424+00:00"
+instance: "kb-cron"
+---
+
+In philosophy, the Grim Reaper paradox is a paradox involving an infinite sequence of Grim Reapers, each tasked with killing a person if no reaper has already killed them. The paradox raises questions about the possibility of continuous time and the infinite past (temporal finitism).
+It is inspired by J. A. Benardete's paradoxes from the 1964 book Infinity: An Essay in Metaphysics. Various formulations of paradoxes involving beginningless sets, whose members perform a function only if no previous member performs it, are all labelled "Benardete Paradoxes". These are examples of supertasks.
+
+
+== Paradox ==
+The paradox supposes there is an infinite sequence of Reapers, each assigned a time to kill a particular person. Each Reaper will only kill this person if no earlier Reaper has already killed them.
+It is 12pm, and the first Reaper is set to kill the person at 1pm. The second Reaper is set to kill them at 12:30pm, the third at 12:15pm, and so on.
+As a consequence of these propositions, the person will certainly be killed by a Reaper before 1pm; however, no individual Reaper can kill them, as there is always an earlier Reaper who would do so first. Therefore, it is impossible for the person to survive but also impossible that any Reaper kills them.
+
+
+== Resolutions and implications ==
+
+
+=== Discrete time ===
+One solution to the paradox is supposing that time must be discrete rather than continuous. If so, an infinite number of Reapers cannot all have a separate time in which they will kill you, as there are only finitely many "moments" in each period of time. A possible issue with this solution is that the Reaper paradox can take different forms that do not rely upon continuous time. One such example appears in Benardete's book, in which a god throws up a wall if a man travels 1/2 mile, another god throws up a wall after 1/4 mile, another at 1/8 mile, ad infinitum. Discrete time would do nothing to prevent this paradox.
+
+
+=== Causal finitism ===
+Another solution is the idea of causal finitism, which asserts that there cannot be an infinite regress of causes. In other words, every causal chain must have a starting point. Thus, there cannot be an infinite number of Reapers whose actions depend on all previous Reapers. All Benardete paradoxes share this feature of an infinite causal chain, and so are all impossible.
+Causal finitism could plausibly imply the discreteness of time, temporal finitism, infinitely large spatial regions, and continuously dense spatial regions, all of which are heavy metaphysical commitments.
+
+
+=== Unsatisfiable Pair Diagnosis ===
+A third potential solution to the Grim Reaper paradox has been suggested, known as the Unsatisfiable Pair Diagnosis (UPD). The UPD asserts that Benardete paradoxes (including the Grim Reaper paradox) are simply logically impossible, and no metaphysical thesis needs to be adopted. In The Form of the Benardete Dichotomy, Nickolas Shackel observes that all Benardete paradoxes involve two conditions:
+
+The linearly ordered set S has no first member.
+For all x in S, E at x iff E nowhere before x.
+Shackel shows these statements to be formally inconsistent—they logically cannot both be true. The paradox assumes that some set of items could satisfy both statements, but no set can.
+
+
+== Relevance to theism ==
+According to Alexander Pruss, the Grim Reaper paradox provides grounds for thinking that the past is finite, i.e., that there must be a first period of time. This would support the Kalam cosmological argument, backing up the premise that the universe began to exist.
+In 2018, Pruss provided a more thorough cosmological argument using causal finitism to motivate a "necessary uncaused cause". The argument is as follows:
+
+Nothing has an infinite causal history.
+There are no causal loops.
+Something has a cause.
+Therefore, there is an uncaused cause.
+Pruss then adds the following causal principle: 5. Every contingent item has a cause. From this, the conclusion can be drawn that there is an uncaused cause that exists necessarily. Pruss states that it is still a major task to argue from a necessary first cause to theism.
+Whilst the Kalam argument opposes sequences that go infinitely backwards in time, this argument denies all causally backwards-infinite sequences.
+
+
+== Notes ==
+
+
+== References ==

data/en.wikipedia.org/wiki/Growing_block_universe-0.md

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+title: "Growing block universe"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Growing_block_universe"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:08.271158+00:00"
+instance: "kb-cron"
+---
+
+The growing block universe, or the growing block view, is a theory of time arguing that the past and present both exist, while the future does not yet exist. The present is the perpetuating factor of time, where new moments are added to the past. By the passage of time more of the world comes into being; therefore, the block universe is said to be growing. The growth of the block is supposed to happen in the present, a very thin slice of spacetime, where more of spacetime is continually coming into being. Growing block theory should not be confused with block universe theory, also known as eternalism.
+The growing block view is an alternative to both eternalism (according to which past, present, and future all exist) and presentism (according to which only the present exists). It is held to be closer to common-sense intuitions than the alternatives. C. D. Broad was a proponent of the theory (1923). Some modern defenders are Michael Tooley (in 1997) and Peter Forrest (in 2004). Fabrice Correia and Sven Rosenkranz (2015) have developed their own distinctive view of this theory.
+
+
+== Overview ==
+Broad first proposed the theory in 1923. He described the theory as follows:
+
+It will be observed that such a theory as this accepts the reality of the present and the past, but holds that the future is simply nothing at all. Nothing has happened to the present by becoming past except that fresh slices of existence have been added to the total history of the world. The past is thus as real as the present. On the other hand, the essence of a present event is, not that it precedes future events, but that there is quite literally nothing to which it has the relation of precedence. The sum total of existence is always increasing, and it is this which gives the time-series a sense as well as an order.  A moment t is later than a moment t'  if the sum total of existence at t includes the sum total of existence at t'  together with something more.
+This dynamic theory of time conforms with the common-sense intuition that the past is fixed, the future is unreal, and the present is constantly changing. The theory resolves the paradox that time has a beginning but does not seem to have an end. There are also other reasons for supporting the growing block view of time that go beyond the common-sense. For example, Tooley bases his argument on the causal relation. His main argument as outlined by Dainton is as follows:
+
+Events in our world are causally related.
+The causal relation is inherently asymmetrical. Effects depend on their causes in a way that causes do not depend on their effects.
+This asymmetry is only possible if a cause's effects are not real as of the time of their cause.
+Causes occur before their effects. "X is earlier than Y" means roughly that some event simultaneous with X causes some event simultaneous with Y.
+Our universe must therefore be a growing block.
+
+
+== Criticism ==
+
+In the 21st century, several philosophers, such as David Braddon-Mitchell (2004), Craig Bourne, and Trenton Merricks, observed that if the growing block view is correct then it must be concluded that it is unknown whether now is now. The first occurrence of "now" is an indexical and the second occurrence of "now" is the objective tensed property. Their observation implies the following sentence: "This part of spacetime has the property of being present." For example, Socrates discussing in the past with Gorgias, and at the same time thinking that the discussion is occurring now. According to the growing block view, tense is a real property of the world so his thought is about now, the objective present. He thinks tenselessly that his thought is occurring on the edge of being but is wrong because he is in the past; he does not know that now is now, yet how can one be sure they are not in the same position. As there is nothing special with Socrates, it cannot be known whether now is now. Some argued that there is an ontological distinction between the past and the present. For instance, Forrest (2004) argues that although there exists a past, it is lifeless and inactive. Consciousness, as well as the flow of time, is not active within the past and can only occur at the boundary of the block universe in which the present exists in all existence.
+This "theory" is in conflict with Special Relativity, where the notion of the present depends on the observer.
+
+
+== See also ==
+An Experiment with Time, which proposes a similar concept
+Eternity
+Philosophy of space and time
+
+
+== References ==
+
+
+== Bibliography ==
+Broad, C. D. (1923). Scientific Thought (pdf). New York: Harcourt, Brace and Co.
+Tooley, Michael (1997). Time, Tense, and Causation (PDF). Oxford University Press. ISBN 9780198235798. Archived from the original (PDF) on 6 October 2016. Retrieved 30 August 2016.
+Bourne, Craig (2002). "When am I?". Australasian Journal of Philosophy. 80 (3): 359–71. doi:10.1080/713659472. hdl:2299/8627. S2CID 169173490.
+Braddon-Mitchell, David (2004). "How do we know it is now now?". Analysis. 64 (283): 199–203. doi:10.1111/j.0003-2638.2004.00485.x.
+Forrest, Peter (2004). "The real but dead past: a reply to Braddon-Mitchell". Analysis. 64 (284): 358–62. doi:10.1111/j.0003-2638.2004.00510.x.
+Merricks, Trenton (2006). Zimmerman, Dean (ed.). Good-Bye Growing Block (PDF). Oxford Studies in Metaphysics. Vol. 2. Oxford University Press. p. 103. ISBN 9780199290598.
+
+
+== External links ==
+"Time". Internet Encyclopedia of Philosophy. UTM.

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+title: "Hauntology"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Hauntology"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:10.113007+00:00"
+instance: "kb-cron"
+---
+
+Hauntology (a portmanteau of haunting and ontology, also spectral studies, spectralities, or the spectral turn) is a range of ideas referring to the return or persistence of elements from the social or cultural past, as if to haunt the present. The term is a neologism first introduced by French philosopher Jacques Derrida in his 1993 book Spectres of Marx. It has since been invoked in fields such as visual arts, philosophy, electronic music, anthropology, criminology, politics, fiction, and literary criticism.
+While Christine Brooke-Rose had previously punned "dehauntological" (on "deontological") in Amalgamemnon (1984), Derrida initially used "hauntology" for his idea of the atemporal nature of Marxism and its tendency to "haunt Western society from beyond the grave". It describes a situation of temporal and ontological disjunction in which presence, especially socially and culturally, is replaced by a deferred non-origin. The concept is derived from deconstruction, in which any attempt to locate the origin of identity or history must inevitably find itself dependent on an always-already existing set of linguistic conditions. Despite being the central focus of Spectres of Marx, the word hauntology appears only three times in the book, and there is little consistency in how other writers define the term.
+In the 2000s, the term was applied to musicians by theorists Simon Reynolds and Mark Fisher, who were said to explore ideas related to temporal disjunction, retrofuturism, cultural memory, and the persistence of the past. Hauntology has been used as a critical lens in various forms of media and theory, including music, aesthetics, political theory, architecture, Africanfuturism, Afrofuturism, neo-futurism, metamodernism, anthropology, and psychoanalysis.  Due to the difficulty in understanding the concept, there is little consistency in how other writers define the term.
+
+== Development ==
+
+=== Precursors ===
+
+Hauntings and ghost stories have existed for millennia, and reached a heyday in the West during the 19th century. In cultural studies, Terry Castle (in The Apparitional Lesbian) and Anthony Vidler (in The Architectural Uncanny) predate Derrida.
+
+=== Spectres of Marx ===
+
+"Hauntology" originates from Derrida's discussion of Karl Marx in Spectres of Marx, specifically Marx's proclamation that "a spectre is haunting Europe—the spectre of communism" in The Communist Manifesto. Derrida calls on Shakespeare's Hamlet, particularly a phrase spoken by the titular character: "the time is out of joint". The word functions as a deliberate near-homophone to "ontology" in Derrida's native French (cf. "hantologie", [ɑ̃tɔlɔʒi] and "ontologie", [ɔ̃tɔlɔʒi]).
+Derrida's prior work on deconstruction, on concepts of trace and différance in particular, serves as the foundation of his formulation of hauntology, fundamentally asserting that there is no temporal point of pure origin but only an "always-already absent present". Derrida sees hauntology as not only more powerful than ontology, but that "it would harbor within itself eschatology and teleology themselves". His writing in Spectres is marked by a preoccupation with the "death" of communism after the 1991 fall of the Soviet Union, in particular after theorists such as Francis Fukuyama asserted that capitalism had conclusively triumphed over other political-economic systems and reached the "end of history".
+
+Despite being the central focus of Spectres of Marx, the word hauntology appears only three times in the book. Peter Buse and Andrew Scott, discussing Derrida's notion of hauntology, explain: Ghosts arrive from the past and appear in the present. However, the ghost cannot be properly said to belong to the past .... Does then the 'historical' person who is identified with the ghost properly belong to the present? Surely not, as the idea of a return from death fractures all traditional conceptions of temporality. The temporality to which the ghost is subject is therefore paradoxical, at once they 'return' and make their apparitional debut [...] any attempt to isolate the origin of language will find its inaugural moment already dependent upon a system of linguistic differences that have been installed prior to the 'originary' moment (11).
+
+=== In music ===
+
+In the 2000s, the term was taken up by critics in reference to paradoxes found in postmodernity, particularly contemporary culture's persistent recycling of retro aesthetics and incapacity to escape old social forms. Writers such as Mark Fisher and Simon Reynolds used the term to describe a musical aesthetic preoccupied with this temporal disjunction and the nostalgia for "lost futures". So-called "hauntological" musicians are described as exploring ideas related to temporal disjunction, retrofuturism, cultural memory, and the persistence of the past.
+
+== In anthropology ==
+Anthropology has seen a widespread usage of hauntology as a methodology across ethnography, archaeology, and psychological anthropology. In 2019 Ethos, the journal of the Society for Psychological Anthropology dedicated a full issue to hauntology, titled Hauntology in Psychological Anthropology, and numerous books and journal articles have since appeared on the topic. In a book titled The Hauntology of Everyday Life, psychological anthropologist Sadeq Rahimi asserts, "the very experience of everyday life is built around a process that we can call hauntogenic, and whose major by-product is a steady stream of ghosts." In 2025, the journal Anthropological Theory Commons published a Special Collection on hauntology titled "Ghostly Lessons."
+
+=== As method ===
+Justin Armstrong, building on Derrida, proposes a "spectral ethnography" that "sees beyond the boundaries of actually spoken language and direct human contact to the interplay between space, place, objects, and temporality". Jeff Ferrell and Theo Kidynis, building on Armstrong, have developed further ideas of "ghost ethnography". Mara Dicenta, building on Gordon and Derrida, proposes "haunting as anti-method," one "that refuses to manage repression through interpretation" and allows for follow ghosts of violence without seeking resolution.
+
+=== Primary and secondary hauntings ===

data/en.wikipedia.org/wiki/Hauntology-1.md

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+---
+title: "Hauntology"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Hauntology"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:10.113007+00:00"
+instance: "kb-cron"
+---
+
+Anthropologists Martha and Bruce Lincoln make a distinction between primary hauntings, in which the haunted recognize the reality and autonomy of metaphysical entities in relatively uncritical, literal manner; and secondary hauntings, which identify "textual residues" history, or as tropes for "collective intrapsychic states" such as trauma and grief. As a case study, they use the example of Ba Chúc's secondary haunting, in which the state-controlled museums display the skulls of the dead and memorabilia, as opposed to traditional Vietnamese burial customs. This is contrasted with the "primary haunting" of Ba Chúc, the paranormal activity said to occur at an execution site marked by a tree.
+Kit Bauserman notes that for literary and critical theorists, the ghost is "pure metaphor" and "a fictional vessel that co-opts their social agenda", whereas ethnographers and anthropologists "come the closest to engaging ghosts as beings". Some scholars have argued that the "neat distinction quickly breaks down in ethnographic analysis" and that "it is far from clear that the presence of ghosts as metaphysical entities is primary."
+
+== See also ==
+Cultural memory
+Eternal return
+National memory
+
+== Notes ==
+
+== References ==
+Blanco, María del Pilar; Peeren, Esther (2013). The spectralities reader : ghosts and haunting in contemporary cultural theory. New York. ISBN 978-1-4411-2478-4. OCLC 859160150.{{cite book}}:  CS1 maint: location missing publisher (link)
+Buse, Peter; Scott, Andrew, eds. (2002). Ghosts: deconstruction, psychoanalysis, history (Digital ed.). Basingstoke London: Palgrave Macmillan. ISBN 978-0-333-71143-9.
+Coverley, Merlin (2021). Hauntology: Ghosts of Futures Past (1st ed.). London: Oldcastle Books, Limited. ISBN 978-0-85730-419-3.
+"Special Issue". Ethos. 47 (4). 2019. doi:10.1111/etho.v47.4. ISSN 0091-2131. Archived from the original on 2025-01-02.
+Derrida, Jacques (2012). Specters of Marx: The State of the Debt, the Work of Mourning and the New International. Hoboken: Taylor and Francis. ISBN 978-0-415-38957-0.
+
+== External links ==
+
+BBC Archive (1 March 2019). "BBC Ideas: What is hauntology? And why is it all around us?". BBC Ideas. BBC. Retrieved 1 May 2020.
+Fisher, Mark (17 January 2006). "k-punk: Hauntology Now". k-punk.abstractdynamics.org.
+Reynolds, Simon (15 October 2017). "ReynoldsRetro: HAUNTOLOGY: the GHOST BOX label (Frieze, 2005)". ReynoldsRetro (Blog). Retrieved 1 May 2020.
+Szrot, Lukas (2019). "Hamlet's Father: Hauntology and the Roots of the Modern Self". Fast Capitalism. 16 (2). ISSN 1930-014X. Archived from the original on 2021-12-02.
+Monk, Jonny (April 21, 2026). "Hauntology". hauntology.co.uk.

data/en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others-0.md

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+---
+title: "If You Find This World Bad, You Should See Some of the Others"
+chunk: 1/4
+source: "https://en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:11.807272+00:00"
+instance: "kb-cron"
+---
+
+"If You Find This World Bad, You Should See Some of the Others", also known as the "Metz speech", is a 1977 speech and essay by science fiction writer Philip K. Dick.  He delivered it as the guest of honor on September 24, 1977, at the Second Metz International Science Fiction Festival in Metz, France. Dick prepared his first version of the speech by May of that year, but was asked to deliver a shorter version due to time constraints. The Metz speech, in its shorter form, was recorded on video and was translated to the French audience by an interpreter. Dick's speech lays out his characteristic, yet arcane thoughts on the philosophy of space and time and the concept of alternate universes within a hypothetical multiverse.  The speech also discusses Dick's strange and unusual visions from 1974, his interpretation of Christian Gnosticism, and the role of alternate history in his published work. 
+The speech was not well received.  Dick spoke in a monotone voice, the interpreter's transcript differed from Dick's speech due to a last minute rewrite, and the audience was left confused and bewildered. At the time, nobody knew exactly what to make of the speech, as it defied conventional wisdom. Some accused Dick of being under the influence, which may have been true, or even of trying to start his own religion, which was a misunderstanding.  Later, some critics argued that Dick had gone insane while writing Valis, which he talks about working on in the speech itself, but this conclusion was heavily debated with no clear resolution one way or the other.  Dick himself admitted in the speech that what he was saying was neither provable nor rational.  Several years later, he admitted that the speech "made no sense whatever". The speech was subsequently published in print form as an edited essay in French, English, German, and Italian, from 1978 to 1991.  The audio portion was first broadcast on the radio in 1978, and again in 1982.
+
+== Background ==
+Biographer Paul Williams describes Dick as a "reluctant convention-goer", who would often cancel at the last minute due to illness when he was supposed to appear in public. Dick was living in the East Bay when the 22nd World Science Fiction Convention came to Oakland in 1964.  He attended, giving rise to many rumors and legends about his life.  Biographer Brian J. Robb notes that "Dick's reputation as a mad, drug-fueled SF prophet emerged almost fully formed from the 1964 WorldCon, and persisted beyond his death."  Williams himself met Dick for the first time in Berkeley at the 26th World Science Fiction Convention in 1968. In correspondence with Andrew I. Porter, Dick spoke about having attended the 30th World Science Fiction Convention in Los Angeles in September 1972, expressing disappointment with his experience.
+His home in Santa Venetia, near San Rafael, California, was burglarized in November 1971, leading him to temporarily leave the U.S. for Canada. With a recommendation from Ursula Le Guin,  Dick attended the Vancouver Science Fiction Convention as the guest of honor from February 18–19, 1972, delivering the speech "The Android and the Human".  Staying in Canada for an extended period, by March he was despondent and attempted suicide, which he survived, enrolling into a rehabilitation program for a month, and then returning to California.  His experience at rehab in Canada later provided material for his novel A Scanner Darkly (1978). In 1974, Dick was asked to attend the West Coast Science Fantasy Conference as the guest of honor, but declined due to health issues.  That same year, he was also asked to be the guest of honor for the future 35th World Science Fiction Convention in 1977, but also declined.  In 1975, Dick was scheduled to give the speech "Man, Android and Machine" at the Institute of Contemporary Arts in London, but never made it due to illness. In his absence, it was published as an essay in Science Fiction at Large (1976).
+By the summer of 1977, Dick was suffering from depression due to the lingering effects of his hallucinations from three years prior, the famous February–March 1974 vision, also known as "2-3-74".  Dick struggled to adapt the strange experience into a novel, as he still had a Bantam contract to meet, which contributed to additional stress.  Several years later, his book A Scanner Darkly was awarded the Graouilly d'Or for Best Novel at the Metz festival when it was published in France in 1979. Dick was invited to attend the International Festival of Science Fiction at Metz for a second time as guest of honor in June 1982, but he died unexpectedly from a stroke in March of that year at the age of 53.  Up to that point, Dick had been writing for 30 years, having released 48 novels, more than 100 short stories, and several essays and speeches.  At the time of his death, Dick was mostly unknown in the U.S. outside of niche academic and literature communities, but was widely read in Australia, Europe, and Japan.

data/en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others-1.md

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+---
+title: "If You Find This World Bad, You Should See Some of the Others"
+chunk: 2/4
+source: "https://en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:11.807272+00:00"
+instance: "kb-cron"
+---
+
+== Invitation and preparation ==
+The first Metz International Science Fiction Festival (Festival International de la Science-Fiction de Metz) took place from May 24–30, 1976, founded by Philippe Hupp, a book reviewer and French translator of Time out of Joint (1959), as well as a columnist for the French version of Galaxy Science Fiction.  Building on the momentum from the success of the first festival, Hupp invited Dick on February 23, 1977, to attend the second festival as the guest of honor.  Dick accepted on March 17, and responded to Hupp with a brief description of his planned speech. Even though Dick had accepted the invitation, his reputation for not showing up to conferences was well known. Hupp flew to the United States to make the case to Dick in person.  They met for lunch at an Italian restaurant near his apartment in Santa Ana, California.  Hupp sold him on the festival, explaining exactly how it would work and how Dick would be taken care of during his time in France.  At the meeting, Dick gave Hupp an audio cassette of the speech he had already prepared (dated May 21), and they enjoyed a meal over two bottles of red wine.  Hupp made note of the fact that Dick appeared to be happy.  Acting also as photographer, Hupp captured several notable images of Dick after lunch, including one of him holding his cat and another where he is seen wearing a large, ornate crucifix. On June 27, Dick wrote a letter to Ralph Vicinanza, his New York literary agent, sending him a copy of his planned Metz speech, with the note, "I hope you enjoy the speech; I hope they do, too. Fortunately for me the French make no clear distinction between genius and madness."
+
+== Metz International Science Fiction Festival ==
+
+The second Metz festival took place on September 19–25, 1977. Dick and Joan Simpson flew out to the festival together as a couple. It was one of only three times Dick had left the country.  Before leaving for the trip, Dick acquired methamphetamine, one of the last known times he would use hard drugs. Hupp picked them up at the airport in Luxembourg and Dick and Simpson checked into the Sofitel hotel. Dick delivered his speech on September 24 at city hall on the Place d'Armes. The speech was titled "If You Find This World Bad, You Should See Some of the Others". 
+Writers Harlan Ellison, Roger Zelazny, John Brunner, Harry Harrison, Robert Sheckley, and Fritz Leiber were all in attendance at Metz. Also invited was film producer Gary Kurtz, who was promoting his film Star Wars (1977) in Europe. The film had just been released in May, and it was screened at the festival, although it was in English as the French version had not yet been made. Dick became a huge fan of the film and later claimed that George Lucas was drawing on the same ideas as he was. German musical duo Cluster, just coming off their Cluster & Eno sessions in June, performed live at Metz, with their performance memorialized in the 22-minute recording titled "Festival International de la Science-Fiction, Metz 1977" (2017). Dick remembered his experience in France fondly, describing the 1977 Metz festival as the greatest time of his life. "I think that there at Metz I was really happy for the first time", he recalled, believing that he had finally come home to his people.

data/en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others-2.md

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+---
+title: "If You Find This World Bad, You Should See Some of the Others"
+chunk: 3/4
+source: "https://en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:11.807272+00:00"
+instance: "kb-cron"
+---
+
+== Synopsis ==
+Dick proposes that novelists have a lot of ideas to work with, but most of these are meaningless and of no value. And yet, try as they might, over their entire career, a novelist may only come across a few great ideas.  Dick argues that the novelist does not find or create these ideas on their own, rather these ideas are like living entities that seek out the novelist to make themselves manifest to the larger world, to history itself.  To support this argument, Dick cites the pre-Socratic philosophers, who proposed the cosmos was a thinking entity, as well as theistic ideas found in works related to Indian Vedanta, Spinoza, and Alfred North Whitehead. Dick points to the existence of a hidden, gnostic, God-like figure, citing the Sufi poet Rumi to make his point: "The workman is invisible within the workshop". From this, Dick places these ideas within a continuum of multiple, parallel universes in a lateral axis of time, independent of linear progression, which he admits is absurd and nonsensical, and undermines his own belief in pantheistic monism.  More importantly, Dick asks, what kind of changes would people experience if these realities were to be altered?  Dick concludes that most people would not notice at all as their memories would adapt to the updated timelines.  
+Dick explains his original theory on "orthogonal or right-angle time".  He presents ideas related to the philosophy of space and time and briefly proposes the existence of alternate universes as a thought experiment in relation to his own personal experiences and published works. Using the extended metaphor of the chessboard, and informed by ideas belonging to mythology and gnostic Christian theology, Dick describes how he believes that many worlds branch off due to a kind of chess game being played that alters the timeline of the "matrix world" by what he calls a "Programmer-Reprogrammer", a god-like entity who maintains an advantage playing against a "dark counterplayer", the personification of evil or death. Dick argues, by way of this metaphor, that the Programmer-Reprogrammer, or god, interferes with the timeline by changing the past to create a better future, and that some people (like himself) can perceive the relics and vestiges of the older timeline or alternate branches by various means, such as writing science fiction (which documents what these other worlds are like), feeling déjà vu, and even religious experience.
+Dick explores the idea of alternate history in his own fiction, with works such as The Man in the High Castle, where the Axis powers won World War II instead of the Allies.  He also connects this to Flow My Tears, the Policeman Said, his novel about a dystopian police state where people slip into more idealized, alternate worlds.  Dick then ties this into Christian theology, arguing that it points to alternate realities itself: the faithful might see the alternate reality of the Kingdom of God, while those who don't will remain in their own alternate reality.  Dick believes that Christ taught the method for traversing these alternate worlds, but that it had been lost, although it might be regained once again. To support his idea, Dick cites Isaiah 65, which speaks of the New Earth and the redemption of humanity: "For I am fashioning a new heaven and a new earth, and the memory of the former things will not enter the mind nor come up into the heart."  Dick speculates that his fictional work forced people to remember the old timelines that a god-like entity had promised to delete with the creation of better timelines. "And perhaps in my novels and stories", Dick concludes, "I have done wrong to urge you to remember."

data/en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others-3.md

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+---
+title: "If You Find This World Bad, You Should See Some of the Others"
+chunk: 4/4
+source: "https://en.wikipedia.org/wiki/If_You_Find_This_World_Bad,_You_Should_See_Some_of_the_Others"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:11.807272+00:00"
+instance: "kb-cron"
+---
+
+== Critical reception ==
+Harlan Ellison did not attend Dick's speech, as they had been estranged since 1975 at the time.  Ellison recalled that the people who heard the speech "looked like they had been stunned by a ball peen hammer...they thought [Dick] was either drunk or doped".  John Brunner, who originally met Dick in 1964 at a party before Worldcon in Oakland, recalled his own confusion on the matter a decade later: "I...failed to figure out how literally [Dick] intended people to regard his claims", wrote Brunner.  "I could not decide whether, after so many years of inner suffering, his reason had been usurped by his own inventions, or whether he had reached the bitter conclusion that the only way to cope with our lunatic world was to treat it as one vast and rather vicious joke, and fight back on the same irrational level." 
+Dick's friend Roger Zelazny, a co-author on the post-apocalyptic science fiction novel Deus Irae (1976), also recalled the confusion of the audience after the speech.  At Metz, Zelazny encountered numerous fans engaged in heated arguments over the meaning of the speech.  One person told Zelazny that he thought Dick intended to start a new religion with himself as the head of the church.  Confused, Zelazny approached Dick to ask him what had happened.  Dick explained that he wasn't sure, but he was asked to cut twenty minutes out of the speech, and that the cuts might have led to the French translator using a different version of the speech, contributing to the bewilderment of the audience.  Zelazny found the situation comically apropos.  "I suddenly felt as if I were in the middle of a Phil Dick story. He had been brought around the world to give a talk, it had been given and now everyone had a different view as to what had been said".
+The question of Dick's state of mind was hotly debated, with people like Eric S. Rabkin arguing that Dick had gone insane after writing Valis, while Umberto Rossi argued against the idea.  Dick notes in the speech, "I am aware that the claims I am making—claims of having retrieved buried memories of an alternate present and to have perceived the agency responsible for arranging that alteration—these claims can neither be proved nor can they even be made to sound rational in the usual sense of the word." 
+Several years later, Dick commented on his Metz performance in the essay "The Lucky Dog Pet Store" (1979), which was edited and republished as a new "Introduction" to "The Golden Man" (1980).  In the essay, he talks about how the Metz speech "typically, made no sense whatever". "Even the French couldn't understand it, despite a translation", Dick writes. "Something goes haywire in my brain when I write speeches; I think I imagine I'm a reincarnation of Zoroaster bringing news of God. So I try to make as few speeches as possible."
+Philosopher Heath Massey believes that the speech addresses one of Dick's most "provocative" ideas about time, the suggestion that we live in many worlds, which Massey compares to the idea of possible worlds discussed in philosophy, as well as metaphysics and theology.  Massey compares and contrasts Dick's use of the multiverse with the concept of eternal return, particularly the interpretation used by Friedrich Nietzsche. "Struggling against a world where ordinary people are dominated by impersonal, inhuman forces", writes Massey, "Dick proposes that not only are there many possible futures, but many alternate presents. Those who can imagine or perhaps even perceive them would, like those who affirm the eternal return, be virtually superhuman—not immortal, not omnipotent, but capable of resisting the supposedly inexorable march of time."
+In his assessment of the audio-only portion of the speech broadcast on the radio, Richard Wolinsky remarked that Dick's monotonous voice made the speech difficult to understand.
+
+== Release ==
+Dick was interviewed several times during the conference.  An interview conducted by Uwe Anton and Werner Fuchs was published three times in Germany, followed by its transcription and English publication in SF Eye in 1996.  Another Metz interview by Yves Breux and François Luxereau appeared on the BBC in 1994 and on French cable television in 2002. To further commemorate Dick's participation at the conference in Metz, his short story "Explorers We" (1959) was reprinted in French as "Le retour des explorateurs" by Henry-Luc Planchât as a limited edition, 16 page booklet.
+A year after Dick delivered his speech, it was published as an essay in French as "Si vous trouvez ce monde mauvais, vous devriez en voir quelques autres" in the work L'année 1977-1978 de la Science-Fiction et du Fantastique. It was later translated into German in 1986 and included in Kosmische Puppen und andere Lebensformen. The Philip K. Dick Society first published the essay in English in 1991, and it was later published in Italian in Se vi pare che questo mondo sia brutto in 1999.  The essay was included in the anthology The Shifting Realities of Philip K. Dick by Pantheon Books in 1995, and later by Vintage Books.
+The original speech delivered at Metz differs in many ways from the published essay, as some significant points raised (often in relation to a question and answer period) were not based on the initial essay, particularly Dick's comments about the simulation hypothesis, where he says "We are living in a computer-programmed reality, and the only clue we have to it is when some variable is changed, and some alteration in our reality occurs". 
+A second followup speech, "How To Build A Universe That Doesn't Fall Apart Two Days Later" (1978) was written a year later, but it is unlikely that it was ever delivered to an audience.  It was first included in I Hope I Shall Arrive Soon (1985).
+Berkeley Pacifica radio station KPFA was allegedly the first to publish, air, and popularize the audio of the Metz speech in 1978, followed by a re-broadcast in 1982.
+
+== Notes and references ==
+Notes
+
+References
+
+== External links ==
+Philip K. Dick, "conférence de Metz": Video of speech, French National Centre for Scientific Research
+If You Find This World Bad, You Should See Some Of The Others at the Internet Archive

data/en.wikipedia.org/wiki/Imaginary_time-0.md

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+---
+title: "Imaginary time"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Imaginary_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:13.037583+00:00"
+instance: "kb-cron"
+---
+
+Imaginary time is a mathematical representation of time that appears in some approaches to special relativity and quantum mechanics. It finds uses in certain cosmological theories.
+Mathematically, imaginary time is real time which has undergone a Wick rotation so that its coordinates are multiplied by the imaginary unit i. Imaginary time is not imaginary in the sense that it is unreal or made-up; it is simply expressed in terms of imaginary numbers.
+
+
+== Origins ==
+In mathematics, the imaginary unit 
+  
+    
+      
+        i
+      
+    
+    {\displaystyle i}
+  
+ is 
+  
+    
+      
+        
+          
+            −
+            1
+          
+        
+      
+    
+    {\displaystyle {\sqrt {-1}}}
+  
+, such that 
+  
+    
+      
+        
+          i
+          
+            2
+          
+        
+      
+    
+    {\displaystyle i^{2}}
+  
+ is defined to be 
+  
+    
+      
+        −
+        1
+      
+    
+    {\displaystyle -1}
+  
+. A number which is a direct multiple of 
+  
+    
+      
+        i
+      
+    
+    {\displaystyle i}
+  
+ is known as an imaginary number. A number that is the sum of an imaginary number and a real number is known as a complex number.
+In certain physical theories, periods of time are multiplied by 
+  
+    
+      
+        i
+      
+    
+    {\displaystyle i}
+  
+ in this way. Mathematically, an imaginary time period 
+  
+    
+      
+        τ
+      
+    
+    {\textstyle \tau }
+  
+ may be obtained from real time 
+  
+    
+      
+        t
+      
+    
+    {\textstyle t}
+  
+ via a Wick rotation by 
+  
+    
+      
+        π
+        
+          /
+        
+        2
+      
+    
+    {\textstyle \pi /2}
+  
+ in the complex plane: 
+  
+    
+      
+        τ
+        =
+        i
+        t
+      
+    
+    {\textstyle \tau =it}
+  
+.
+Stephen Hawking popularized the concept of imaginary time in his book The Universe in a Nutshell.
+
+"One might think this means that imaginary numbers are just a mathematical game having nothing to do with the real world. From the viewpoint of positivist philosophy, however, one cannot determine what is real. All one can do is find which mathematical models describe the universe we live in. It turns out that a mathematical model involving imaginary time predicts not only effects we have already observed but also effects we have not been able to measure yet nevertheless believe in for other reasons. So what is real and what is imaginary? Is the distinction just in our minds?"
+In fact, the terms "real" and "imaginary" for numbers are just a historical accident, much like the terms "rational" and "irrational":
+
+"...the words real and imaginary are picturesque relics of an age when the nature of complex numbers was not properly understood."
+
+
+== In cosmology ==
+
+
+=== Derivation ===
+In the Minkowski spacetime model adopted by the theory of relativity, spacetime is represented as a four-dimensional surface or manifold. Its four-dimensional equivalent of a distance in three-dimensional space is called an interval. Assuming that a specific time period is represented as a real number in the same way as a distance in space, an interval 
+  
+    
+      
+        d
+      
+    
+    {\displaystyle d}
+  
+ in relativistic spacetime is given by the usual formula but with time negated:
+
+  
+    
+      
+        
+          d
+          
+            2
+          
+        
+        =
+        
+          x
+          
+            2
+          
+        
+        +
+        
+          y
+          
+            2
+          
+        
+        +
+        
+          z
+          
+            2
+          
+        
+        −
+        
+          t
+          
+            2
+          
+        
+      
+    
+    {\displaystyle d^{2}=x^{2}+y^{2}+z^{2}-t^{2}}
+  
+
+where 
+  
+    
+      
+        x
+      
+    
+    {\displaystyle x}
+  
+, 
+  
+    
+      
+        y
+      
+    
+    {\displaystyle y}
+  
+ and 
+  
+    
+      
+        z
+      
+    
+    {\displaystyle z}
+  
+ are distances along each spatial axis and 
+  
+    
+      
+        t
+      
+    
+    {\displaystyle t}
+  
+ is a period of time or "distance" along the time axis (Strictly, the time coordinate is 
+  
+    
+      
+        (
+        c
+        t
+        
+          )
+          
+            2
+          
+        
+      
+    
+    {\displaystyle (ct)^{2}}
+  
+ where 
+  
+    
+      
+        c
+      
+    
+    {\displaystyle c}
+  
+ is the speed of light, however we conventionally choose units such that 
+  
+    
+      
+        c
+        =
+        1
+      
+    
+    {\displaystyle c=1}
+  
+).
+Mathematically this is equivalent to writing
+
+  
+    
+      
+        
+          d
+          
+            2
+          
+        
+        =
+        
+          x
+          
+            2
+          
+        
+        +
+        
+          y
+          
+            2
+          
+        
+        +
+        
+          z
+          
+            2
+          
+        
+        +
+        (
+        i
+        t
+        
+          )
+          
+            2
+          
+        
+      
+    
+    {\displaystyle d^{2}=x^{2}+y^{2}+z^{2}+(it)^{2}}
+  
+
+In this context, 
+  
+    
+      
+        i
+      
+    
+    {\displaystyle i}
+  
+ may be either accepted as a feature of the relationship between space and real time, as above, or it may alternatively be incorporated into time itself, such that the value of time is itself an imaginary number, denoted by 
+  
+    
+      
+        τ
+      
+    
+    {\displaystyle \tau }
+  
+. The equation may then be rewritten in normalised form: 
+  
+    
+      
+        
+          d
+          
+            2
+          
+        
+        =
+        
+          x
+          
+            2
+          
+        
+        +
+        
+          y
+          
+            2
+          
+        
+        +
+        
+          z
+          
+            2
+          
+        
+        +
+        
+          τ
+          
+            2
+          
+        
+      
+    
+    {\displaystyle d^{2}=x^{2}+y^{2}+z^{2}+\tau ^{2}}
+  
+
+Similarly its four vector may then be written as 
+  
+    
+      
+        (
+        
+          x
+          
+            0
+          
+        
+        ,
+        
+          x
+          
+            1
+          
+        
+        ,
+        
+          x
+          
+            2
+          
+        
+        ,
+        
+          x
+          
+            3
+          
+        
+        )
+      
+    
+    {\displaystyle (x_{0},x_{1},x_{2},x_{3})}
+  
+ where distances are represented as 
+  
+    
+      
+        
+          x
+          
+            n
+          
+        
+      
+    
+    {\displaystyle x_{n}}
+  
+, and 
+  
+    
+      
+        
+          x
+          
+            0
+          
+        
+        =
+        i
+        c
+        t
+      
+    
+    {\displaystyle x_{0}=ict}
+  
+ where 
+  
+    
+      
+        c
+      
+    
+    {\displaystyle c}
+  
+ is the speed of light and time is imaginary.
+
+
+=== Application to cosmology ===
+Hawking noted the utility of rotating time intervals into an imaginary metric in certain situations, in 1971.
+In physical cosmology, imaginary time may be incorporated into certain models of the universe which are solutions to the equations of general relativity. In particular, imaginary time can help to smooth out gravitational singularities, where known physical laws break down, to remove the singularity and avoid such breakdowns (see Hartle–Hawking state). The Big Bang, for example, appears as a singularity in ordinary time but, when modelled with imaginary time, the singularity can be removed and the Big Bang functions like any other point in four-dimensional spacetime. Any boundary to spacetime is a form of singularity, where the smooth nature of spacetime breaks down. With all such singularities removed from the Universe, it thus can have no boundary and Stephen Hawking speculated that "the boundary condition to the Universe is that it has no boundary".
+However, the unproven nature of the relationship between actual physical time and imaginary time incorporated into such models has raised criticisms. Roger Penrose has noted that there needs to be a transition from the Riemannian metric (often referred to as "Euclidean" in this context) with imaginary time at the Big Bang to a Lorentzian metric with real time for the evolving Universe. Also, modern observations suggest that the Universe is open and will never shrink back to a Big Crunch. If this proves true, then the end-of-time boundary still remains.
+
+
+== See also ==
+Euclidean quantum gravity
+Multiple time dimensions
+
+
+== References ==
+
+
+== Further reading ==
+Hawking, Stephen W. (1998). A Brief History of Time (Tenth Anniversary Commemorative ed.). Bantam Books. p. 157. ISBN 978-0-553-10953-5.
+Mahan, Gerald D. (2000). "Chapter 3". Many-Particle Physics (3rd ed.). Springer. ISBN 0-306-46338-5.
+Zee, A. (2003). "Chapter V.2". Quantum field theory in a nutshell. Princeton University Press. ISBN 0-691-01019-6.
+
+
+== External links ==
+The Beginning of Time — Lecture by Stephen Hawking which discusses imaginary time.
+Stephen Hawking's Universe: Strange Stuff Explained Archived 2016-03-03 at the Wayback Machine — PBS site on imaginary time.

data/en.wikipedia.org/wiki/Immediacy_(philosophy)-0.md

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+---
+title: "Immediacy (philosophy)"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Immediacy_(philosophy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:14.226570+00:00"
+instance: "kb-cron"
+---
+
+Immediacy is a philosophical concept related to time and temporal perspectives, both visual, and cognitive. Considerations of immediacy reflect on how we experience the world and what reality is. It implies a direct experience of an event or object bereft of any intervening medium. An example would be looking at a painting, losing awareness of the medium, and seeing the depiction as real. The medium is an important concept, and somewhat paradoxical, as it is both necessary and yet forgotten. Plato deals with a similar concept in the purity of experience. He tells us that speech is more immediate than writing, because the words emerge more directly from the speaker's mind. Immediacy also possesses characteristics of both of the homophonic heterographs 'immanent' and 'imminent', and what entails to both within ontology.
+Immediacy also relates to the philosophy of phenomenology, as they are schools of thought which both concern subjective perceptions of objects and time.

data/en.wikipedia.org/wiki/Moving_spotlight_theory_of_time-0.md

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+---
+title: "Moving spotlight theory of time"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Moving_spotlight_theory_of_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:15.388049+00:00"
+instance: "kb-cron"
+---
+
+The moving spotlight theory of time is a theory of time which says that the complete history of the world exists (in much the same sense as the block universe, that is, past, present, and future all exist), but that there is an absolute, objective present moment. On this view, what is present really changes as time passes. This is a "moving spotlight" theory because the objective present "moves" through the block analogous to how a spotlight might traverse (e.g.) a row of houses in a street. This view is suggested, though not endorsed, by C.D. Broad in his Scientific Thought (1923). (Broad endorses the growing block theory of time, though this view can trace it roots further back to Samuel Alexander.)
+Moving spotlight theory and growing block theory are both taken as intermediary positions between either presentism and eternalism, or the A-theory and B-theory of time. It is common to see the moving spotlight theory described as a "non-presentist A-theory." The moving spotlight theory accepts the dynamic thesis of the A-theory that time really passes, what is objectively present changes, and accepts the static ontology of the B-theory that past, present, and future entities exist: the totality of what exists does not change.
+According to Ross Cameron and Daniel Deasy, the label "moving spotlight theory" encompasses at least three distinct views in the contemporary debate in the philosophy of time: the traditional moving spotlight theory, due to Broad, which can be understood as an enriched B-theory of time; permanentist propositional temporalism, due to Meghan Sullivan and Deasy, which combines permanentism (always, everything always exists) and temporalism, the view that propositions can change truth-values over time; and, a form of enriched presentism, due to Cameron. Other variations of the theory have been put forward.
+The moving spotlight theory can be extended to cover not only the distinction between one time and another, but also the distinction between one consciousness and another. A variant of this theory is a principal component of the plot of Fred Hoyle's novel October the First Is Too Late, which combines the idea of the moving spotlight with open individualism.
+
+
+== References ==

data/en.wikipedia.org/wiki/Non-simultaneity-0.md

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+---
+title: "Non-simultaneity"
+chunk: 1/3
+source: "https://en.wikipedia.org/wiki/Non-simultaneity"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:16.563486+00:00"
+instance: "kb-cron"
+---
+
+Non-simultaneity or nonsynchronism (German: Ungleichzeitigkeit, sometimes also translated as non-synchronicity) is a concept in the writings of Ernst Bloch which denotes the time lag, or uneven temporal development, produced in the social sphere by the processes of capitalist modernization and/or the incomplete nature of those processes. The term, especially in the phrase "the simultaneity of the non-simultaneous", has been used subsequently in predominantly Marxist theories of modernity, world-systems, postmodernity and globalization.
+
+== In the work of Ernst Bloch ==
+The phrase "the non-simultaneity of the simultaneous" (die 'Ungleichzeitigkeit' des Gleichzeitigen) was first used by the German art historian Wilhelm Pinder in his 1926 book Das Problem der Generation in der Kunstgeschichte Europas ("The Problem of Generation in European Art History").
+Bloch's principal use of the term "non-simultaneity" was in an essay from 1932 which attempted to explain the rise and popularity of Nazism in Germany in the light of the capitalist economic crisis of the Great Depression and which became a chapter of his influential 1935 study Heritage of our Times (Erbschaft dieser Zeit). The essay's central idea is that heterogeneous stages of social and economic development coexist simultaneously in 1930s Germany. Because of uneven modernization, Bloch argues, there remained in Germany, "this classical land of non-simultaneity", significant traces of pre-capitalist relations of production:
+
+"Not all people exist in the same Now. They do so only externally, by virtue of the fact that they may all be seen today. But that does not mean that they are living at the same time with others.
+Rather, they carry earlier things with them, things which are intricately involved. One has one's times according to where one stands corporeally, above all in terms of classes. Times older than the present continue to effect older strata; here it is easy to return or dream one's way back to older times. [...] In general, different years resound in the one that has just been recorded and prevails. Moreover, they do not emerge in a hidden way as previously but rather, they contradict the Now in a very peculiar way, awry, from the rear. [...] Many earlier forces, from quite a different Below, are beginning to slip between. [...]
+
+Over and above a great deal of false nonsynchronism [non-simultaneity] there is this one in particular: Nature, and more than that, the ghost of history comes very easily to the desperate peasant, to the bankrupt petty bourgeois; the depression which releases the ghost takes place in a country with a particularly large amount of pre-capitalist material. It is important to ask whether Germany is not more undeveloped, even more vulcanic than, for instance, France, in terms of its power. Certainly it has not formed and evened out capitalist ratio nearly as synchronously."
+The text signals that to some extent these ideas derive from Marx's Critique of Political Economy, and in particular his notion of "the unequal rate of development", or "uneven development". Marx had also used the term "simultaneity" (Gleichzeitigkeit) in his explanation of the concentration of production processes under the demands of commodity production in the first volume of Das Kapital (see below). But Bloch's argument is also an attempt to counter simplistic interpretations of Hegelian and Marxist teleology, by introducing what he terms "the polyrhythm and the counterpoint of such dialectics", a "polyphonous", "multispatial" and "multitemporal" dialectics, not in order to deny the possibility of proletarian revolution, but in order to "gain additional revolutionary force from the incomplete wealth of the past":
+
+The still subversive and utopian contents in the relations of people to people and nature, which are not past because they were never quite attained, can only be of use in this way. These contents are, as it were, the goldbearing gravel in the course of previous labor processes and their superstructures in the form of works. Polyphonous dialectics, as a dialectics of the "contradictions" which are more concentrated today than ever, has in any case enough questions and contents in capitalism that are not yet "superseded by the course of economic development".
+This argument touches on the need to understand the spatial dynamics of capitalism that would be taken up in the 1960s and 1970s by Marxist urban philosopher Henri Lefebvre, with his analysis of the dialectics of (urban) space, and his work on "rhythmanalysis". It also anticipates the study of the subaltern's "contradicted" relationship to Western modernity undertaken by subaltern studies and postcolonial theory (see below).
+
+== The simultaneity of the non-simultaneous ==
+Although often attributed to "Nonsynchronism and the Obligation to its Dialectics", the phrase die Gleichzeitigkeit des Ungleichzeitigen ("the simultaneity of the non-simultaneous" or "the synchronism/synchronicity of the nonsynchronous") — i.e., a reversal of Pinder's "non-simultaneity of the simultaneous" — is not explicitly used in this work. Bloch elaborates instead the idea of synchronous and nonsynchronous contradictions with "the Now". By "synchronous contradiction" he means those forces of contradiction (to capital) that capitalism itself generates, principally the contemporary industrialized proletariat (as analysed by Marx). "Nonsynchronous contradiction" refers to the atavistic survival of an "uncompleted past which has not yet been 'sublated' by capitalism" as discussed above.
+
+== In the work of Marx ==
+
+After the posthumous publication of Marx's Grundrisse in 1939, it became clear that a dialectic of simultaneity and non-simultaneity had been implicit in Marx's thinking on the spatiality and geography of capitalism. Das Kapital (1867–94) had argued on the one hand that the money form had arisen in order to allow for non-simultaneous or delayed exchange of commodities (as opposed to face-to-face bartering), and on the other that "simultaneity" (Gleichzeitigkeit) was a requirement of (and a phenomenon produced by) the demands of commodity production (the capitalist has to be able to synchronize the disparate activities required to manufacture a product). The powerful spatio-temporal effects of the dual demands of exchange and commodity production were summarized in the Grundrisse with the concept of "the annihilation of space by time", i.e. with the imposition of simultaneity or synchronicity over spatial separation and geographical diversity:

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+---
+title: "Non-simultaneity"
+chunk: 2/3
+source: "https://en.wikipedia.org/wiki/Non-simultaneity"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:16.563486+00:00"
+instance: "kb-cron"
+---
+
+The more production comes to rest on exchange value, hence on exchange, the more important do the physical conditions of exchange — the means of communication and transport — become for the costs of circulation. Capital by its nature drives beyond every spatial barrier. Thus the creation of the physical conditions of exchange — of the means of communication and transport — the annihilation of space by time — becomes an extraordinary necessity for it.
+At the same time, Marx showed himself to be acutely aware of the resistances to this overcoming of spatio-temporal barriers, and, more importantly, to the fact that capitalism itself generates its own resistances, or contradictions, to the universalization of its mode of production:
+
+But from the fact that capital posits every such limit as a barrier and hence gets ideally beyond it, it does not by any means follow that it has really overcome it, and, since every such barrier contradicts its character, its production moves in contradictions which are constantly overcome but just as constantly posited. Furthermore. The universality towards which it irresistibly strives encounters barriers in its own nature, which will, at a certain stage of its development, allow it to be recognized as being itself the greatest barrier to this tendency, and hence will drive towards its own suspension.
+Due to the late publication of the Grundrisse, Bloch would not have been acquainted with these precise words at the time of the writing of "Nonsynchronism", although the similarity of concepts relating to the way in which capitalism posits its own (simultaneous and non-simultaneous) contradictions to production ultimately derives from Das Kapital as discussed above.
+
+== Subsequent use ==
+
+=== In structural Marxism ===
+The problematic of simultaneity/non-simultaneity and synchronism/nonsynchronism was taken up in the work of post-Second-World-War Marxist sociologists and philosophers, such as Theodor Adorno, Nicos Poulantzas, Louis Althusser and Étienne Balibar.
+As structural Marxists, Althusser and Balibar were concerned to understand how "the problems of diachrony" in the transition from one mode of production to another could be related to the overall structure or "synchrony" of production. In Reading Capital (1970), they argue, in similar vein to Bloch, that the succession of different modes of production as theorized by Marx is not a teleological process driven by "the forward march of the productive forces", but that instead periods of transition are marked by "the coexistence of several modes of production":
+
+Thus it seems that the dislocation [décalage] between the connexions and instances in transition periods merely reflects the coexistence of two (or more) modes of production in a single 'simultaneity ', and the dominance of one of them over the other. This confirms the fact that the problems of diachrony, too, must be thought within the problematic of a theoretical 'synchrony': the problems of the transition and of the forms of the transition from one mode of production to another are problems of a more general synchrony than that of the mode of production itself, englobing several systems and their relations.
+For the Greek political sociologist and structural Marxist Nicos Poulantzas, forms of socio-cultural difference such as "territory and historico-cultural tradition [...] produce the uneven development of capitalism as an unevenness of historical moments affecting those differentiated, classified and distinct spaces that are called nations". In State, Power, Socialism (1978), he argues that such differences are in fact a precondition for global capitalist development.
+
+=== Henri Lefebvre and Ernest Mandel ===
+Althusser and Balibar's contemporary, Henri Lefebvre, was sharply critical of what he saw as these writers' fetishization of a fixed, abstract and purely structural notion of "general" synchronic space subsuming diachronic or historical processes. By contrast, Lefebvre's own "turbulent spatiality" which "would restore geography to history, history to geography", together with his rhythmanalysis, shares at least a common vocabulary with Bloch's multispatial and multitemporal dialectics. Lefebvre was also one of the first commentators to link uneven development to the production of space on a global scale: "The law of unevenness of growth and development, so far from becoming obsolete, is becoming world-wide in its application — or, more precisely is presiding over the globalization of a world market".
+Meanwhile, Belgian Marxist Ernest Mandel was developing, at the same time as Lefebvre, a characterization of "late capitalism" which also refuses the idea that (global) capitalism produces homogeneity. Instead, he argues, capitalism must produce "underdevelopment" in order to maximize the production of surplus profit:
+
+The entire capitalist system thus appears as a hierarchical structure of different levels of productivity, and as the outcome of the uneven and combined development of states, regions, branches of industry and firms, unleashed by the quest for surplus-profit. It forms an integrated unity, but it is an integrated unity of non-homogeneous parts, and it is precisely the unity that here determines the lack of homogeneity. In this whole system development and underdevelopment reciprocally determine each other, for while the quest for surplus-profits constitutes the prime motive power behind the mechanisms of growth, surplus-profit can only be achieved at the expense of less productive regions and branches of production.
+
+=== In Marxist sociology and geography ===
+Thinkers as diverse as Immanuel Wallerstein, with his world-systems theory, David Harvey with his analysis of the Limits to Capital (1982) and time–space compression, and Harvey's erstwhile student Neil Smith with his Uneven Development, can all be seen to develop one or other aspect of this line of Marxist thought. The early work of Anthony Giddens and in particular his concept of "time-space distanciation", e.g. in his Critique of Historical Materialism (1981), has also been influential in this area.
+
+=== In theories of modernity and postmodernity ===
+Perhaps the most famous use of Bloch's terminology to date is that made by the Marxist cultural critic Fredric Jameson when describing the economic basis of modernism in Postmodernism, or the Cultural Logic of Late Capitalism (1991):

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+---
+title: "Non-simultaneity"
+chunk: 3/3
+source: "https://en.wikipedia.org/wiki/Non-simultaneity"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:16.563486+00:00"
+instance: "kb-cron"
+---
+
+Modernism must thus be seen as uniquely corresponding to an uneven moment of social development, or to what Ernst Bloch called the "simultaneity of the nonsimultaneous," the "synchronicity of the nonsynchronous" (Gleichzeitigkeit des Ungleichzeitigen): the coexistence of realities from radically different moments of history — handicrafts alongside the great cartels, peasant fields with the Krupp factories or the Ford plant in the distance.
+Jameson goes on, however, to argue that with the advent of postmodernity and its attendant postmodernisms, the "uneven moment" of modernity has been completely replaced by the mass standardization and homogenization of the third, multinational, phase of capitalist development:
+
+the postmodern must be characterized as a situation in which the survival, the residue, the holdover, the archaic, has finally been swept away without a trace. In the postmodern, then, the past itself has disappeared (along with the well-known "sense of the past" or historicity and collective memory). Where its buildings still remain, renovation and restoration allow them to be transferred to the present in their entirety as those other, very different and postmodern things called simulacra. Everything is now organized and planned; nature has been triumphantly blotted out, along with peasants, petit-bourgeois commerce, handicraft, feudal aristocracies and imperial bureaucracies. Ours is a more homogeneously modernized condition; we no longer are encumbered with the embarrassment of non-simultaneities and non-synchronicities. Everything has reached the same hour on the great clock of development or rationalization (at least from the perspective of the "West"). This is the sense in which we can affirm, either that modernism is characterized by a situation of incomplete modernization, or that postmodernism is more modern than modernism itself.
+
+=== In postcolonial theory ===
+Subaltern studies and postcolonial theory, however, tend to maintain that the idea of a globally homogenized space, even under postmodernity, is undercut precisely by Bloch's "nonsynchronous remnants" and diverse temporalities. Homi K. Bhabha, commenting on Jameson, claims that
+
+What is manifestly new about this version of international space and its social (in)visibility is its temporal measure [...] The non-synchronous temporality of global and national cultures opens up a cultural space — a third space — where the negotiation of incommensurable differences creates a tension peculiar to borderline existences.
+Postcolonial anthropologist Arjun Appadurai makes a similar point in his book Modernity at Large (1996) via an implicit critique of Wallerstein: "The new global cultural economy has to be seen as a complex, overlapping, disjunctive order that cannot any longer be understood in terms of existing center-periphery models (even those that might account for multiple centers and peripheries)".
+
+== See also ==
+Ernst Bloch
+Uneven development
+Ungleichzeitigkeit
+Time-space compression
+
+== References ==

data/en.wikipedia.org/wiki/Past-0.md

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+---
+title: "Past"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Past"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:17.747415+00:00"
+instance: "kb-cron"
+---
+
+The past is the set of all events that occurred before a given point in time. The past is contrasted with and defined by the present and the future. The concept of the past is derived from the linear fashion in which human observers experience time, and is accessed through memory and recollection. In addition, humans have recorded the past since the advent of written language.
+In English, the word past was one of the many variant forms and spellings of passed, the past participle of the Middle English verb passen (whence Modern English pass), among ypassed, ypassyd, i-passed, passyd, passid, pass'd, paste, etc. It developed into an adjective and preposition in the 14th century, and a noun (as in the past or a past, through ellipsis with the adjective past) in the 15th century.
+
+
+== Grammar ==
+
+In English grammar, actions are classified according to one of the following twelve verb tenses: past (past, uses of English verb forms, past perfect, or past perfect continuous), present (present, present continuous, present perfect, or present perfect continuous), or future (future, future continuous, future perfect, or future perfect continuous). The past tense refers to actions that have already happened. For example, "she is walking" refers to a girl who is currently walking (present tense), while "she walked" refers to a girl who was walking before now (past tense).
+The past continuous tense refers to actions that continued for a period of time, as in the sentence "she was walking," which describes an action that was still happening in a prior window of time to which a speaker is presently referring. The past perfect tense is used to describe actions that were already completed by a specific point in the past. For example, "she had walked" describes an action that took place in the past and was also completed in the past.
+The past perfects continuous tense refers to an action that was happening up until a particular point in the past but was completed. It is different from the past perfect tense because the emphasis of past perfect continuous verbs is not on the action having been completed by the present moment, but rather on its having taken place actively over a time period before another moment in the past. The verb tense used in the sentence "She had been walking in the park regularly before I met her" is past perfect continuous because it describes an action ("walking") that was actively happening before a time when something else in the past was happening (when "I met her").
+Depending on its usage in a sentence, "past" can be described using a variety of terms. Synonyms for "past" as an adjective include, "former," "bygone," "earlier," "preceding," and "previous." Synonyms for "past" as a noun include, "history, "background," "life story," and "biography." Synonyms of "past" as a preposition include, "in front of," "beyond," "by," and "in excess of."
+
+
+== Other uses ==
+
+The word "past" can also be used to describe the offices of those who have previously served in an organization, group, or event such as, "past president," or, "past champions." "Past" can also refer to something or someone being at or in a position that is further than a particular point. For instance, in the sentence, "I live on Fielding Road, just past the train station," the word "past" is used to describe a location (the speaker's residence) beyond a certain point (the train station). Alternatively, the sentence, "He ran past us at full speed," utilizes the concept of the past to describe the position of someone ("He") that is further than the speaker.
+The "past" is also used to define a time that is a certain number of minute before or after a particular hour, as in "We left the party at half-past twelve." People also use "past" to refer to being beyond a particular biological age or phase of being, as in, "The boy was past the age of needing a babysitter," or, "I'm past caring about that problem." The "past" is commonly used to refer to history, either generally or with regard to specific time periods or events, as in, "Past monarchs had absolute power to determine the law in contrast to many European Kings and Queens of today."
+Nineteenth-century British author Charles Dickens created one of the best-known fictional personifications of the "past" in his short book, "A Christmas Carol." In the story, the Ghost of Christmas Past is an apparition that shows the main character, a cold-hearted and tight-fisted man named Ebenezer Scrooge, vignettes from his childhood and early adult life to teach him that joy does not necessarily come from wealth.
+
+
+== Fields of study ==
+The past is the object of study within such fields as time, life, history, nostalgia, archaeology, archaeoastronomy, chronology, astronomy, physics, geology, historical geology, historical linguistics, ontology, paleontology, paleobotany, paleoethnobotany, palaeogeography, paleoclimatology, etymology and physical cosmology.
+
+
+== See also ==
+
+
+== References ==

data/en.wikipedia.org/wiki/Past_hypothesis-0.md

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+---
+title: "Past hypothesis"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Past_hypothesis"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:18.928263+00:00"
+instance: "kb-cron"
+---
+
+In cosmology, the past hypothesis is a fundamental law of physics that postulates that the universe started in a low-entropy state, in accordance with the second law of thermodynamics. The second law states that any closed system follows the arrow of time, meaning its entropy never decreases. Applying this idea to the entire universe, the hypothesis argues that the universe must have started from a special event with less entropy than is currently observed, in order to preserve the arrow of time globally.
+This idea has been discussed since the development of statistical mechanics, but the term "past hypothesis" was coined by philosopher David Albert in 2000. Philosophical and theoretical efforts focus on trying to explain the consistency and the origin of the postulate.
+The past hypothesis is an exception to the principle of indifference, according to which every possible microstate within a certain macrostate would have an equal probability. The past hypothesis allows only those microstates that are compatible with a much-lower-entropy past, although these states are assigned equal probabilities. If the principle of indifference is applied without taking into account the past hypothesis, a low- or medium-entropy state would have likely evolved both from and toward higher-entropy macrostates, as there are more ways statistically to be high-entropy than low-entropy. The low- or medium-entropy state would have appeared as a "statistical fluctuation" amid a higher-entropy past and a higher-entropy future.
+Common theoretical frameworks have been developed in order to explain the origin of the past hypothesis based on inflationary models or the anthropic principle. The Weyl curvature hypothesis, an alternative model by Roger Penrose, argues a link between entropy, the arrow of time and the curvature of spacetime (encoded in the Weyl tensor).
+
+
+== See also ==
+Loschmidt's paradox
+Entropy as an arrow of time
+
+
+== Notes ==
+
+
+== References ==

data/en.wikipedia.org/wiki/Perspectival_realism-0.md

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+---
+title: "Perspectival realism"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Perspectival_realism"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:20.083010+00:00"
+instance: "kb-cron"
+---
+
+In Caspar Hare's theory of perspectival realism, there is a defining intrinsic property that the things that are in perceptual awareness have.  Consider seeing object A but not object B.  Of course, we can say that the visual experience of A is present to you, and no visual experience of B is present to you.  But, it can be argued, this misses the fact that the visual experience of A is simply present, not relative to anything.  This is what Hare's perspectival realism attempts to capture, resulting in a weak version of metaphysical solipsism.
+As Hare points out, the same type of argument is often used in the philosophy of time to support theories such as presentism.  Of course, we can say that A is happening on [insert today's date].  But, it can be argued, this misses the fact that A is simply happening (right now), not relative to anything.
+Hare's theory of perspectival realism is closely related to his theory of egocentric presentism.
+Several other philosophers have written reviews of Hare's work on this topic.
+
+
+== See also ==
+Metaphysical subjectivism
+Centered worlds
+Benj Hellie's vertiginous question
+J.J. Valberg's personal horizon
+
+
+== References ==
+
+
+== External links ==
+Hare, Caspar. Self-Bias, Time-Bias, and the Metaphysics of Self and Time. Preprint of article in The Journal of Philosophy (2007).
+Hare, Caspar. On Myself, and Other, Less Important Subjects. Early draft of book published by Princeton University Press (2009).
+Hare, Caspar. Realism About Tense and Perspective. Preprint of article in Philosophy Compass (2010).

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+title: "Philosophy of Time Society"
+chunk: 1/1
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+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:21.247470+00:00"
+instance: "kb-cron"
+---
+
+The Philosophy of Time Society is an organization which grew out of a National Endowment for the Humanities Summer Seminar on the Philosophy of Time offered by George N. Schlesinger in 1991. The organization itself was formed in 1993. Its stated goal is "to promote the study of the philosophy of time from a broad analytic perspective, and to provide a forum as an affiliated group with the American Philosophical Association, to discuss the issues in and related to the philosophy of time." The current President of the Society is Nina Emery.
+The Philosophy of Time Society's meetings are held at the division meetings of the American Philosophical Association. In the past, they have included many notable scholars such as Craig Callender, L. A. Paul, Robin Le Poidevin, Ned Markosian, D. H. Mellor, John Perry, Theodore Sider, Michael Tooley, and Dean Zimmerman. Topics of papers have varied widely.
+
+
+== References ==
+
+
+== External links ==
+Philosophy of Time Society
+George Schlesinger Memorial Site

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+---
+title: "Physical cosmology"
+chunk: 1/5
+source: "https://en.wikipedia.org/wiki/Physical_cosmology"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:22.441984+00:00"
+instance: "kb-cron"
+---
+
+Physical cosmology is a branch of physics concerned with modeling the universe based on the laws of physics. A cosmological model provides a mathematical description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood.
+Physical cosmology, as it is now understood, began in 1915 with the development of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way; then, work by Vesto Slipher and others showed that the universe is expanding. These advances made it possible to speculate about the origin of the universe, and allowed the establishment of the Big Bang theory, by Georges Lemaître, as the leading cosmological model. A few researchers still advocate a handful of alternative cosmologies; however, most cosmologists agree that the Big Bang theory best explains the observations.
+Dramatic advances in observational cosmology since the 1990s, including the cosmic microwave background, distant supernovae and galaxy redshift surveys, have led to the development of a standard model of cosmology. Although this model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, it gives detailed predictions that are in excellent agreement with many diverse observations.
+Cosmology draws heavily on the work of many disparate areas of research in theoretical and applied physics. Areas relevant to cosmology include particle physics experiments and theory, theoretical and observational astrophysics, general relativity, quantum mechanics, and plasma physics.
+
+== Subject history ==
+
+Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time. At the time, Einstein believed in a static universe, but found that his original formulation of the theory did not permit it. This is because masses distributed throughout the universe gravitationally attract, and move toward each other over time. However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his field equations in order to force them to model a static universe. The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). However, this so-called Einstein model is unstable to small perturbations—it will eventually start to expand or contract. It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the cosmological principle. The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s. His equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed.
+
+In the 1910s, Vesto Slipher (and later Carl Wilhelm Wirtz) interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth. However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its angular size, but a physical size must be assumed in order to do this. Another method is to measure the brightness of an object and assume an intrinsic luminosity, from which the distance may be determined using the inverse-square law. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—which was later called the Big Bang. In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance from Earth. This fact is now known as Hubble's law, though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables.
+Given the cosmological principle, Hubble's law suggested that the universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other explanation was Fred Hoyle's steady state model in which new matter is created as the galaxies move away from each other. In this model, the universe is roughly the same at any point in time.
+For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model, and since the precise measurements of the cosmic microwave background by the Cosmic Background Explorer in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos.

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+---
+title: "Physical cosmology"
+chunk: 2/5
+source: "https://en.wikipedia.org/wiki/Physical_cosmology"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:22.441984+00:00"
+instance: "kb-cron"
+---
+
+== Energy of the cosmos ==
+The lightest chemical elements, primarily hydrogen and helium, were created during the Big Bang through the process of nucleosynthesis. In a sequence of stellar nucleosynthesis reactions, smaller atomic nuclei are then combined into larger atomic nuclei, ultimately forming stable iron group elements such as iron and nickel, which have the highest nuclear binding energies. The net process results in a later energy release, meaning subsequent to the Big Bang. Such reactions of nuclear particles can lead to sudden energy releases from cataclysmic variable stars such as novae. Gravitational collapse of matter into black holes also powers the most energetic processes, generally seen in the nuclear regions of galaxies, forming quasars and active galaxies.
+Cosmologists cannot explain all cosmic phenomena exactly, such as those related to the accelerating expansion of the universe, using conventional forms of energy. Instead, cosmologists propose a new form of energy called dark energy that permeates all space. One hypothesis is that dark energy is just the vacuum energy, a component of empty space that is associated with the virtual particles that exist due to the uncertainty principle.
+There is no clear way to define the total energy in the universe using the most widely accepted theory of gravity, general relativity. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each photon that travels through intergalactic space loses energy due to the redshift effect. This energy is not transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of conservation of energy.
+Different forms of energy may dominate the cosmos—relativistic particles which are referred to as radiation, or non-relativistic particles referred to as matter. Relativistic particles are particles whose rest mass is zero or negligible compared to their kinetic energy, and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light.
+As the universe expands, both matter and radiation become diluted. However, the energy densities of radiation and matter dilute at different rates. As a particular volume expands, mass-energy density is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in the wavelength of the photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands. The very early universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well analytically. As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe's expansion.
+
+== History of the universe ==
+
+The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the Lambda-CDM model.
+
+=== Equations of motion ===
+
+Within the standard cosmological model, the equations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant. The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool and become diluted. At first, the expansion is slowed down by gravitation attracting the radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago.
+
+=== Particle physics in cosmology ===
+
+During the earliest moments of the universe, the average energy density was very high, making knowledge of particle physics critical to understanding this environment. Hence, scattering processes and decay of unstable elementary particles are important for cosmological models of this period.
+As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe. The time scale that describes the expansion of the universe is 
+  
+    
+      
+        1
+        
+          /
+        
+        H
+      
+    
+    {\displaystyle 1/H}
+  
+ with 
+  
+    
+      
+        H
+      
+    
+    {\displaystyle H}
+  
+ being the Hubble parameter, which varies with time. The expansion timescale 
+  
+    
+      
+        1
+        
+          /
+        
+        H
+      
+    
+    {\displaystyle 1/H}
+  
+ is roughly equal to the age of the universe at each point in time.
+
+=== Timeline of the Big Bang ===
+
+Observations suggest that the universe began around 13.8 billion years ago. Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth. Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses.
+Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known, but according to the ΛCDM model it will continue expanding forever.

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+---
+title: "Physical cosmology"
+chunk: 3/5
+source: "https://en.wikipedia.org/wiki/Physical_cosmology"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:22.441984+00:00"
+instance: "kb-cron"
+---
+
+== Areas of study ==
+Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of the Big Bang cosmology, which is presented in Timeline of the Big Bang.
+
+=== Very early universe ===
+
+The early, hot universe appears to be well explained by the Big Bang from roughly 10−33 seconds onwards, but there are several problems. One is that there is no compelling reason, using current particle physics, for the universe to be flat, homogeneous, and isotropic (see the cosmological principle). Moreover, grand unified theories of particle physics suggest that there should be magnetic monopoles in the universe, which have not been found. These problems are resolved by a brief period of cosmic inflation, which drives the universe to flatness, smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles. The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and quantum field theory. Some cosmologists think that string theory and brane cosmology will provide an alternative to inflation.
+Another major problem in cosmology is what caused the universe to contain far more matter than antimatter. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be X-rays and gamma rays produced as a result of annihilation, but this is not observed. Therefore, some process in the early universe must have created a small excess of matter over antimatter, and this (currently not understood) process is called baryogenesis. Three required conditions for baryogenesis were derived by Andrei Sakharov in 1967, and requires a violation of the particle physics symmetry, called CP-symmetry, between matter and antimatter. However, particle accelerators measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists look for additional violations of the CP-symmetry in the early universe that might account for the baryon asymmetry.
+Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and experiment, rather than through observations of the universe.
+
+=== Big Bang Theory ===
+
+Big Bang nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its temperature dropped below that at which nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from hydrogen ions (protons), it principally produced deuterium, helium-4, and lithium. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by George Gamow, Ralph Asher Alpher, and Robert Herman. It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe. Specifically, it can be used to test the equivalence principle, to probe dark matter, and test neutrino physics. Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.
+
+==== Standard model of Big Bang cosmology ====
+The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda (Greek Λ), associated with dark energy, and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology.
+
+=== Cosmic microwave background ===
+
+The cosmic microwave background is radiation left over from decoupling after the epoch of recombination when neutral atoms first formed. At this point, radiation produced in the Big Bang stopped Thomson scattering from charged ions. The radiation, first observed in 1965 by Arno Penzias and Robert Woodrow Wilson, has a perfect thermal black-body spectrum. It has a temperature of 2.7 kelvins today and is isotropic to one part in 105. Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular power spectrum of the radiation, and it has been measured by the recent satellite experiments (COBE and WMAP) and many ground and balloon-based experiments (such as Degree Angular Scale Interferometer, Cosmic Background Imager, and Boomerang). One of the goals of these efforts is to measure the basic parameters of the Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The results of measurements made by WMAP, for example, have placed limits on the neutrino masses.
+Newer experiments, such as QUIET and the Atacama Cosmology Telescope, are trying to measure the polarization of the cosmic microwave background. These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies, such as the Sunyaev-Zel'dovich effect and Sachs-Wolfe effect, which are caused by interaction between galaxies and clusters with the cosmic microwave background.
+On 17 March 2014, astronomers of the BICEP2 Collaboration announced the apparent detection of B-mode polarization of the CMB, considered to be evidence of primordial gravitational waves that are predicted by the theory of inflation to occur during the earliest phase of the Big Bang. However, later that year the Planck collaboration provided a more accurate measurement of cosmic dust, concluding that the B-mode signal from dust is the same strength as that reported from BICEP2. On 30 January 2015, a joint analysis of BICEP2 and Planck data was published and the European Space Agency announced that the signal can be entirely attributed to interstellar dust in the Milky Way.
+
+=== Formation and evolution of large-scale structure ===

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+title: "Physical cosmology"
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+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:22.441984+00:00"
+instance: "kb-cron"
+---
+
+Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies, clusters and superclusters) is one of the largest efforts in cosmology. Cosmologists study a model of hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling. One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter power spectrum. This is the approach of the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.
+Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments, superclusters and voids. Most simulations contain only non-baryonic cold dark matter, which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.
+
+Other, complementary observations to measure the distribution of matter in the distant universe and to probe reionization include:
+
+The Lyman-alpha forest, which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.
+The 21-centimeter absorption line of neutral atomic hydrogen also provides a sensitive test of cosmology.
+Weak lensing, the distortion of a distant image by gravitational lensing due to dark matter.
+These will help cosmologists settle the question of when and how structure formed in the universe.
+
+=== Dark matter ===
+
+Evidence from Big Bang nucleosynthesis, the cosmic microwave background, structure formation, and galaxy rotation curves suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible, baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like a cold, non-radiative fluid that forms haloes around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable supersymmetric particle, a weakly interacting massive particle, a gravitationally-interacting massive particle, an axion, and a massive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (MOND) or an effect from brane cosmology. TeVeS is a version of MOND that can explain gravitational lensing.
+
+=== Dark energy ===
+
+If the universe is flat, there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate.
+Apart from its density and its clustering properties, nothing is known about dark energy. Quantum field theory predicts a cosmological constant (CC) much like dark energy, but 120 orders of magnitude larger than that observed. Steven Weinberg and a number of string theorists (see string landscape) have invoked the 'weak anthropic principle': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in a universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant (CC) which allows for life to exist) it does not attempt to explain the context of that universe. For example, the weak anthropic principle alone does not distinguish between:
+
+Only one universe will ever exist and there is some underlying principle that constrains the CC to the value we observe.
+Only one universe will ever exist and although there is no underlying principle fixing the CC, we got lucky.
+Lots of universes exist (simultaneously or serially) with a range of CC values, and of course ours is one of the life-supporting ones.
+Other possible explanations for dark energy include quintessence or a modification of gravity on the largest scales. The effect on cosmology of the dark energy that these models describe is given by the dark energy's equation of state, which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.
+A better understanding of dark energy is likely to solve the problem of the ultimate fate of the universe. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than superclusters from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a Big Rip, or whether it will eventually reverse, lead to a Big Freeze, or follow some other scenario.

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+---
+title: "Physical cosmology"
+chunk: 5/5
+source: "https://en.wikipedia.org/wiki/Physical_cosmology"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:14:22.441984+00:00"
+instance: "kb-cron"
+---
+
+=== Gravitational waves ===
+Gravitational waves are ripples in the curvature of spacetime that propagate as waves at the speed of light, generated in certain gravitational interactions that propagate outward from their source. Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about sources of detectable gravitational waves such as binary star systems composed of white dwarfs, neutron stars, and black holes; and events such as supernovae, and the formation of the early universe shortly after the Big Bang.
+In 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had made the first observation of gravitational waves, originating from a pair of merging black holes using the Advanced LIGO detectors. On 15 June 2016, a second detection of gravitational waves from coalescing black holes was announced. Besides LIGO, many other gravitational-wave observatories (detectors) are under construction.
+
+=== Other areas of inquiry ===
+Cosmologists also study:
+
+Whether primordial black holes were formed in our universe, and what happened to them.
+Detection of cosmic rays with energies above the GZK cutoff, and whether it signals a failure of special relativity at high energies.
+The equivalence principle, whether or not Einstein's general theory of relativity is the correct theory of gravitation, and if the fundamental laws of physics are the same everywhere in the universe.
+Biophysical cosmology: a type of physical cosmology that studies and understands life as part or an inherent part of physical cosmology. It stresses that life is inherent to the universe and therefore frequent.
+
+== See also ==
+
+== References ==
+
+== Further reading ==
+
+=== Popular ===
+Greene, Brian (2005). The Fabric of the Cosmos. Penguin Books Ltd. ISBN 978-0-14-101111-0.
+Guth, Alan (1997). The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Random House. ISBN 978-0-224-04448-6.
+Hawking, Stephen W. (1988). A Brief History of Time: From the Big Bang to Black Holes. Bantam Books, Inc. ISBN 978-0-553-38016-3.
+Hawking, Stephen W. (2001). The Universe in a Nutshell. Bantam Books, Inc. ISBN 978-0-553-80202-3.
+Ostriker, Jeremiah P.; Mitton, Simon (2013). Heart of Darkness: Unraveling the mysteries of the invisible Universe. Princeton, NJ: Princeton University Press. ISBN 978-0-691-13430-7.
+Singh, Simon (2005). Big Bang: The Origin of the Universe. Fourth Estate. Bibcode:2004biba.book.....S. ISBN 978-0-00-716221-5.
+Weinberg, Steven (1993) [1978]. The First Three Minutes. Basic Books. ISBN 978-0-465-02437-7.
+
+=== Textbooks ===
+Cheng, Ta-Pei (2005). Relativity, Gravitation and Cosmology: a Basic Introduction. Oxford and New York: Oxford University Press. ISBN 978-0-19-852957-6. Introductory cosmology and general relativity without the full tensor apparatus, deferred until the last part of the book.
+Baumann, Daniel (2022). Cosmology. Cambridge: Cambridge University Press. ISBN 978-0-19-852957-6. Modern introduction to cosmology covering the homogeneous and inhomogeneous universe as well as inflation and the CMB.
+Dodelson, Scott (2003). Modern Cosmology. Academic Press. ISBN 978-0-12-219141-1. An introductory text, released slightly before the WMAP results.
+Gal-Or, Benjamin (1987) [1981]. Cosmology, Physics and Philosophy. Springer Verlag. ISBN 0-387-90581-2.
+Grøn, Øyvind; Hervik, Sigbjørn (2007). Einstein's General Theory of Relativity with Modern Applications in Cosmology. New York: Springer. ISBN 978-0-387-69199-2.
+Harrison, Edward (2000). Cosmology: the science of the universe. Cambridge University Press. ISBN 978-0-521-66148-5. For undergraduates; mathematically gentle with a strong historical focus.
+Kutner, Marc (2003). Astronomy: A Physical Perspective. Cambridge University Press. ISBN 978-0-521-52927-3. An introductory astronomy text.
+Kolb, Edward; Michael Turner (1988). The Early Universe. Addison-Wesley. ISBN 978-0-201-11604-5. The classic reference for researchers.
+Liddle, Andrew (2003). An Introduction to Modern Cosmology. John Wiley. ISBN 978-0-470-84835-7. Cosmology without general relativity.
+Liddle, Andrew; David Lyth (2000). Cosmological Inflation and Large-Scale Structure. Cambridge. ISBN 978-0-521-57598-0. An introduction to cosmology with a thorough discussion of inflation.
+Mukhanov, Viatcheslav (2005). Physical Foundations of Cosmology. Cambridge University Press. ISBN 978-0-521-56398-7.
+Padmanabhan, T. (1993). Structure formation in the universe. Cambridge University Press. ISBN 978-0-521-42486-8. Discusses the formation of large-scale structures in detail.
+Peacock, John (1998). Cosmological Physics. Cambridge University Press. ISBN 978-0-521-42270-3. An introduction including more on general relativity and quantum field theory than most.
+Peebles, P. J. E. (1993). Principles of Physical Cosmology. Princeton University Press. ISBN 978-0-691-01933-8. Strong historical focus.
+Peebles, P. J. E. (1980). The Large-Scale Structure of the Universe. Princeton University Press. ISBN 978-0-691-08240-0. The classic work on large-scale structure and correlation functions.
+Rees, Martin (2002). New Perspectives in Astrophysical Cosmology. Cambridge University Press. ISBN 978-0-521-64544-7.
+Weinberg, Steven (1971). Gravitation and Cosmology. John Wiley. ISBN 978-0-471-92567-5. A standard reference for the mathematical formalism.
+Weinberg, Steven (2008). Cosmology. Oxford University Press. ISBN 978-0-19-852682-7.
+
+== External links ==
+
+=== From groups ===
+Cambridge Cosmology – from Cambridge University (public home page)
+Cosmology 101 – from the NASA WMAP group
+Center for Cosmological Physics Archived 11 February 2011 at the Wayback Machine. University of Chicago, Chicago, Illinois
+Origins, Nova Online – Provided by PBS
+
+=== From individuals ===
+Gale, George, "Cosmology: Methodological Debates in the 1930s and 1940s", The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.)
+Madore, Barry F., "Level 5 : A Knowledgebase for Extragalactic Astronomy and Cosmology". Caltech and Carnegie. Pasadena, California.
+Tyler, Pat, and Newman, Phil, "Beyond Einstein Archived 29 April 2004 at the Wayback Machine". Laboratory for High Energy Astrophysics (LHEA) NASA Goddard Space Flight Center.
+Wright, Ned. "Cosmology tutorial and FAQ". Division of Astronomy & Astrophysics, UCLA.
+Musser, George (February 2004). "Four Keys to Cosmology". Scientific American. Retrieved 22 March 2015.
+Burgess, Cliff; Quevedo, Fernando (November 2007). "The Great Cosmic Roller-Coaster Ride". Scientific American (print). pp. 52–59. (subtitle) Could cosmic inflation be a sign that our universe is embedded in a far vaster realm?

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data/en.wikipedia.org/wiki/Relaxation_(physics)-0.md

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+---
+title: "Relaxation (physics)"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Relaxation_(physics)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:24.533010+00:00"
+instance: "kb-cron"
+---
+
+In the physical sciences, relaxation usually means the return of a perturbed system into equilibrium.
+Each relaxation process can be categorized by a relaxation time τ. The simplest theoretical description of relaxation as function of time t is an exponential law exp(−t/τ) (exponential decay).
+
+== In simple linear systems ==
+
+=== Mechanics: Damped unforced oscillator ===
+
+Let the homogeneous differential equation:
+
+  
+    
+      
+        m
+        
+          
+            
+              
+                d
+                
+                  2
+                
+              
+              y
+            
+            
+              d
+              
+                t
+                
+                  2
+                
+              
+            
+          
+        
+        +
+        γ
+        
+          
+            
+              d
+              y
+            
+            
+              d
+              t
+            
+          
+        
+        +
+        k
+        y
+        =
+        0
+      
+    
+    {\displaystyle m{\frac {d^{2}y}{dt^{2}}}+\gamma {\frac {dy}{dt}}+ky=0}
+  
+
+model damped unforced oscillations of a weight on a spring.
+The displacement will then be of the form 
+  
+    
+      
+        y
+        (
+        t
+        )
+        =
+        A
+        
+          e
+          
+            −
+            t
+            
+              /
+            
+            T
+          
+        
+        cos
+        ⁡
+        (
+        μ
+        t
+        −
+        δ
+        )
+      
+    
+    {\displaystyle y(t)=Ae^{-t/T}\cos(\mu t-\delta )}
+  
+. The constant T (
+  
+    
+      
+        =
+        2
+        m
+        
+          /
+        
+        γ
+      
+    
+    {\displaystyle =2m/\gamma }
+  
+) is called the relaxation time of the system and the constant μ is the quasi-frequency.
+
+=== Electronics: RC circuit ===
+In an RC circuit containing a charged capacitor and a resistor, the voltage decays exponentially:
+
+  
+    
+      
+        V
+        (
+        t
+        )
+        =
+        
+          V
+          
+            0
+          
+        
+        
+          e
+          
+            −
+            
+              
+                t
+                
+                  R
+                  C
+                
+              
+            
+          
+        
+         
+        ,
+      
+    
+    {\displaystyle V(t)=V_{0}e^{-{\frac {t}{RC}}}\ ,}
+  
+
+The constant 
+  
+    
+      
+        τ
+        =
+        R
+        C
+         
+      
+    
+    {\displaystyle \tau =RC\ }
+  
+ is called the relaxation time or RC time constant of the circuit. A nonlinear oscillator circuit which generates a repeating waveform by the repetitive discharge of a capacitor through a resistance is called a relaxation oscillator.
+
+== In condensed matter physics ==
+In condensed matter physics, relaxation is usually studied as a linear response to a small external perturbation. Since the underlying microscopic processes are active even in the absence of external perturbations, one can also study "relaxation in equilibrium" instead of the usual "relaxation into equilibrium" (see fluctuation-dissipation theorem).
+
+=== Stress relaxation ===
+In continuum mechanics, stress relaxation is the gradual disappearance of stresses from a viscoelastic medium after it has been deformed.
+
+=== Dielectric relaxation time ===
+In dielectric materials, the dielectric polarization P depends on the electric field E. If E changes, P(t) reacts: the polarization relaxes towards a new equilibrium, i.e., the surface charges equalize. It is important in dielectric spectroscopy. Very long relaxation times are responsible for dielectric absorption.
+The dielectric relaxation time is closely related to the electrical conductivity. In a semiconductor it is a measure of how long it takes to become neutralized by conduction process. This relaxation time is small in metals and can be large in semiconductors and insulators.
+
+=== Liquids and amorphous solids ===
+An amorphous solid such as amorphous indomethacin displays a temperature dependence of molecular motion, which can be quantified as the average relaxation time for the solid in a metastable supercooled liquid or glass to approach the molecular motion characteristic of a crystal. Differential scanning calorimetry can be used to quantify enthalpy change due to molecular structural relaxation.
+The term "structural relaxation" was introduced in the scientific literature in 1947/48 without any explanation, applied to NMR, and meaning the same as "thermal relaxation".
+
+=== Spin relaxation in NMR ===
+
+In nuclear magnetic resonance (NMR), various relaxations are the properties that it measures.
+
+== Chemical relaxation methods ==
+
+In chemical kinetics, relaxation methods are used for the measurement of very fast reaction rates. A system initially at equilibrium is perturbed by a rapid change in a parameter such as the temperature (most commonly), the pressure, the electric field or the pH of the solvent. The return to equilibrium is then observed, usually by spectroscopic means, and the relaxation time measured. In combination with the chemical equilibrium constant of the system, this enables the determination of the rate constants for the forward and reverse reactions.
+
+=== Monomolecular first-order reversible reaction ===
+A monomolecular, first order reversible reaction which is close to equilibrium can be visualized by the following symbolic structure:
+
+  
+    
+      
+        
+          A
+        
+         
+        
+          
+            →
+            k
+          
+        
+         
+        
+          B
+        
+         
+        
+          
+            →
+            
+              k
+              ′
+            
+          
+        
+         
+        
+          A
+        
+      
+    
+    {\displaystyle {\ce {A}}~{\overset {k}{\rightarrow }}~{\ce {B}}~{\overset {k'}{\rightarrow }}~{\ce {A}}}
+  
+
+  
+    
+      
+        
+          A
+          
+            
+              
+                
+                  
+                    
+                      ↽
+                    
+                    
+                    
+                    
+                      −
+                    
+                  
+                
+              
+              
+                
+                  
+                    
+                      −
+                    
+                    
+                    
+                    
+                      ⇀
+                    
+                  
+                
+              
+            
+          
+          B
+        
+      
+    
+    {\displaystyle {\ce {A <=> B}}}
+  
+
+In other words, reactant A and product B are forming into one another based on reaction rate constants k and k'.
+To solve for the concentration of A, recognize that the forward reaction (
+  
+    
+      
+        
+          A
+          
+            
+              →
+              
+                
+                  k
+                
+              
+            
+          
+          B
+        
+      
+    
+    {\displaystyle {\ce {A ->[{k}] B}}}
+  
+) causes the concentration of A to decrease over time, whereas the reverse reaction (
+  
+    
+      
+        
+          B
+          
+            
+              →
+              
+                
+                  
+                    k
+                    ′
+                  
+                
+              
+            
+          
+          A
+        
+      
+    
+    {\displaystyle {\ce {B ->[{k'}] A}}}
+  
+) causes the concentration of A to increase over time.
+Therefore, 
+  
+    
+      
+        
+          
+            
+              d
+              
+                
+                  [
+                  A
+                  ]
+                
+              
+            
+            
+              d
+              t
+            
+          
+        
+        =
+        −
+        k
+        
+          
+            [
+            A
+            ]
+          
+        
+        +
+        
+          k
+          ′
+        
+        
+          
+            [
+            B
+            ]
+          
+        
+      
+    
+    {\displaystyle {d{\ce {[A]}} \over dt}=-k{\ce {[A]}}+k'{\ce {[B]}}}
+  
+, where brackets around A and B indicate concentrations.
+If we say that at 
+  
+    
+      
+        t
+        =
+        0
+        ,
+        
+          
+            [
+            A
+            ]
+          
+        
+        (
+        t
+        )
+        =
+        
+          
+            
+              [
+              A
+              ]
+            
+          
+          
+            0
+          
+        
+      
+    
+    {\displaystyle t=0,{\ce {[A]}}(t)={\ce {[A]}}_{0}}
+  
+, and applying the law of conservation of mass, we can say that at any time, the sum of the concentrations of A and B must be equal to the concentration of 
+  
+    
+      
+        
+          A
+          
+            0
+          
+        
+      
+    
+    {\displaystyle A_{0}}
+  
+, assuming the volume into which A and B are dissolved does not change:
+
+  
+    
+      
+        
+          
+            [
+            A
+            ]
+          
+        
+        +
+        
+          
+            [
+            B
+            ]
+          
+        
+        =
+        
+          
+            
+              [
+              A
+              ]
+            
+          
+          
+            0
+          
+        
+        ⇒
+        
+          
+            [
+            B
+            ]
+          
+        
+        =
+        
+          
+            
+              [
+              A
+              ]
+            
+          
+          
+            0
+          
+        
+        −
+        
+          
+            [
+            A
+            ]
+          
+        
+      
+    
+    {\displaystyle {\ce {[A]}}+{\ce {[B]}}={\ce {[A]}}_{0}\Rightarrow {\ce {[B]}}={\ce {[A]}}_{0}-{\ce {[A]}}}
+  
+
+Substituting this value for [B] in terms of [A]0 and [A](t) yields
+
+  
+    
+      
+        
+          
+            
+              d
+              
+                
+                  [
+                  A
+                  ]
+                
+              
+            
+            
+              d
+              t
+            
+          
+        
+        =
+        −
+        k
+        
+          
+            [
+            A
+            ]
+          
+        
+        +
+        
+          k
+          ′
+        
+        
+          
+            [
+            B
+            ]
+          
+        
+        =
+        −
+        k
+        
+          
+            [
+            A
+            ]
+          
+        
+        +
+        
+          k
+          ′
+        
+        (
+        
+          
+            
+              [
+              A
+              ]
+            
+          
+          
+            0
+          
+        
+        −
+        
+          
+            [
+            A
+            ]
+          
+        
+        )
+        =
+        −
+        (
+        k
+        +
+        
+          k
+          ′
+        
+        )
+        
+          
+            [
+            A
+            ]
+          
+        
+        +
+        
+          k
+          ′
+        
+        
+          
+            
+              [
+              A
+              ]
+            
+          
+          
+            0
+          
+        
+        ,
+      
+    
+    {\displaystyle {d{\ce {[A]}} \over dt}=-k{\ce {[A]}}+k'{\ce {[B]}}=-k{\ce {[A]}}+k'({\ce {[A]}}_{0}-{\ce {[A]}})=-(k+k'){\ce {[A]}}+k'{\ce {[A]}}_{0},}
+  
+ which becomes the separable differential equation 
+  
+    
+      
+        
+          
+            
+              d
+              
+                
+                  [
+                  A
+                  ]
+                
+              
+            
+            
+              −
+              (
+              k
+              +
+              
+                k
+                ′
+              
+              )
+              
+                
+                  [
+                  A
+                  ]
+                
+              
+              +
+              
+                k
+                ′
+              
+              
+                
+                  
+                    [
+                    A
+                    ]
+                  
+                
+                
+                  0
+                
+              
+            
+          
+        
+        =
+        d
+        t
+      
+    
+    {\displaystyle {\frac {d{\ce {[A]}}}{-(k+k'){\ce {[A]}}+k'{\ce {[A]}}_{0}}}=dt}
+  

data/en.wikipedia.org/wiki/Relaxation_(physics)-1.md

@@ -0,0 +1,294 @@
+---
+title: "Relaxation (physics)"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Relaxation_(physics)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:24.533010+00:00"
+instance: "kb-cron"
+---
+
+This equation can be solved by substitution to yield 
+  
+    
+      
+        
+          
+            [
+            A
+            ]
+          
+        
+        =
+        
+          
+            
+              
+                k
+                ′
+              
+              −
+              k
+              
+                e
+                
+                  −
+                  (
+                  k
+                  +
+                  
+                    k
+                    ′
+                  
+                  )
+                  t
+                
+              
+            
+            
+              k
+              +
+              
+                k
+                ′
+              
+            
+          
+        
+        
+          
+            
+              [
+              A
+              ]
+            
+          
+          
+            0
+          
+        
+      
+    
+    {\displaystyle {\ce {[A]}}={k'-ke^{-(k+k')t} \over k+k'}{\ce {[A]}}_{0}}
+  
+
+== In atmospheric sciences ==
+
+=== Desaturation of clouds ===
+Consider a supersaturated portion of a cloud. Then shut off the updrafts, entrainment, and any other vapor sources/sinks and things that would induce the growth of the particles (ice or water). Then wait for this supersaturation to reduce and become just saturation (relative humidity = 100%), which is the equilibrium state. The time it takes for the supersaturation to dissipate is called relaxation time. It will happen as ice crystals or liquid water content grow within the cloud and will thus consume the contained moisture. The dynamics of relaxation are very important in cloud physics for accurate mathematical modelling.
+In water clouds where the concentrations are larger (hundreds per cm3) and the temperatures are warmer (thus allowing for much lower supersaturation rates as compared to ice clouds), the relaxation times will be very low (seconds to minutes).
+In ice clouds the concentrations are lower (just a few per liter) and the temperatures are colder (very high supersaturation rates) and so the relaxation times can be as long as several hours. Relaxation time is given as
+
+ where:
+D = diffusion coefficient [m2/s]
+N = concentration (of ice crystals or water droplets) [m−3]
+R = mean radius of particles [m]
+K = capacitance [unitless].
+
+== In astronomy ==
+In astronomy, relaxation time relates to clusters of gravitationally interacting bodies, for instance, stars in a galaxy. The relaxation time is a measure of the time it takes for one object in the system (the "test star") to be significantly perturbed by other objects in the system (the "field stars"). It is most commonly defined as the time for the test star's velocity to change by of order itself.
+Suppose that the test star has velocity v. As the star moves along its orbit, its motion will be randomly perturbed by the gravitational field of nearby stars. The relaxation time can be shown to be
+
+  
+    
+      
+        
+          
+            
+              
+                
+                  T
+                  
+                    r
+                  
+                
+              
+              
+                
+                =
+                
+                  
+                    
+                      0.34
+                      
+                        σ
+                        
+                          3
+                        
+                      
+                    
+                    
+                      
+                        G
+                        
+                          2
+                        
+                      
+                      m
+                      ρ
+                      ln
+                      ⁡
+                      Λ
+                    
+                  
+                
+              
+            
+            
+              
+              
+                
+                ≈
+                0.95
+                ×
+                
+                  10
+                  
+                    10
+                  
+                
+                
+                
+                  
+                    (
+                    
+                      
+                        σ
+                        
+                          200
+                          
+                          
+                            
+                              k
+                              m
+                              
+                              s
+                            
+                            
+                              −
+                              1
+                            
+                          
+                        
+                      
+                    
+                    )
+                  
+                  
+                    
+                    3
+                  
+                
+                
+                
+                
+                  
+                    (
+                    
+                      
+                        ρ
+                        
+                          
+                            10
+                            
+                              6
+                            
+                          
+                          
+                          
+                            M
+                            
+                              ⊙
+                            
+                          
+                          
+                          
+                            
+                              p
+                              c
+                            
+                            
+                              −
+                              3
+                            
+                          
+                        
+                      
+                    
+                    )
+                  
+                  
+                    
+                    −
+                    1
+                  
+                
+                
+                
+                
+                  
+                    (
+                    
+                      
+                        m
+                        
+                          M
+                          
+                            ⊙
+                          
+                        
+                      
+                    
+                    )
+                  
+                  
+                    
+                    −
+                    1
+                  
+                
+                
+                
+                
+                  
+                    (
+                    
+                      
+                        
+                          ln
+                          ⁡
+                          Λ
+                        
+                        15
+                      
+                    
+                    )
+                  
+                  
+                    
+                    −
+                    1
+                  
+                
+                
+                
+                  y
+                  r
+                
+              
+            
+          
+        
+      
+    
+    {\displaystyle {\begin{aligned}T_{r}&={0.34\sigma ^{3} \over G^{2}m\rho \ln \Lambda }\\&\approx 0.95\times 10^{10}\!\left({\sigma  \over 200\,\mathrm {km\,s} ^{-1}}\right)^{\!3}\!\!\left({\rho  \over 10^{6}\,M_{\odot }\,\mathrm {pc} ^{-3}}\right)^{\!-1}\!\!\left({m \over M_{\odot }}\right)^{\!-1}\!\!\left({\ln \Lambda  \over 15}\right)^{\!-1}\!\mathrm {yr} \end{aligned}}}
+  
+
+where ρ is the mean density, m is the test-star mass, σ is the 1d velocity dispersion of the field stars, and ln Λ is the Coulomb logarithm.
+Various events occur on timescales relating to the relaxation time, including core collapse, energy equipartition, and formation of a Bahcall-Wolf cusp around a supermassive black hole.
+
+== See also ==
+Relaxation oscillator
+Time constant
+
+== References ==

data/en.wikipedia.org/wiki/Rotation_period_(astronomy)-0.md

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+---
+title: "Rotation period (astronomy)"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Rotation_period_(astronomy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:25.739752+00:00"
+instance: "kb-cron"
+---
+
+In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the sidereal rotation period (or sidereal day), i.e., the time that the object takes to complete a full rotation around its axis relative to the background stars (inertial space). The other type of commonly used "rotation period" is the object's synodic rotation period (or solar day), which may differ, by a fraction of a rotation or more than one rotation, to accommodate the portion of the object's orbital period around a star or another body during one day.
+
+
+== Measuring rotation ==
+For solid objects, such as rocky planets and asteroids, the rotation period is a single value. For gaseous or fluid bodies, such as stars and giant planets, the period of rotation varies from the object's equator to its pole due to a phenomenon called differential rotation. Typically, the stated rotation period for a giant planet (such as Jupiter, Saturn, Uranus, Neptune) is its internal rotation period, as determined from the rotation of the planet's magnetic field. For objects that are not spherically symmetrical, the rotation period is, in general, not fixed, even in the absence of gravitational or tidal forces. This is because, although the rotation axis is fixed in space (by the conservation of angular momentum), it is not necessarily fixed in the body of the object itself. As a result of this, the moment of inertia of the object around the rotation axis can vary, and hence the rate of rotation can vary (because the product of the moment of inertia and the rate of rotation is equal to the angular momentum, which is fixed).  For example, Hyperion, a moon of Saturn, exhibits this behaviour, and its rotation period is described as chaotic.
+
+
+== Rotation period of selected objects ==
+
+Orbital velocity values are mean orbital speeds along each planet's orbit around the Sun.
+
+
+== See also ==
+
+
+== Notes ==
+
+
+== References ==
+
+
+== External links ==
+Murray, Carl D. & Dermott, Stanley F. (1999). Solar System Dynamics. Cambridge University Press. p. 531. ISBN 0-521-57295-9.Note, the rotation periods for Mercury and Earth in this work may be inaccurate.

data/en.wikipedia.org/wiki/Saros_(astronomy)-0.md

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+---
+title: "Saros (astronomy)"
+chunk: 1/3
+source: "https://en.wikipedia.org/wiki/Saros_(astronomy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:27.065142+00:00"
+instance: "kb-cron"
+---
+
+In astronomy, the saros ( ) is a length of time covering exactly 223 synodic months (18 years 11 days and 8 hours), 242 draconic months and 239 anomalistic months. Arising naturally due to synchronization between lunar phase, nodal precession, and apsidal precession, it can be used to predict eclipses of the Sun and Moon. One saros period after an eclipse, the Sun, Earth, and Moon return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur, in what is referred to as an eclipse cycle. Every eclipse has an associated saros series and all succeeding or preceding eclipses have a different saros series associated with them - as the eclipse of the same series occurs or occurred with a gap of one saros only. Solar and lunar eclipses have different saros series.
+A series of eclipses that are separated by one saros is called a saros series. It corresponds to:
+
+6,585.321347 solar days
+18.029 years
+223 synodic months
+241.999 draconic months
+18.999 eclipse years (38 eclipse seasons of 173.31 days)
+238.992 anomalistic months
+241.029 sidereal months
+The 19 eclipse years means that if there is a solar eclipse (or lunar eclipse), then after one saros a new moon will take place at the same node of the orbit of the Moon, and under these circumstances another eclipse can occur.
+
+== History ==
+The earliest discovered historical record of what is known as the saros is by Chaldean (neo-Babylonian) astronomers in the last several centuries BCE. It was later known to Hipparchus, Pliny and Ptolemy.
+The name "saros" (Greek: σάρος) was applied to the eclipse cycle by Edmond Halley in 1686, who took it from the Suda, a Byzantine lexicon of the 11th century. The Suda says, "[The saros is] a measure and a number among Chaldeans. For 120 saroi make 2220 years (years of 12 lunar months) according to the Chaldeans' reckoning, if indeed the saros makes 222 lunar months, which are 18 years and 6 months (i.e. years of 12 lunar months)." The information in the Suda in turn was derived directly or otherwise from the Chronicle of Eusebius of Caesarea, which quoted Berossus. (Guillaume Le Gentil claimed that Halley's usage was incorrect in 1756, but the name continues to be used.) The Greek word apparently either comes from the Babylonian word sāru meaning the number 3600 or the Greek verb saro (σαρῶ) that means "sweep (the sky with the series of eclipses)".
+
+The Saros period of 223 lunar months (in Greek numerals, ΣΚΓ′) is in the Antikythera Mechanism  user manual on this instrument, made around 150–100 BCE in Greece, as seen in the picture. This number is one of a few inscriptions of the mechanism that are visible with the unaided eye. Above it, the period of the Metonic cycle and the Callippic cycle are also visible.
+
+== Description ==
+
+  
+    
+      
+        
+          
+            
+              
+                1
+                
+                   saros
+                
+              
+              
+                
+                =
+                6585.3211
+                
+                   days
+                
+              
+            
+            
+              
+              
+                
+                =
+                15
+                
+                   common years
+                
+                +
+                3
+                
+                   leap years
+                
+                +
+                12.321
+                
+                   days
+                
+              
+            
+            
+              
+              
+                
+                =
+                14
+                
+                   common years
+                
+                +
+                4
+                
+                   leap years
+                
+                +
+                11.321
+                
+                   days
+                
+              
+            
+            
+              
+              
+                
+                =
+                13
+                
+                   common years
+                
+                +
+                5
+                
+                   leap years
+                
+                +
+                10.321
+                
+                   days
+                
+              
+            
+          
+        
+      
+    
+    {\displaystyle {\begin{aligned}1{\text{ saros}}&=6585.3211{\text{ days}}\\&=15{\text{ common years}}+3{\text{ leap years}}+12.321{\text{ days}}\\&=14{\text{ common years}}+4{\text{ leap years}}+11.321{\text{ days}}\\&=13{\text{ common years}}+5{\text{ leap years}}+10.321{\text{ days}}\end{aligned}}}
+  
+
+The saros, a period of 6585.3211 days, is useful for predicting the times at which nearly identical eclipses will occur. Three periodicities related to lunar orbit, the synodic month, the draconic month, and the anomalistic month coincide almost perfectly each saros cycle. For an eclipse to occur, either the Moon must be located between the Earth and Sun (for a solar eclipse) or the Earth must be located between the Sun and Moon (for a lunar eclipse). This can happen only when the Moon is new or full, respectively, and repeat occurrences of these lunar phases result from solar and lunar orbits producing the Moon's synodic period of 29.53059 days. During most full and new moons, however, the shadow of the Earth or Moon falls to the north or south of the other body. Eclipses occur when the three bodies form a nearly straight line. Because the plane of the lunar orbit is inclined to that of the Earth, this condition occurs only when a full or new Moon is near or in the ecliptic plane, that is when the Moon is at one of the two nodes (the ascending or descending node). The period of time for two successive lunar passes through the ecliptic plane (returning to the same node) is termed the draconic month, a 27.21222 day period. The three-dimensional geometry of an eclipse, when the new or full moon is near one of the nodes, occurs every five or six months when the Sun is in conjunction or opposition to the Moon and coincidentally also near a node of the Moon's orbit at that time, or twice per eclipse year. Two eclipses separated by one saros have very similar appearance and duration due to the distance between the Earth and Moon being nearly the same for each event: this is because the saros is also an integer multiple of the anomalistic month of 27.5545 days, the period of the moon with respect to the lines of apsides in its orbit.

data/en.wikipedia.org/wiki/Saros_(astronomy)-1.md

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+---
+title: "Saros (astronomy)"
+chunk: 2/3
+source: "https://en.wikipedia.org/wiki/Saros_(astronomy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:27.065142+00:00"
+instance: "kb-cron"
+---
+
+After one saros, the Moon will have completed roughly an integer number of synodic, draconic, and anomalistic periods (223, 242, and 239) and the Earth-Sun-Moon geometry will be nearly identical: the Moon will have the same phase and be at the same node and the same distance from the Earth. In addition, because the saros is close to 18 years in length (about 11 days longer), the Earth will be nearly the same distance from the Sun, and tilted to it in nearly the same orientation (same season). Given the date of an eclipse, one saros later a nearly identical eclipse can be predicted. During this 18-year period, about 40 other solar and lunar eclipses take place, but with a somewhat different geometry. One saros equaling 18.03 years is not equal to a perfect integer number of lunar orbits (Earth revolutions with respect to the fixed stars of 27.32166 days sidereal month), therefore, even though the relative geometry of the Earth–Sun–Moon system will be nearly identical after a saros, the Moon will be in a slightly different position with respect to the stars for each eclipse in a saros series. The axis of rotation of the Earth–Moon system exhibits a precession period of 18.59992 years.
+The saros is not an integer number of days, but contains the fraction of +1⁄3 of a day. Thus each successive eclipse in a saros series occurs about eight hours later in the day. In the case of an eclipse of the Sun, this means that the region of visibility will shift westward about 120°, or about one third of the way around the globe, and the two eclipses will thus not be visible from the same place on Earth. In the case of an eclipse of the Moon, the next eclipse might still be visible from the same location as long as the Moon is above the horizon. Given three saros eclipse intervals, the local time of day of an eclipse will be nearly the same. This three saros interval (19,755.96 days) is known as a triple saros or exeligmos (Greek: "turn of the wheel") cycle.
+
+== Saros series ==
+
+Each solar saros series starts with a partial eclipse (Sun first enters the end of the node, when new Moon occurs east of a node) - which means the first eclipse of the saros series occurs within about 15° to 18° of the node before Earth reaches the node and the moon's elongation is also within the gamma range for an eclipse to occur. In each successive saros, the path of the Moon is shifted either northward (when near the descending node) or southward (when near the ascending node) due to the fact that the saros is not an exact integer of draconic months (223 synodic months of a saros is about one hour short of 242 draconic months - hence, the Moon's node shifts eastward by about 0.5º with each cycle). At some point, eclipses are no longer possible and the series terminates (Sun leaves the beginning of the node). An arbitrary solar saros series was designated as solar saros series 1 by compilers of eclipse statistics. This series has finished, but the eclipse of November 16, 1990 BC (Julian calendar) for example is in solar saros series 1. There are different saros series for solar and lunar eclipses. For lunar saros series, the lunar eclipse occurring 58.5 synodic months earlier (February 23, 1994 BC) was assigned the number 1. If there is an eclipse one inex (29 years minus about 20 days) after an eclipse of a particular saros series then it is a member of the next series. For example, the eclipse of October 26, 1961 BC is in solar saros series 2. Saros series, of course, went on before these dates, and it is necessary to extend the saros series numbers backwards to negative numbers even just to accommodate eclipses occurring in the years following 2000 BC (up till the last eclipse with a negative saros number in 1367 BC). It takes between 1226 and 1550 years for the members of a saros series to traverse the Earth's surface from north to south (or vice versa). These extremes allow from 69 to 87 eclipses in each series (most series have 71 or 72 eclipses). From 39 to 59 (mostly about 43) eclipses in a given series will be central (that is, total, annular, or hybrid). At any given time, approximately 40 different saros series will be in progress.
+Saros series, as mentioned, are numbered according to the type of eclipse (lunar or solar). In odd numbered series (for solar eclipses) the Sun is near the ascending node, whereas in even numbered series it is near the descending node (this is reversed for lunar eclipse saros series). Generally, the ordering of these series determines the time at which each series peaks, which corresponds to when an eclipse is closest to one of the lunar nodes. For solar eclipses, the 40 series numbered between 117 and 156 are active (series 117 will end in 2054), whereas for lunar eclipses, there are now 41 active saros series numbered between 110 and 150 (series 110 will end in 2027). These numbers can be derived by counting the number of eclipses listed over an 18-year (saros) period from the eclipse catalog sites.
+
+=== Example ===

data/en.wikipedia.org/wiki/Saros_(astronomy)-2.md

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+---
+title: "Saros (astronomy)"
+chunk: 3/3
+source: "https://en.wikipedia.org/wiki/Saros_(astronomy)"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:27.065142+00:00"
+instance: "kb-cron"
+---
+
+As an example of a single saros series, this table gives the dates of some of the 72 lunar eclipses for saros series 131. This eclipse series began in AD 1427 with a partial eclipse at the southern edge of the Earth's shadow when the Moon was close to its descending node. In each successive saros, the Moon's orbital path is shifted northward with respect to the Earth's shadow, with the first total eclipse occurring in 1950. For the following 252 years, total eclipses occur, with the central eclipse in 2078. The first partial eclipse after this will occur in the year 2220, and the final partial eclipse of the series will occur in 2707. The total lifetime of lunar saros series 131 is 1280 years. Solar saros 138 interleaves with this lunar saros with an event occurring every 9 years 5 days alternating between each saros series.
+Because of the +1⁄3 fraction of days in a saros, the visibility of each eclipse will differ for an observer at a given locale. For the lunar saros series 131, the first total eclipse of 1950 had its best visibility for viewers in Eastern Europe and the Middle East because mid-eclipse was at 20:44 UT. The following eclipse in the series occurred about 8 hours later in the day with mid-eclipse at 4:47 UT, and was best seen from North America and South America. The third total eclipse occurred about 8 hours later in the day than the second eclipse with mid-eclipse at 12:43 UT, and had its best visibility for viewers in the Western Pacific, East Asia, Australia and New Zealand. This cycle of visibility repeats from the start to the end of the series, with minor variations. Solar saros 138 interleaves with this lunar saros with an event occurring every 9 years 5 days alternating between each saros series.
+For a similar example for solar saros see solar saros 136.
+
+== Relationship between lunar and solar saros (sar) ==
+After a given lunar or solar eclipse, after 9 years and 5+1⁄2 days (a half saros, or sar) an eclipse will occur that is lunar instead of solar, or vice versa, with similar properties.
+For example, if the Moon's penumbra partially covers the southern limb of the Earth during a solar eclipse, 9 years and 5+1⁄2 days later a lunar eclipse will occur in which the Moon is partially covered by the southern limb of the Earth's penumbra. Likewise, 9 years and 5+1⁄2 days after a total solar eclipse or an annular solar eclipse occurs, a total lunar eclipse will also occur. This 9-year period is referred to as a sar. It includes 111+1⁄2 synodic months, or 111 synodic months plus one fortnight. The fortnight accounts for the alternation between solar and lunar eclipse. For a visual example see this chart (each row is one sar apart).
+
+== See also ==
+List of saros series for solar eclipses
+List of saros series for lunar eclipses
+Eclipse cycle
+Exeligmos
+Inex
+Solar eclipse
+Lunar eclipse
+Metonic cycle
+
+== Notes ==
+
+== References ==
+
+== Bibliography ==
+Jean Meeus and Hermann Mucke (1983) Canon of Lunar Eclipses. Astronomisches Büro, Vienna
+Theodor von Oppolzer (1887). Canon der Finsternisse. Vienna
+Jean Meeus, Mathematical Astronomy Morsels, Willmann-Bell, Inc., 1997 (Chapter 9, p. 51, Table 9. A Some eclipse Periodicities)
+
+== External links ==
+
+List of all active saros cycles
+NASA – Eclipses and the Saros
+Solar and Lunar Eclipses – Xabier Jubier – Interactive eclipse search
+Eclipse Search – Search 5,000 years of eclipse data by various attributes
+Eclipses, Cosmic Clockwork of the Ancients – Fundamental astronomy of eclipses

data/en.wikipedia.org/wiki/Sidereal_time-0.md

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+---
+title: "Sidereal time"
+chunk: 1/3
+source: "https://en.wikipedia.org/wiki/Sidereal_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:28.221801+00:00"
+instance: "kb-cron"
+---
+
+Sidereal time ("sidereal" pronounced  sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars". A sidereal day (also known as the sidereal rotation period) represents the time for one rotation about the planet axis relative to the stars.
+Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same time of day (or night), if the day is defined as a sidereal day. This is similar to how the time kept by a sundial (Solar time) can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis: solar time is reckoned according to the position of the Sun in the sky while sidereal time is based approximately on the position of the fixed stars on the theoretical celestial sphere.
+More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox (the northern hemisphere's vernal equinox) and both celestial poles, and is usually expressed in hours, minutes, and seconds. In the context of sidereal time, "March equinox" or "equinox" or "first point of Aries" is currently the direction from the center of the Earth along the line formed by the intersection of the Earth's equator and the ecliptic, toward the constellation Pisces; during ancient times it was toward the constellation Aries. Common time on a typical clock (using mean Solar time) measures a slightly longer cycle, affected not only by Earth's axial rotation but also by Earth's orbit around the Sun.
+The March equinox itself precesses slowly westward relative to the fixed stars, completing one revolution in about 25,800 years, so the misnamed "sidereal" day ("sidereal" is derived from the Latin sidus meaning "star") is 0.0084 seconds shorter than the stellar day, Earth's actual period of rotation relative to the fixed stars. The slightly longer stellar period is measured as the Earth rotation angle (ERA), formerly the stellar angle. An increase of 360° in the ERA is a full rotation of the Earth.
+A sidereal day on Earth is approximately 86164.0905 seconds (23 h 56 min 4.0905 s or 23.9344696 h). (Seconds are defined as per International System of Units and are not to be confused with ephemeris seconds.) Each day, the sidereal time at any given place and time will be about four minutes shorter than local civil time (which is based on solar time), so that for a complete year the number of sidereal "days" is one more than the number of solar days.
+
+== Comparison to solar time ==
+
+Solar time is measured by the apparent diurnal motion of the Sun. Local noon in apparent solar time is the moment when the Sun is exactly due south or north (depending on the observer's latitude and the season). A mean solar day (what we normally measure as a "day") is the average time between local solar noons ("average" since this varies slightly over a year).
+Earth makes one rotation around its axis each sidereal day; during that time it moves a short distance (about 1°) along its orbit around the Sun. So after a sidereal day has passed, Earth still needs to rotate slightly more before the Sun reaches local noon according to solar time. A mean solar day is, therefore, nearly 4 minutes longer than a sidereal day.
+The stars are so far away that Earth's movement along its orbit makes nearly no difference to their apparent direction (except for the nearest stars if measured with extreme accuracy; see parallax), and so they return to their highest point at the same time each sidereal day.
+Another way to understand this difference is to notice that, relative to the stars, as viewed from Earth, the position of the Sun at the same time each day appears to move around Earth once per year. A year has about 365.24 solar days but 366.24 sidereal days. Therefore, there is one fewer solar day per year than there are sidereal days, similar to an observation of the coin rotation paradox.

data/en.wikipedia.org/wiki/Sidereal_time-1.md

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+---
+title: "Sidereal time"
+chunk: 2/3
+source: "https://en.wikipedia.org/wiki/Sidereal_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:28.221801+00:00"
+instance: "kb-cron"
+---
+
+== Effects of precession ==
+Earth's rotation is not a simple rotation around an axis that remains always parallel to itself. Earth's rotational axis itself rotates about a second axis, orthogonal to the plane of Earth's orbit, taking about 25,800 years to perform a complete rotation. This phenomenon is termed the precession of the equinoxes. Because of this precession, the stars appear to move around Earth in a manner more complicated than a simple constant rotation.
+For this reason, to simplify the description of Earth's orientation in astronomy and geodesy, it was conventional to chart the positions of the stars in the sky according to right ascension and declination, which are based on a frame of reference that follows Earth's precession, and to keep track of Earth's rotation, through sidereal time, relative to this frame as well. (The conventional reference frame, for purposes of star catalogues, was replaced in 1998 with the International Celestial Reference Frame, which is fixed with respect to extra-galactic radio sources. Because of the great distances, these sources have no appreciable proper motion.) In this frame of reference, Earth's rotation is close to constant, but the stars appear to rotate slowly with a period of about 25,800 years. It is also in this frame of reference that the tropical year (or solar year), the year related to Earth's seasons, represents one orbit of Earth around the Sun. The precise definition of a sidereal day is the time taken for one rotation of Earth in this precessing frame of reference.
+
+== Modern definitions ==
+During the past, time was measured by observing stars with instruments such as photographic zenith tubes and Danjon astrolabes, and the passage of stars across defined lines would be timed with the observatory clock. Then, using the right ascension of the stars from a star catalog, the time when the star should have passed through the meridian of the observatory was computed, and a correction to the time kept by the observatory clock was computed. Sidereal time was defined such that the March equinox would transit the meridian of the observatory at 0 hours local sidereal time.
+Beginning during the 1970s, the radio astronomy methods very-long-baseline interferometry (VLBI) and pulsar timing overtook optical instruments for the most precise astrometry. This resulted in the determination of UT1 (mean solar time at 0° longitude) using VLBI, a new measure of the Earth Rotation Angle, and new definitions of sidereal time. These changes became effective 1 January 2003.
+
+=== Earth rotation angle ===
+The Earth rotation angle (ERA) measures the rotation of the Earth from an origin on the celestial equator, the Celestial Intermediate Origin, also termed the Celestial Ephemeris Origin, that has no instantaneous motion along the equator; it was originally referred to as the non-rotating origin. This point is very close to the equinox of J2000.
+ERA, measured in radians, is related to UT1 by a simple linear relation:
+  
+    
+      
+        θ
+        (
+        
+          t
+          
+            U
+          
+        
+        )
+        =
+        2
+        π
+        (
+        0.779
+        
+        057
+        
+        273
+        
+        2640
+        +
+        1.002
+        
+        737
+        
+        811
+        
+        911
+        
+        354
+        
+        48
+        ⋅
+        
+          t
+          
+            U
+          
+        
+        )
+      
+    
+    {\displaystyle \theta (t_{U})=2\pi (0.779\,057\,273\,2640+1.002\,737\,811\,911\,354\,48\cdot t_{U})}
+  
+where tU is the Julian UT1 date (JD) minus 2451545.0.
+The linear coefficient represents the Earth's rotation speed around its own axis.
+ERA replaces Greenwich Apparent Sidereal Time (GAST). The origin on the celestial equator for GAST, termed the true equinox, does move, due to the movement of the equator and the ecliptic. The lack of motion of the origin of ERA is considered a significant advantage.
+The ERA may be converted to other units; for example, the Astronomical Almanac for the Year 2017 tabulated it in degrees, minutes, and seconds.
+As an example, the Astronomical Almanac for the Year 2017 gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″. Since Coordinated Universal Time (UTC) is within a second or two of UT1, this can be used as an anchor to give the ERA approximately for a given civil time and date.
+
+=== Mean and apparent varieties ===
+
+Although ERA is intended to replace sidereal time, there is a need to maintain definitions for sidereal time during the transition, and when working with older data and documents.
+Similarly to mean solar time, every location on Earth has its own local sidereal time (LST), depending on the longitude of the point. Since it is not feasible to publish tables for every longitude, astronomical tables use Greenwich sidereal time (GST), which is sidereal time on the IERS Reference Meridian, less precisely termed the Greenwich, or Prime meridian. There are two varieties, mean sidereal time if the mean equator and equinox of date are used, and apparent sidereal time if the apparent equator and equinox of date are used. The former ignores the effect of astronomical nutation while the latter includes it. When the choice of location is combined with the choice of including astronomical nutation or not, the acronyms GMST, LMST, GAST, and LAST result.
+The following relationships are true:
+
+The new definitions of Greenwich mean and apparent sidereal time (since 2003, see above) are:
+
+  
+    
+      
+        
+          G
+          M
+          S
+          T
+        
+        (
+        
+          t
+          
+            U
+          
+        
+        ,
+        t
+        )
+        =
+        θ
+        (
+        
+          t
+          
+            U
+          
+        
+        )
+        −
+        
+          E
+          
+            
+              P
+              R
+              E
+              C
+            
+          
+        
+        (
+        t
+        )
+      
+    
+    {\displaystyle \mathrm {GMST} (t_{U},t)=\theta (t_{U})-E_{\mathrm {PREC} }(t)}
+  
+
+  
+    
+      
+        
+          G
+          A
+          S
+          T
+        
+        (
+        
+          t
+          
+            U
+          
+        
+        ,
+        t
+        )
+        =
+        θ
+        (
+        
+          t
+          
+            U
+          
+        
+        )
+        −
+        
+          E
+          
+            0
+          
+        
+        (
+        t
+        )
+      
+    
+    {\displaystyle \mathrm {GAST} (t_{U},t)=\theta (t_{U})-E_{0}(t)}
+  
+such that θ is the Earth Rotation Angle, EPREC is the accumulated precession, and E0 is equation of the origins, which represents accumulated precession and nutation. The calculation of precession and nutation was described in Chapter 6 of Urban & Seidelmann.
+As an example, the Astronomical Almanac for the Year 2017 gave the ERA at 0 h 1 January 2017 UT1 as 100° 37′ 12.4365″ (6 h 42 m 28.8291 s). The GAST was 6 h 43 m 20.7109 s. For GMST the hour and minute were the same but the second was 21.1060.

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+---
+title: "Sidereal time"
+chunk: 3/3
+source: "https://en.wikipedia.org/wiki/Sidereal_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:28.221801+00:00"
+instance: "kb-cron"
+---
+
+=== Relationship between solar time and sidereal time intervals ===
+
+If a certain interval I is measured in both mean solar time (UT1) and sidereal time, the numerical value will be greater in sidereal time than in UT1, because sidereal days are shorter than UT1 days. The ratio is:
+  
+    
+      
+        
+          
+            
+              I
+              
+                
+                  m
+                  e
+                  a
+                  n
+                  
+                  s
+                  i
+                  d
+                  e
+                  r
+                  e
+                  a
+                  l
+                
+              
+            
+            
+              I
+              
+                
+                  U
+                  T
+                  1
+                
+              
+            
+          
+        
+        =
+        
+          r
+          ′
+        
+        =
+        1.002
+        
+        737
+        
+        379
+        
+        093
+        
+        507
+        
+        95
+        +
+        5.9006
+        ×
+        
+          10
+          
+            −
+            11
+          
+        
+        t
+        −
+        5.9
+        ×
+        
+          10
+          
+            −
+            15
+          
+        
+        
+          t
+          
+            2
+          
+        
+      
+    
+    {\displaystyle {\frac {I_{\mathrm {mean\,sidereal} }}{I_{\mathrm {UT1} }}}=r'=1.002\,737\,379\,093\,507\,95+5.9006\times 10^{-11}t-5.9\times 10^{-15}t^{2}}
+  
+such that t represents the number of Julian centuries elapsed since noon 1 January 2000 Terrestrial Time.
+
+== Sidereal days compared to solar days on other planets ==
+Six of the eight solar planets have prograde rotation—that is, they rotate more than once per year in the same direction as they orbit the Sun, so the Sun rises in the east. Venus and Uranus, however, have retrograde rotation. For prograde rotation, the formula relating the lengths of the sidereal and solar days is:
+
+or, equivalently:
+
+When calculating the formula for a retrograde rotation, the operator of the denominator will be a plus sign (put another way, in the original formula the length of the sidereal day must be treated as negative). This is due to the solar day being shorter than the sidereal day for retrograde rotation, as the rotation of the planet would be against the direction of orbital motion.
+If a planet rotates prograde, and the sidereal day exactly equals the orbital period, then the formula above gives an infinitely long solar day (division by zero). This is the case for a planet in synchronous rotation; in the case of zero eccentricity, one hemisphere experiences eternal day, the other eternal night, with a "twilight belt" separating them.
+All the solar planets more distant from the Sun than Earth are similar to Earth in that, since they experience many rotations per revolution around the Sun, there is only a small difference between the length of the sidereal day and that of the solar day – the ratio of the former to the latter never being less than Earth's ratio of 0.997. But the situation is quite different for Mercury and Venus. Mercury's sidereal day is about two-thirds of its orbital period, so by the prograde formula its solar day lasts for two revolutions around the Sun – three times as long as its sidereal day. Venus rotates retrograde with a sidereal day lasting about 243.0 Earth days, or about 1.08 times its orbital period of 224.7 Earth days; hence by the retrograde formula its solar day is about 116.8 Earth days, and it has about 1.9 solar days per orbital period.
+By convention, rotation periods of planets are given in sidereal terms unless otherwise specified.
+
+== See also ==
+Anti-sidereal time
+Earth's rotation
+International Celestial Reference Frame
+Nocturnal (instrument)
+Sidereal month
+Sidereal year
+Synodic day
+Transit instrument
+
+== Citations ==
+
+== References ==
+Astronomical Almanac for the Year 2017. Washington and Taunton: US Government Printing Office and The UK Hydrographic Office. 2016. ISBN 978-0-7077-41666.
+Bakich, Michael E. (2000). The Cambridge Planetary Handbook. Cambridge University Press. ISBN 0-521-63280-3.
+"Earth Rotation Angle". International Earth Rotation and Reference System Service. 2013. Retrieved 20 March 2018.
+Explanatory Supplement to the Ephemeris. London, England: Her Majesty's Stationery Office. 1961.
+"Time and Frequency from A to Z, S to So". National Institute of Standards and Technology. 12 May 2010.
+Urban, Sean E.; Seidelmann, P. Kenneth, eds. (2013). Explanatory Supplement to the Astronomical Almanac (3rd ed.). Mill Valley, California: University Science Books. ISBN 978-1-891389-85-6.
+
+== External links ==
+
+Web-based Sidereal time calculator

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+---
+title: "Solar time"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Solar_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:29.423240+00:00"
+instance: "kb-cron"
+---
+
+Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time (discussed in this article), and sidereal time, which is based on the apparent motions of stars other than the Sun.
+
+== Introduction ==
+
+A tall pole vertically fixed in the ground casts a shadow on any sunny day. At one moment during the day, the shadow will point exactly north or south (or disappear when and if the Sun moves directly overhead). That instant is called local apparent noon, or 12:00 local apparent time. About 24 hours later the shadow will again point north–south, the Sun seeming to have covered a 360-degree arc around Earth's axis. When the Sun has covered exactly 15 degrees (1/24 of a circle, both angles being measured in a plane perpendicular to Earth's axis), local apparent time is 13:00 exactly; after 15 more degrees it will be 14:00 exactly.
+The problem is that in September the Sun takes less time (as measured by an accurate clock) to make an apparent revolution than it does in December; 24 "hours" of solar time can be 21 seconds less or 29 seconds more than 24 hours of clock time. This change is quantified by the equation of time, and is due to the eccentricity of Earth's orbit (as in, Earth's orbit is not perfectly circular, meaning that the Earth–Sun distance varies throughout the year), and the fact that Earth's axis is not perpendicular to the plane of its orbit (the so-called obliquity of the ecliptic).
+The effect of this is that a clock running at a constant rate – e.g. completing the same number of pendulum swings in each hour – cannot follow the actual Sun; instead it follows an imaginary "mean Sun" that moves along the celestial equator at a constant rate that matches the real Sun's average rate over the year. This is "mean solar time", which is still not perfectly constant from one century to the next but is close enough for most purposes. As of 2008, a mean solar day is about 86,400.002 SI seconds, i.e., about 24.0000006 hours.
+
+== Apparent solar time ==
+
+The apparent sun is the true sun as seen by an observer on Earth. Apparent solar time or true solar time is based on the apparent motion of the actual Sun. It is based on the apparent solar day, the interval between two successive returns of the Sun to the local meridian. Apparent solar time can be crudely measured by a sundial.
+The length of a solar day varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the mean. The effect has two main causes. First, due to the eccentricity of Earth's orbit, Earth moves faster when it is nearest the Sun (perihelion) and slower when it is farthest from the Sun (aphelion) (see Kepler's laws of planetary motion). Second, due to Earth's axial tilt (known as the obliquity of the ecliptic), the Sun's annual motion is along a great circle (the ecliptic) that is tilted to Earth's celestial equator.  When the Sun crosses the equator at both equinoxes, the Sun's daily shift (relative to the background stars) is at an angle to the equator, so the projection of this shift onto the equator is less than its average for the year; when the Sun is farthest from the equator at both solstices, the Sun's shift in position from one day to the next is parallel to the equator, so the projection onto the equator of this shift is larger than the average for the year (see tropical year). In June and December when the sun is farthest from the celestial equator, a given shift along the ecliptic corresponds to a large shift at the equator. Therefore, apparent solar days are shorter in March and September than in June or December.
+
+These lengths will change slightly in a few years and significantly in thousands of years.
+
+== Mean solar time ==
+
+Mean solar time is the hour angle of the mean position of the Sun, plus 12 hours. This 12 hour offset comes from the decision to make each day start at midnight for civil purposes, whereas the hour angle or the mean sun is measured from the local meridian. As of 2009, this is realized with the UT1 time scale, constructed mathematically from very-long-baseline interferometry observations of the diurnal motions of radio sources located in other galaxies, and other observations. The duration of daylight varies during the year but the length of a mean solar day is nearly constant, unlike that of an apparent solar day. An apparent solar day can be 20 seconds shorter or 30 seconds longer than a mean solar day. Long or short days occur in succession, so the difference builds up until mean time is ahead of apparent time by about 14 minutes near February 6, and behind apparent time by about 16 minutes near November 3. The equation of time is this difference, which is cyclical and does not accumulate from year to year.
+Mean time follows the mean sun. Jean Meeus describes the mean sun as follows:
+
+Consider a first fictitious Sun travelling along the ecliptic with a constant speed and coinciding with the true sun at the perigee and apogee (when the Earth is in perihelion and aphelion, respectively). Then consider a second fictitious Sun travelling along the celestial equator at a constant speed and coinciding with the first fictitious Sun at the equinoxes. This second fictitious sun is the mean Sun.
+The length of the mean solar day is slowly increasing due to the tidal acceleration of the Moon by Earth and the corresponding slowing of Earth's rotation by the Moon.
+
+== History ==

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+---
+title: "Solar time"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Solar_time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:29.423240+00:00"
+instance: "kb-cron"
+---
+
+The sun has always been visible in the sky, and its position forms the basis of apparent solar time, the timekeeping method used in antiquity. An Egyptian obelisk constructed c. 3500 BC, a gnomon in China dated 2300 BC, and an Egyptian sundial dated 1500 BC are some of the earliest methods for measuring the sun's position.
+Babylonian astronomers knew that the hours of daylight varied throughout the year. A tablet from 649 BC shows that they used a 2:1 ratio for the longest day to the shortest day, and estimated the variation using a linear zigzag function. It is not clear if they knew of the variation in the length of the solar day and the corresponding equation of time. Ptolemy clearly distinguishes the mean solar day and apparent solar day in his Almagest (2nd century), and he tabulated the equation of time in his Handy Tables. 
+Apparent solar time grew less useful as commerce increased and mechanical clocks improved. Mean solar time was introduced in almanacs in England in 1834 and in France in 1835. Because the sun was difficult to observe directly due to its large size in the sky, mean solar time was determined as a fixed ratio of time as observed by the stars, which used point-like observations. A specific standard for measuring "mean solar time" from midnight came to be called Universal Time.
+Conceptually Universal Time is the rotation of the Earth with respect to the sun and hence is mean solar time. However, UT1, the version in common use since 1955, uses a slightly different definition of rotation that corrects for the motion of Earth's poles as it rotates. The difference between this corrected mean solar time and Coordinated Universal Time (UTC) determines whether a leap second is needed. (Since 1972 the UTC time scale has run on SI seconds, and the SI second, when adopted, was already a little shorter than the current value of the second of mean solar time.)
+
+== See also ==
+Local mean time
+Meridian circle
+Earth rotation
+Synodic day
+
+== Notes ==
+
+== References ==
+
+== External links ==
+Sunrise and Sunset and maximum Sun altitude, all year long, anywhere
+Astrarium Solar Tempometer: Apparent solar time in a digital display.

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+---
+title: "Solstice"
+chunk: 1/4
+source: "https://en.wikipedia.org/wiki/Solstice"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:30.644057+00:00"
+instance: "kb-cron"
+---
+
+A solstice is the time when the Sun reaches its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around 20–22 June and 20–22 December. In many countries, the seasons of the year are defined by reference to the solstices and the equinoxes.
+The term solstice can also be used in a broader sense, as the day when this occurs. For locations not too close to the equator or the poles, the dates with the longest and shortest periods of daylight are the summer and winter solstices, respectively. Terms with no ambiguity as to which hemisphere is the context are "June solstice" and "December solstice", referring to the months in which they take place every year.
+
+== Etymology ==
+The word solstice is derived from the Latin sol ('sun') and sistere ('to stand still'), because at the solstices, the Sun's declination appears to "stand still"; that is, the seasonal movement of the Sun's daily path (as seen from Earth) reaches a northern or southern limit before reversing direction.
+Solstice first entered into English in the Middle English period. An older term in English is its calque sunstead (Old English: sunstede), which became rare after the 17th century. Sunstead is cognate with other terms with the same meaning in other Germanic language such as Old Norse: sólstaðr and Middle High German: sunnenstat. A similar English calque of the Latin term is sunstay which was first used in the 16th century and is now also rare.
+
+== Definitions and frames of reference ==
+
+For an observer at the North Pole, the Sun reaches the highest position in the sky once a year in June. The day this occurs is called the June solstice day. Similarly, for an observer on the South Pole, the Sun reaches the highest position on the December solstice day. When it is the summer solstice at one Pole, it is the winter solstice on the other. The Sun's westerly motion never ceases as Earth is continually in rotation. However, at the moment of solstice the Sun's motion in declination (i.e. vertically) appears to stop for an instant, and then reverse. In that sense, solstice means "sun-standing".
+This modern scientific word descends from a Latin scientific word in use in the late Roman Republic of the 1st century BC: solstitium. Pliny uses it a number of times in his Natural History with a similar meaning that it has today. It contains two Latin-language morphemes, sol, "sun", and -stitium, "stoppage". The Romans used "standing" to refer to a component of the relative velocity of the Sun as it is observed in the sky. Relative velocity is the motion of an object from the point of view of an observer in a frame of reference. From a fixed position on the ground, the Sun appears to orbit around Earth.
+To an observer in an inertial frame of reference, planet Earth is seen to rotate about an axis and orbit around the Sun in an elliptical path with the Sun at one focus. Earth's axis is tilted with respect to the plane of Earth's orbit and this axis maintains a position that changes little with respect to the background of stars. An observer on Earth therefore sees a solar path that is the result of both rotation and revolution.
+
+The component of the Sun's motion seen by an earthbound observer caused by the revolution of the tilted axis—which, keeping the same angle in space, is oriented toward or away from the Sun—is an observed daily increment (and lateral offset) of the elevation of the Sun at noon for approximately six months and observed daily decrement for the remaining six months. At maximum or minimum elevation, the relative yearly motion of the Sun perpendicular to the horizon stops and reverses direction.
+Outside of the tropics, the maximum elevation occurs at the summer solstice and the minimum at the winter solstice. The path of the Sun, or ecliptic, sweeps north and south between the northern and southern hemispheres. The lengths of time when the sun is up are longer around the summer solstice and shorter around the winter solstice, except near the equator. When the Sun's path crosses the equator, the length of the nights at latitudes +L° and −L° are of equal length. This is known as an equinox. There are two solstices and two equinoxes in a tropical year.
+
+Because of the variation in the rate at which the sun's right ascension changes, the days of longest and shortest daylight do not coincide with the solstices for locations very close to the equator. At the equator, the longest day is around 23 December and the shortest around 16 September (see graph). Inside the Arctic or Antarctic Circles the sun is up all the time for days or even months.
+
+== Relationship to seasons ==
+
+The seasons occur because the Earth's axis of rotation is not perpendicular to its orbital plane (the plane of the ecliptic) but currently makes an angle of about 23.44° (called the obliquity of the ecliptic), and because the axis keeps its orientation with respect to an inertial frame of reference. As a consequence, for half the year the Northern Hemisphere is inclined toward the Sun while for the other half year the Southern Hemisphere has this distinction. The two moments when the inclination of Earth's rotational axis has maximum effect are the solstices.
+At the June solstice the subsolar point is further north than any other time: at latitude 23.44° north, known as the Tropic of Cancer. Similarly at the December solstice the subsolar point is further south than any other time: at latitude 23.44° south, known as the Tropic of Capricorn. The subsolar point will cross every latitude between these two extremes exactly twice per year.

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+---
+title: "Solstice"
+chunk: 2/4
+source: "https://en.wikipedia.org/wiki/Solstice"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:30.644057+00:00"
+instance: "kb-cron"
+---
+
+Also during the June solstice, places on the Arctic Circle (latitude 66.56° north) will see the Sun just on the horizon during midnight, and all places north of it will see the Sun above horizon for 24 hours. That is the midnight sun or midsummer-night sun or polar day. On the other hand, places on the Antarctic Circle (latitude 66.56° south) will see the Sun just on the horizon during midday, and all places south of it will not see the Sun above horizon at any time of the day. That is the polar night. During the December Solstice, the effects on both hemispheres are just the opposite. This sees polar sea ice re-grow annually due to lack of sunlight on the air above and surrounding sea. The warmest and coldest periods of the year in temperate regions are offset by about one month from the solstices, delayed by the earth's thermal inertia.
+
+== Cultural aspects ==
+
+=== Ancient Greek names and concepts ===
+The concept of the solstices was embedded in ancient Greek celestial navigation. As soon as they discovered that the Earth was spherical they devised the concept of the celestial sphere, an imaginary spherical surface rotating with the heavenly bodies (ouranioi) fixed in it (the modern one does not rotate, but the stars in it do). As long as no assumptions are made concerning the distances of those bodies from Earth or from each other, the sphere can be accepted as real and is in fact still in use. The Ancient Greeks use the term "ηλιοστάσιο" (heliostāsio), meaning stand of the Sun.
+The stars move across the inner surface of the celestial sphere along the circumferences of circles in parallel planes perpendicular to the Earth's axis extended indefinitely into the heavens and intersecting the celestial sphere in a celestial pole. The Sun and the planets do not move in these parallel paths but along another circle, the ecliptic, whose plane is at an angle, the obliquity of the ecliptic, to the axis, bringing the Sun and planets across the paths of and in among the stars.*
+Cleomedes states:
+
+The band of the Zodiac (zōdiakos kuklos, "zodiacal circle") is at an oblique angle (loksos) because it is positioned between the tropical circles and equinoctial circle touching each of the tropical circles at one point ... This Zodiac has a determinable width (set at 8° today) ... that is why it is described by three circles: the central one is called "heliacal" (hēliakos, "of the sun").
+The term heliacal circle is used for the ecliptic, which is in the center of the zodiacal circle, conceived as a band including the noted constellations named on mythical themes. Other authors use Zodiac to mean ecliptic, which first appears in a gloss of unknown author in a passage of Cleomedes where he is explaining that the Moon is in the zodiacal circle as well and periodically crosses the path of the Sun. As some of these crossings represent eclipses of the Moon, the path of the Sun is given a synonym, the ekleiptikos (kuklos) from ekleipsis, "eclipse".
+
+=== English names ===
+
+The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress.
+
+Summer solstice and winter solstice are the most common names, referring to the seasons they are associated with. However, these can be ambiguous since the Northern Hemisphere's summer is the Southern Hemisphere's winter, and vice versa. The Latinate names estival solstice (summer) and hibernal solstice (winter) are sometimes used to the same effect, as are midsummer and midwinter.
+June solstice and December solstice refer to the months of year in which they take place, with no ambiguity as to which hemisphere is the context. They are still not universal, however, as not all cultures use a solar-based calendar where the solstices occur every year in the same month (as they do not in the Islamic calendar and Hebrew calendar, for example).
+Northern solstice and southern solstice indicate the hemisphere of the Sun's location. The northern solstice is in June, when the Sun is directly over the Tropic of Cancer in the Northern Hemisphere, and the southern solstice is in December, when the Sun is directly over the Tropic of Capricorn in the Southern Hemisphere. These terms can be used unambiguously for other planets.
+First point of Cancer and first point of Capricorn refer to the astrological signs that the sun "is entering" (a system rooted in Roman Classical period dates). Due to the precession of the equinoxes, the constellations the sun appears in at solstices are currently Taurus in June and Sagittarius in December.
+
+=== Solstice terms in East Asia ===
+
+The traditional East Asian calendars divide a year into 24 solar terms (節氣). Xiàzhì (pīnyīn) or Geshi (rōmaji) (Chinese and Japanese: 夏至; Korean: 하지(Haji); Vietnamese: Hạ chí; lit. summer's extreme) is the 10th solar term, and marks the summer solstice. It begins when the Sun reaches the celestial longitude of 90° (around 21 June) and ends when the Sun reaches the longitude of 105° (around 7 July). Xiàzhì more often refers in particular to the day when the Sun is exactly at the celestial longitude of 90°.
+Dōngzhì (pīnyīn) or Tōji (rōmaji) (Chinese and Japanese: 冬至; Korean: 동지(Dongji); Vietnamese: Đông chí; lit. winter's extreme) is the 22nd solar term, and marks the winter solstice. It begins when the Sun reaches the celestial longitude of 270° (around 23 December) and ends when the Sun reaches the longitude of 285° (around 5 January). Dōngzhì more often refers in particular to the day when the Sun is exactly at the celestial longitude of 270°.
+The solstices (as well as the equinoxes) mark the middle of the seasons in East Asian calendars. Here, the Chinese character 至 means "extreme", so the terms for the solstices directly signify the summits of summer and winter.
+
+=== Solstice celebrations ===

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+---
+title: "Solstice"
+chunk: 3/4
+source: "https://en.wikipedia.org/wiki/Solstice"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:30.644057+00:00"
+instance: "kb-cron"
+---
+
+The term solstice can also be used in a wider sense, as the date (day) that such a passage happens. The solstices, together with the equinoxes, are connected with the seasons. In some languages they are considered to start or separate the seasons; in others they are considered to be centre points (in England, in the Northern Hemisphere, for example, the period around the northern solstice is known as midsummer). Midsummer's Day, defined as St. Johns Day by the Christian Church, is 24 June, about three days after the solstice itself). Similarly 25 December is the start of the Christmas celebration, and is the day the Sun begins to return to the Northern Hemisphere. The traditional British and Irish main rent and meeting days of the year, "the usual quarter days," were often those of the solstices and equinoxes.
+Many cultures celebrate various combinations of the winter and summer solstices, the equinoxes, and the midpoints between them, leading to various holidays arising around these events. During the southern or winter solstice, Christmas is the most widespread contemporary holiday, while Yalda, Saturnalia, Karachun, Hanukkah, Kwanzaa, and Yule are also celebrated around this time. In East Asian cultures, the Dongzhi Festival is celebrated on the winter solstice. For the northern or summer solstice, Christian cultures celebrate the feast of St. John from June 23 to 24 (see St. John's Eve, Ivan Kupala Day), while Modern Pagans observe Midsummer, known as Litha among Wiccans. For the vernal (spring) equinox, several springtime festivals are celebrated, such as the Persian Nowruz, the observance in Judaism of Passover, the rites of Easter in most Christian churches, as well as the Wiccan Ostara. The autumnal equinox is associated with the Jewish holiday of Sukkot and the Wiccan Mabon. 
+In the southern tip of South America, the Mapuche people celebrate We Tripantu (the New Year) a few days after the northern solstice, on 24 June. Further north, the Atacama people formerly celebrated this date with a noise festival, to call the Sun back. Further east, the Aymara people celebrate their New Year on 21 June. A celebration occurs at sunrise, when the sun shines directly through the Gate of the Sun in Tiwanaku. Other Aymara New Year feasts occur throughout Bolivia, including at the site of El Fuerte de Samaipata.
+In the Hindu calendar, two sidereal solstices are named Makara Sankranti which marks the start of Uttarayana and Karka Sankranti which marks the start of Dakshinayana. The former occurs around 14 January each year, while the latter occurs around 14 July each year. These mark the movement of the Sun along a sidereally fixed zodiac (precession is ignored) into Makara, the zodiacal sign which corresponds with Capricorn, and into Karka, the zodiacal sign which corresponds with Cancer, respectively.
+The Amundsen–Scott South Pole Station celebrates every year on 21 June a midwinter party, to celebrate that the Sun is at its lowest point and coming back.
+The Fremont Solstice Parade takes place every summer solstice in Fremont, Seattle, Washington in the United States.
+The reconstructed Cahokia Woodhenge, a large timber circle located at the Mississippian culture Cahokia archaeological site near Collinsville, Illinois, is the site of annual equinox and solstice sunrise observances. Out of respect for Native American beliefs these events do not feature ceremonies or rituals of any kind.

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+---
+title: "Solstice"
+chunk: 4/4
+source: "https://en.wikipedia.org/wiki/Solstice"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:30.644057+00:00"
+instance: "kb-cron"
+---
+
+=== Solstice determination ===
+Unlike the equinox, the solstice time is not easy to determine. The changes in solar declination become smaller as the Sun gets closer to its maximum/minimum declination. The days before and after the solstice, the declination speed is less than 30 arcseconds per day which is less than 1⁄60 of the angular size of the Sun, or the equivalent to just 2 seconds of right ascension.
+This difference is hardly detectable with indirect viewing based devices like sextant equipped with a vernier, and impossible with more traditional tools like a gnomon or an astrolabe. It is also hard to detect the changes in sunrise/sunset azimuth due to the atmospheric refraction changes. Those accuracy issues render it impossible to determine the solstice day based on observations made within the 3 (or even 5) days surrounding the solstice without the use of more complex tools.
+Accounts do not survive but Greek astronomers must have used an approximation method based on interpolation, which is still used by some amateurs. This method consists of recording the declination angle at noon during some days before and after the solstice, trying to find two separate days with the same declination. When those two days are found, the halfway time between both noons is estimated solstice time. An interval of 45 days has been postulated as the best one to achieve up to a quarter-day precision, in the solstice determination.
+In 2012, the journal DIO found that accuracy of one or two hours with balanced errors can be attained by observing the Sun's equal altitudes about S = twenty degrees (or d = about 20 days) before and after the summer solstice because the average of the two times will be early by q arc minutes where q is (πe cosA)/3 times the square of S in degrees (e = earth orbit eccentricity, A = earth's perihelion or Sun's apogee), and the noise in the result will be about 41 hours divided by d if the eye's sharpness is taken as one arc minute.
+Guo Shoujing, a Chinese astronomer, found that the taller the gnomon, the more acurately the journey of the sun could be measured. He designed the 12.6 metres (41 ft) gnomon constructed at Gaocheng Astronomical Observatory in 1276. The measurements from Gaocheng determined the length of the year to within one minute of the current measurement, a value in accord with the value of the Gregorian Calendar, but obtained 300 years earlier..
+Astronomical almanacs define the solstices as the moments when the Sun passes through the solstitial colure, i.e. the times when the apparent geocentric celestial longitude of the Sun is equal to 90° (June solstice) or 270° (December solstice). The dates of the solstice varies each year and may occur a day earlier or later depending on the time zone. Because the earth's orbit takes slightly longer than a calendar year of 365 days, the solstices occur slightly later each calendar year, until a leap day re-aligns the calendar with the orbit. Thus the solstices always occur between June 20 and 22 and between December 20 and 23  in a four-year-long cycle with the 21st and 22nd being the most common dates, as can be seen in the schedule at the start of the article.
+Currently, government organizations like USNO
+and IMCCE
+publish the date and time of the solstice.
+
+== In the constellations ==
+Using the current official IAU constellation boundaries—and taking into account the variable precession speed and the rotation of the ecliptic—the solstices shift through the constellations as follows (expressed in astronomical year numbering in which the year 0 = 1 BC, −1 = 2 BC, etc.):
+
+The northern solstice passed from Leo into Cancer in year −1458, passed into Gemini in year −10, passed into Taurus in December 1989, and is expected to pass into Aries in year 4609.
+The southern solstice passed from Capricornus into Sagittarius in year −130, is expected to pass into Ophiuchus in year 2269, and is expected to pass into Scorpius in year 3597.
+
+== See also ==
+
+Analemma
+Geocentric view of the seasons
+Iranian calendars
+Perihelion and aphelion
+Wheel of the Year
+Zoroastrian calendar
+
+== References ==
+
+== External links ==
+
+Equinoxes and Solstices Calculator (1600 to 2400)
+"Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion (2000–2025)". United States Naval Observatory, Astronomical Applications Department. Retrieved December 9, 2015.
+Weisstein, Eric (1996–2007). "Summer Solstice". Eric Weisstein's World of Astronomy. Retrieved October 24, 2008. The above plots show how the date of the summer solstice shifts through the Gregorian calendar according to the insertion of leap years.

data/en.wikipedia.org/wiki/Sunrise_equation-0.md

@@ -0,0 +1,596 @@
+---
+title: "Sunrise equation"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Sunrise_equation"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:31.804583+00:00"
+instance: "kb-cron"
+---
+
+The sunrise equation or sunset equation can be used to derive the time of sunrise or sunset for any solar declination and latitude in terms of local solar time when sunrise and sunset actually occur. 
+
+== Geometric equation ==
+The time at which a celestial object crosses the horizon can be calculated by converting its coordinates from the equatorial coordinate system to the horizontal coordinate system, and then solving the equation for an altitude of zero. We then obtain
+
+  
+    
+      
+        cos
+        ⁡
+        
+          H
+          
+            0
+          
+        
+        =
+        −
+        tan
+        ⁡
+        ϕ
+        
+        tan
+        ⁡
+        δ
+      
+    
+    {\displaystyle \cos H_{0}=-\tan \phi \,\tan \delta }
+  
+
+where:
+
+  
+    
+      
+        
+          H
+          
+            0
+          
+        
+      
+    
+    {\displaystyle H_{0}}
+  
+ is the solar hour angle at either sunrise (when the negative value is taken) or sunset (when the positive value is taken);
+
+  
+    
+      
+        ϕ
+      
+    
+    {\displaystyle \phi }
+  
+ is the latitude of the observer on the Earth;
+
+  
+    
+      
+        δ
+      
+    
+    {\displaystyle \delta }
+  
+ is the Sun's declination.
+This gives the geometric rise or set time (ignoring refraction) of the center of the Sun. See below for an equation which accounts for these effects.
+The Earth rotates at an angular velocity of 15°/hour. Therefore, the expression 
+  
+    
+      
+        
+          H
+          
+            0
+          
+        
+        
+          /
+        
+        
+          
+            15
+          
+          
+            ∘
+          
+        
+      
+    
+    {\displaystyle H_{0}/\mathrm {15} ^{\circ }}
+  
+, where 
+  
+    
+      
+        
+          H
+          
+            0
+          
+        
+      
+    
+    {\displaystyle H_{0}}
+  
+ is in degrees, gives the interval of time in hours from sunrise to local solar noon or from local solar noon to sunset.
+The sign convention is that the observer latitude 
+  
+    
+      
+        ϕ
+      
+    
+    {\displaystyle \phi }
+  
+ is 0 at the equator, positive for the Northern Hemisphere and negative for the Southern Hemisphere, and the solar declination 
+  
+    
+      
+        δ
+      
+    
+    {\displaystyle \delta }
+  
+ is 0 when the Sun is exactly above the equator, positive during the Northern Hemisphere summer and negative during the Northern Hemisphere winter.The declination of the Sun is nearly, but not exactly, zero at the equinoxes.
+The equation has no solution when 
+  
+    
+      
+        
+          |
+        
+        tan
+        ⁡
+        ϕ
+        
+        tan
+        ⁡
+        δ
+        
+          |
+        
+        >
+        1
+      
+    
+    {\displaystyle |\tan \phi \,\tan \delta |>1}
+  
+. This occurs north of the Arctic Circle or south of the Antarctic Circle, during the polar night, when the Sun is not visible above the horizon at local midday.
+
+=== Hemispheric relation ===
+Suppose 
+  
+    
+      
+        
+          ϕ
+          
+            N
+          
+        
+      
+    
+    {\displaystyle \phi _{N}}
+  
+ is a given latitude in Northern Hemisphere, and 
+  
+    
+      
+        
+          H
+          
+            N
+          
+        
+      
+    
+    {\displaystyle H_{N}}
+  
+ is the corresponding sunrise hour angle that has a negative value, and similarly, 
+  
+    
+      
+        
+          ϕ
+          
+            S
+          
+        
+      
+    
+    {\displaystyle \phi _{S}}
+  
+ is the same latitude but in Southern Hemisphere, which means 
+  
+    
+      
+        
+          ϕ
+          
+            S
+          
+        
+        =
+        −
+        
+          ϕ
+          
+            N
+          
+        
+      
+    
+    {\displaystyle \phi _{S}=-\phi _{N}}
+  
+, and 
+  
+    
+      
+        
+          H
+          
+            S
+          
+        
+      
+    
+    {\displaystyle H_{S}}
+  
+ is the corresponding sunrise hour angle, then it is apparent that
+
+  
+    
+      
+        cos
+        ⁡
+        
+          H
+          
+            S
+          
+        
+        =
+        −
+        cos
+        ⁡
+        
+          H
+          
+            N
+          
+        
+        =
+        cos
+        ⁡
+        (
+        −
+        
+          180
+          
+            ∘
+          
+        
+        −
+        
+          H
+          
+            N
+          
+        
+        )
+      
+    
+    {\displaystyle \cos H_{S}=-\cos H_{N}=\cos(-180^{\circ }-H_{N})}
+  
+,
+which means 
+
+  
+    
+      
+        
+          H
+          
+            N
+          
+        
+        +
+        
+          H
+          
+            S
+          
+        
+        =
+        −
+        
+          180
+          
+            ∘
+          
+        
+      
+    
+    {\displaystyle H_{N}+H_{S}=-180^{\circ }}
+  
+.
+The above relation implies that on the same day, the lengths of daytime from sunrise to sunset at 
+  
+    
+      
+        
+          ϕ
+          
+            N
+          
+        
+      
+    
+    {\displaystyle \phi _{N}}
+  
+ and 
+  
+    
+      
+        
+          ϕ
+          
+            S
+          
+        
+      
+    
+    {\displaystyle \phi _{S}}
+  
+ sum to 24 hours if 
+  
+    
+      
+        
+          ϕ
+          
+            S
+          
+        
+        =
+        −
+        
+          ϕ
+          
+            N
+          
+        
+      
+    
+    {\displaystyle \phi _{S}=-\phi _{N}}
+  
+, and this also applies to regions where polar days and polar nights occur. This further suggests that the global average of length of daytime on any given day is 12 hours without considering the effect of atmospheric refraction.
+
+== Generalized equation ==
+
+The equation above neglects the influence of atmospheric refraction and the non-zero angle subtended by the solar disc — i.e. the apparent diameter of the sun. The times of the rising and the setting of the upper solar limb as given in astronomical almanacs correct for this by using the more general equation:
+
+  
+    
+      
+        cos
+        ⁡
+        
+          H
+          
+            0
+          
+        
+        =
+        
+          
+            
+              
+                sin
+                ⁡
+                
+                  h
+                  
+                    0
+                  
+                
+                −
+                sin
+                ⁡
+                ϕ
+                
+                sin
+                ⁡
+                δ
+              
+              
+                cos
+                ⁡
+                ϕ
+                
+                cos
+                ⁡
+                δ
+              
+            
+          
+        
+      
+    
+    {\displaystyle \cos H_{0}={\dfrac {\sin h_{0}-\sin \phi \,\sin \delta }{\cos \phi \,\cos \delta }}}
+  
+
+where 
+  
+    
+      
+        
+          h
+          
+            0
+          
+        
+      
+    
+    {\displaystyle h_{0}}
+  
+ is the geometric altitude angle of the center of the Sun at the time of rising or setting, which is approximately −0.833° or −50.0 arcminutes, although the exact figure depends on atmospheric conditions along the line of sight.
+This equation, as given by Jean Meeus, can be also used for any other solar altitude. The NOAA provides additional approximate expressions for refraction corrections at these other altitudes. There are also alternative formulations, such as a non-piecewise expression by G.G. Bennett used in the U.S. Naval Observatory's "Vector Astronomy Software".
+The dip of the horizon in radians, including refraction and the geometric correction for the observer's height above the apparent horizon, can be approximated by:
+
+  
+    
+      
+        ψ
+        =
+        
+          
+            2
+            
+              
+                h
+                
+                  R
+                  
+                    0
+                  
+                
+              
+            
+            (
+            1
+            −
+            k
+            )
+          
+        
+      
+    
+    {\displaystyle \psi ={\sqrt {2{\frac {h}{R_{0}}}(1-k)}}}
+  
+
+where h is the height of the observer, 
+  
+    
+      
+        
+          R
+          
+            0
+          
+        
+      
+    
+    {\displaystyle R_{0}}
+  
+is the radius of the Earth, and k is the ratio of the radius of the ray's curvature to the radius of the Earth. This assumes that light rays follow a circular path, which is approximately true when the lapse rate is constant. For a typical value of k of 0.17, this gives
+
+  
+    
+      
+        ψ
+        =
+        
+          1.75
+          
+            ′
+          
+        
+        
+          
+            height in metres
+          
+        
+      
+    
+    {\displaystyle \psi =1.75^{\prime }{\sqrt {\text{height in metres}}}}
+  
+
+or
+
+  
+    
+      
+        ψ
+        =
+        
+          0.97
+          
+            ′
+          
+        
+        
+          
+            height in feet
+          
+        
+      
+    
+    {\displaystyle \psi =0.97^{\prime }{\sqrt {\text{height in feet}}}}
+  
+
+where the prime (
+  
+    
+      
+        
+          
+          
+            ′
+          
+        
+      
+    
+    {\displaystyle ^{\prime }}
+  
+) indicates arcminutes, i.e. 1/60 °. This should be subtracted from the altitude angle. In summary, at sunrise or sunset:
+
+  
+    
+      
+        
+          h
+          
+            0
+          
+        
+        =
+        −
+        s
+        −
+        ψ
+      
+    
+    {\displaystyle h_{0}=-s-\psi }
+  
+
+where s is the semidiameter of the Sun, about 16 arcminutes.
+
+== In Universal Time ==
+To calculate the time of the sunrise in Universal Time, Meeus recommends the following procedure. The position of the Sun in equatorial coordinates should first be calculated or looked up for the day of interest. For the day D, find:
+
+  
+    
+      
+        
+          θ
+          
+            0
+          
+        
+      
+    
+    {\displaystyle \theta _{0}}
+  
+: the apparent sidereal time (or Earth Rotation Angle) at 0h Universal Time on day 
+  
+    
+      
+        D
+      
+    
+    {\displaystyle D}
+  
+,

data/en.wikipedia.org/wiki/Sunrise_equation-1.md

@@ -0,0 +1,552 @@
+---
+title: "Sunrise equation"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Sunrise_equation"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:31.804583+00:00"
+instance: "kb-cron"
+---
+
+  
+    
+      
+        
+          α
+          
+            1
+          
+        
+      
+    
+    {\displaystyle \alpha _{1}}
+  
+ and 
+  
+    
+      
+        
+          δ
+          
+            1
+          
+        
+      
+    
+    {\displaystyle \delta _{1}}
+  
+, the right ascension and declination on day 
+  
+    
+      
+        D
+        −
+        1
+      
+    
+    {\displaystyle D-1}
+  
+ at 0h Universal Time,
+
+  
+    
+      
+        
+          α
+          
+            2
+          
+        
+      
+    
+    {\displaystyle \alpha _{2}}
+  
+ and 
+  
+    
+      
+        
+          δ
+          
+            2
+          
+        
+      
+    
+    {\displaystyle \delta _{2}}
+  
+, the right ascension and declination on day 
+  
+    
+      
+        D
+      
+    
+    {\displaystyle D}
+  
+,
+
+  
+    
+      
+        
+          α
+          
+            3
+          
+        
+      
+    
+    {\displaystyle \alpha _{3}}
+  
+ and 
+  
+    
+      
+        
+          δ
+          
+            3
+          
+        
+      
+    
+    {\displaystyle \delta _{3}}
+  
+, the right ascension and declination on day 
+  
+    
+      
+        D
+        +
+        1
+      
+    
+    {\displaystyle D+1}
+  
+.
+Calculate the approximate time of the sunset using
+
+  
+    
+      
+        cos
+        ⁡
+        
+          H
+          
+            0
+          
+        
+        =
+        
+          
+            
+              
+                sin
+                ⁡
+                
+                  h
+                  
+                    0
+                  
+                
+                −
+                sin
+                ⁡
+                ϕ
+                
+                sin
+                ⁡
+                
+                  δ
+                  
+                    2
+                  
+                
+              
+              
+                cos
+                ⁡
+                ϕ
+                
+                cos
+                ⁡
+                
+                  δ
+                  
+                    2
+                  
+                
+              
+            
+          
+        
+        .
+      
+    
+    {\displaystyle \cos H_{0}={\dfrac {\sin h_{0}-\sin \phi \,\sin \delta _{2}}{\cos \phi \,\cos \delta _{2}}}.}
+  
+
+If the right hand side has an absolute value greater than 1, then the Sun does not go below the horizon on that day and 
+  
+    
+      
+        
+          H
+          
+            0
+          
+        
+      
+    
+    {\displaystyle H_{0}}
+  
+ does not exist.
+Calculate the transit, sunrise and sunset time in fractions of a day:
+
+  
+    
+      
+        
+          m
+          
+            0
+          
+        
+        =
+        
+          
+            
+              
+                α
+                
+                  2
+                
+              
+              +
+              L
+              −
+              
+                θ
+                
+                  0
+                
+              
+            
+            
+              360
+              
+                ∘
+              
+            
+          
+        
+      
+    
+    {\displaystyle m_{0}={\frac {\alpha _{2}+L-\theta _{0}}{360^{\circ }}}}
+  
+
+  
+    
+      
+        
+          m
+          
+            1
+          
+        
+        =
+        
+          m
+          
+            0
+          
+        
+        −
+        
+          
+            
+              H
+              
+                0
+              
+            
+            
+              360
+              
+                ∘
+              
+            
+          
+        
+      
+    
+    {\displaystyle m_{1}=m_{0}-{\frac {H_{0}}{360^{\circ }}}}
+  
+
+  
+    
+      
+        
+          m
+          
+            2
+          
+        
+        =
+        
+          m
+          
+            0
+          
+        
+        +
+        
+          
+            
+              H
+              
+                0
+              
+            
+            
+              360
+              
+                ∘
+              
+            
+          
+        
+      
+    
+    {\displaystyle m_{2}=m_{0}+{\frac {H_{0}}{360^{\circ }}}}
+  
+
+where L is the geographical longitude expressed as an angle increasing westwards from Greenwich, i.e. the opposite sign convention than is typically used in geography.
+These values of m can be multiplied by 24 to give the time of each event in hours, accurate to about ±0.01 days (14 minutes). For greater accuracy, the elevation angle of the Sun should be calculated at the proposed time, and then an adjustment applied to bring it to the desired elevation. The adjustment is:
+
+  
+    
+      
+        Δ
+        m
+        =
+        
+          
+            
+              h
+              −
+              
+                h
+                
+                  0
+                
+              
+            
+            
+              
+                360
+                
+                  ∘
+                
+              
+              cos
+              ⁡
+              δ
+              cos
+              ⁡
+              ϕ
+              sin
+              ⁡
+              H
+            
+          
+        
+      
+    
+    {\displaystyle \Delta m={\frac {h-h_{0}}{360^{\circ }\cos \delta \cos \phi \sin H}}}
+  
+
+where
+
+  
+    
+      
+        H
+        =
+        θ
+        −
+        L
+        −
+        α
+      
+    
+    {\displaystyle H=\theta -L-\alpha }
+  
+ is the hour angle,
+
+  
+    
+      
+        θ
+        =
+        
+          θ
+          
+            0
+          
+        
+        +
+        
+          360.985647
+          
+            ∘
+          
+        
+        m
+      
+    
+    {\displaystyle \theta =\theta _{0}+360.985647^{\circ }m}
+  
+ is the sidereal time at Greenwich in degrees,
+h is the altitude of the Sun in degrees at m,
+
+  
+    
+      
+        α
+      
+    
+    {\displaystyle \alpha }
+  
+ is the result of linear interpolation between 
+  
+    
+      
+        
+          α
+          
+            1
+          
+        
+      
+    
+    {\displaystyle \alpha _{1}}
+  
+, 
+  
+    
+      
+        
+          α
+          
+            2
+          
+        
+      
+    
+    {\displaystyle \alpha _{2}}
+  
+ and 
+  
+    
+      
+        
+          α
+          
+            3
+          
+        
+      
+    
+    {\displaystyle \alpha _{3}}
+  
+,
+
+  
+    
+      
+        δ
+      
+    
+    {\displaystyle \delta }
+  
+ is similarly interpolated between 
+  
+    
+      
+        
+          δ
+          
+            1
+          
+        
+      
+    
+    {\displaystyle \delta _{1}}
+  
+, 
+  
+    
+      
+        
+          δ
+          
+            2
+          
+        
+      
+    
+    {\displaystyle \delta _{2}}
+  
+ and 
+  
+    
+      
+        
+          δ
+          
+            3
+          
+        
+      
+    
+    {\displaystyle \delta _{3}}
+  
+.
+The final time is then 
+  
+    
+      
+        m
+        +
+        Δ
+        m
+      
+    
+    {\displaystyle m+\Delta m}
+  
+. 
+Temperature variations in the atmosphere unpredictably affect the amount of refraction, limiting accuracy to about two minutes.
+
+== See also ==
+Daytime
+Equation of time
+
+== References ==
+
+== External links ==
+Sunrise, sunset, or sun position for any location — U.S. only (U.S. NOAA)
+Sunrise, sunset and day length for any location — worldwide
+Rise/Set/Transit/Twilight Data — U.S. only (U.S. Naval Observatory)
+Astronomical Information Center (U.S. Naval Observatory)
+Converting Between Julian Dates and Gregorian Calendar Dates
+Approximate Solar Coordinates
+Algorithms for Computing Astronomical Phenomena
+Astronomy Answers: Position of the Sun
+A Simple Expression for the Equation of Time
+The Equation of Time
+Equation of Time
+Long-Term Almanac for Sun, Moon, and Polaris V1.11
+Evaluating the Effectiveness of Current Atmospheric Refraction Models in Predicting Sunrise and Sunset Times

data/en.wikipedia.org/wiki/Terrestrial_Time-0.md

@@ -0,0 +1,409 @@
+---
+title: "Terrestrial Time"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Terrestrial_Time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:32.984785+00:00"
+instance: "kb-cron"
+---
+
+Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.
+For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD), which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.
+The unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock, but TT is not itself defined by atomic clocks.  It is a theoretical ideal, and real clocks can only approximate it.
+TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC).  TT is indirectly the basis of UTC, via International Atomic Time (TAI).  Because of the historical difference between TAI and ET when TT was introduced, TT is 32.184 s ahead of TAI.
+
+== History ==
+A definition of a terrestrial time standard was adopted by the International Astronomical Union (IAU) in 1976 at its XVI General Assembly and later named Terrestrial Dynamical Time (TDT). It was the counterpart to Barycentric Dynamical Time (TDB), which was a time standard for Solar system ephemerides, to be based on a dynamical time scale. Both of these time standards turned out to be imperfectly defined. Doubts were also expressed about the meaning of 'dynamical' in the name TDT.
+In 1991, in Recommendation IV of the XXI General Assembly, the IAU redefined TDT, also renaming it "Terrestrial Time". TT was formally defined in terms of Geocentric Coordinate Time (TCG), defined by the IAU on the same occasion. TT was defined to be a linear scaling of TCG, such that the unit of TT is the "SI second on the geoid", i.e. the rate approximately matched the rate of proper time on the Earth's surface at mean sea level. Thus the exact ratio between TT time and TCG time was 
+  
+    
+      
+        1
+        −
+        
+          L
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle 1-L_{\mathrm {G} }}
+  
+, where 
+  
+    
+      
+        
+          L
+          
+            
+              G
+            
+          
+        
+        =
+        
+          U
+          
+            
+              G
+            
+          
+        
+        
+          /
+        
+        
+          c
+          
+            2
+          
+        
+      
+    
+    {\displaystyle L_{\mathrm {G} }=U_{\mathrm {G} }/c^{2}}
+  
+ was a constant and 
+  
+    
+      
+        
+          U
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle U_{\mathrm {G} }}
+  
+ was the gravitational potential at the geoid surface, a value measured by physical geodesy. In 1991 the best available estimate of 
+  
+    
+      
+        
+          L
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle L_{\mathrm {G} }}
+  
+ was 6.969291×10−10.
+In 2000, the IAU very slightly altered the definition of TT by adopting an exact value, LG = 6.969290134×10−10.
+
+== Current definition ==
+TT differs from Geocentric Coordinate Time (TCG) by a constant rate.  Formally it is defined by the equation
+
+  
+    
+      
+        
+          T
+          T
+        
+        =
+        
+          
+            (
+          
+        
+        1
+        −
+        
+          L
+          
+            
+              G
+            
+          
+        
+        
+          
+            )
+          
+        
+        ×
+        
+          T
+          C
+          G
+        
+        +
+        E
+        ,
+      
+    
+    {\displaystyle \mathrm {TT} ={\bigl (}1-L_{\mathrm {G} }{\bigr )}\times \mathrm {TCG} +E,}
+  
+
+where TT and TCG are linear counts of SI seconds in Terrestrial Time and Geocentric Coordinate Time respectively, 
+  
+    
+      
+        
+          L
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle L_{\mathrm {G} }}
+  
+ is the constant difference in the rates of the two time scales, and 
+  
+    
+      
+        E
+      
+    
+    {\displaystyle E}
+  
+ is a constant to resolve the epochs (see below). 
+  
+    
+      
+        
+          L
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle L_{\mathrm {G} }}
+  
+ is defined as exactly 6.969290134×10−10. Due to the term 
+  
+    
+      
+        1
+        −
+        
+          L
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle 1-L_{\mathrm {G} }}
+  
+ the rate of TT is very slightly slower than that of TCG.
+The equation linking TT and TCG more commonly has the form given by the IAU,
+
+  
+    
+      
+        
+          T
+          T
+        
+        =
+        
+          T
+          C
+          G
+        
+        −
+        
+          L
+          
+            
+              G
+            
+          
+        
+        ×
+        
+          
+            (
+          
+        
+        
+          J
+          
+            D
+            
+              T
+              C
+              G
+            
+          
+        
+        −
+        2443144.5003725
+        
+          
+            )
+          
+        
+        ×
+        86400
+        ,
+      
+    
+    {\displaystyle \mathrm {TT} =\mathrm {TCG} -L_{\mathrm {G} }\times {\bigl (}\mathrm {JD_{TCG}} -2443144.5003725{\bigr )}\times 86400,}
+  
+
+where 
+  
+    
+      
+        
+          J
+          
+            D
+            
+              T
+              C
+              G
+            
+          
+        
+      
+    
+    {\displaystyle \mathrm {JD_{TCG}} }
+  
+ is the TCG time expressed as a Julian date (JD). The Julian Date is a linear transformation of the raw count of seconds represented by the variable TCG, so this form of the equation is not simplified. The use of a Julian Date specifies the epoch fully. The above equation is often given with the Julian Date 2443144.5 for the epoch, but that is inexact (though inappreciably so, because of the small size of the multiplier 
+  
+    
+      
+        
+          L
+          
+            
+              G
+            
+          
+        
+      
+    
+    {\displaystyle L_{\mathrm {G} }}
+  
+). The value 2443144.5003725 is exactly in accord with the definition.
+Time coordinates on the TT and TCG scales are specified conventionally using traditional means of specifying days, inherited from non-uniform time standards based on the rotation of Earth. Specifically, both Julian Dates and the Gregorian calendar are used. For continuity with their predecessor Ephemeris Time (ET), TT and TCG were set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z). More precisely, it was defined that TT instant 1977-01-01T00:00:32.184 and TCG instant 1977-01-01T00:00:32.184 exactly correspond to the International Atomic Time (TAI) instant 1977-01-01T00:00:00.000. This is also the instant at which TAI introduced corrections for gravitational time dilation.
+TT and TCG expressed as Julian Dates can be related precisely and most simply by the equation
+
+  
+    
+      
+        
+          J
+          
+            D
+            
+              T
+              T
+            
+          
+        
+        =
+        
+          E
+          
+            
+              J
+              D
+            
+          
+        
+        +
+        
+          
+            (
+          
+        
+        
+          J
+          
+            D
+            
+              T
+              C
+              G
+            
+          
+        
+        −
+        
+          E
+          
+            
+              J
+              D
+            
+          
+        
+        
+          
+            )
+          
+        
+        ×
+        
+          
+            (
+          
+        
+        1
+        −
+        
+          L
+          
+            
+              G
+            
+          
+        
+        
+          
+            )
+          
+        
+        ,
+      
+    
+    {\displaystyle \mathrm {JD_{TT}} =E_{\mathrm {JD} }+{\bigl (}\mathrm {JD_{TCG}} -E_{\mathrm {JD} }{\bigr )}\times {\bigl (}1-L_{\mathrm {G} }{\bigr )},}
+  
+
+where 
+  
+    
+      
+        
+          E
+          
+            
+              J
+              D
+            
+          
+        
+      
+    
+    {\displaystyle E_{\mathrm {JD} }}
+  
+ is 2443144.5003725 exactly.
+
+== Realizations ==
+TT is a theoretical ideal, not dependent on a particular realization. For practical use, physical clocks must be measured and their readings processed to estimate TT. A simple offset calculation is sufficient for most applications, but in demanding applications, detailed modeling of relativistic physics and measurement uncertainties may be needed.
+
+=== TAI ===

data/en.wikipedia.org/wiki/Terrestrial_Time-1.md

@@ -0,0 +1,71 @@
+---
+title: "Terrestrial Time"
+chunk: 2/2
+source: "https://en.wikipedia.org/wiki/Terrestrial_Time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:32.984785+00:00"
+instance: "kb-cron"
+---
+
+The main realization of TT is supplied by TAI. The BIPM TAI service, performed since 1958, estimates TT using measurements from an ensemble of atomic clocks spread over the surface and low orbital space of Earth. TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings shown by that particular group of atomic clocks at the time.  Estimates of TAI are also provided in real time by the institutions that operate the participating clocks.  Because of the historical difference between TAI and ET when TT was introduced, the TAI realization of TT is defined thus:
+
+  
+    
+      
+        
+          T
+          T
+          (
+          T
+          A
+          I
+          )
+          =
+          T
+          A
+          I
+          +
+          32.184
+           
+          s
+        
+        .
+      
+    
+    {\displaystyle \mathrm {TT(TAI)=TAI+32.184~s} .}
+  
+
+The offset 32.184 s arises from history. The atomic time scale A1 (a predecessor of TAI) was set equal to UT2 at its conventional starting date of 1 January 1958, when ΔT (ET − UT) was about 32 seconds. The offset 32.184 seconds was the 1976 estimate of the difference between Ephemeris Time (ET) and TAI, "to provide continuity with the current values and practice in the use of Ephemeris Time".
+TAI is never revised once published and TT(TAI) has small errors relative to TT(BIPM), on the order of 10-50 microseconds.
+The GPS time scale has a nominal difference from atomic time (TAI − GPS time = +19 seconds), so that TT ≈ GPS time + 51.184 seconds. This realization introduces up to a microsecond of additional error, as the GPS signal is not precisely synchronized with TAI, but GPS receiving devices are widely available.
+
+=== TT(BIPM) ===
+Approximately annually since 1992, the International Bureau of Weights and Measures (BIPM) has produced better realizations of TT based on reanalysis of historical TAI data.  BIPM's realizations of TT  are named in the form "TT(BIPM08)", with the digits indicating the year of publication.  They are published in the form of a table of differences from TT(TAI), along with an extrapolation equation that may be used for dates later than the table. The latest as of July 2024 is TT(BIPM23).
+
+=== Pulsars ===
+Researchers from the International Pulsar Timing Array collaboration have created a realization TT(IPTA16) of TT based on observations of an ensemble of pulsars up to 2012. This new pulsar time scale is an independent means of computing TT. The researchers observed that their scale was within 0.5 microseconds of TT(BIPM17), with significantly lower errors since 2003. The data used was insufficient to analyze long-term stability, and contained several anomalies, but as more data is collected and analyzed, this realization may eventually be useful to identify defects in TAI and TT(BIPM).
+
+=== Other standards ===
+TT is in effect a continuation of (but is more precisely uniform than) the former Ephemeris Time (ET).  It was designed for continuity with ET, and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations). The JPL ephemeris time argument Teph is within a few milliseconds of TT.
+TT is slightly ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as ΔT = TT − UT1.  ΔT was measured at +67.6439 seconds (TT ahead of UT1) at 0 h UTC on 1 January 2015; and by retrospective calculation, ΔT was close to zero about the year 1900. ΔT is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future. In fine detail, ΔT is somewhat unpredictable, with 10-year extrapolations diverging by 2-3 seconds from the actual value.
+
+== Relativistic relationships ==
+Observers in different locations, that are in relative motion or at different altitudes, can disagree about the rates of each other's clocks, owing to effects described by the theory of relativity.  As a result, TT (even as a theoretical ideal) does not match the proper time of all observers.
+In relativistic terms, TT is described as the proper time of a clock located on the geoid (essentially mean sea level).
+However,
+TT is now actually defined as a coordinate time scale.
+The redefinition did not quantitatively change TT, but rather made the existing definition more precise.  In effect it defined the geoid (mean sea level) in terms of a particular level of gravitational time dilation relative to a notional observer located at infinitely high altitude.
+The present definition of TT is a linear scaling of Geocentric Coordinate Time (TCG), which is the proper time of a notional observer who is infinitely far away (so not affected by gravitational time dilation) and at rest relative to Earth. TCG is used to date mainly for theoretical purposes in astronomy. From the point of view of an observer on Earth's surface the second of TCG passes in slightly less than the observer's SI second. The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster.
+
+== See also ==
+Barycentric Coordinate Time
+Geocentric Coordinate Time
+
+== Notes ==
+
+== References ==
+
+== External links ==
+BIPM technical services: Time Metrology
+Time and Frequency from A to Z

data/en.wikipedia.org/wiki/Tropical_year-0.md

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+---
+title: "Tropical year"
+chunk: 1/4
+source: "https://en.wikipedia.org/wiki/Tropical_year"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:34.155727+00:00"
+instance: "kb-cron"
+---
+
+A tropical year, or solar year (or tropical period), is the time that the Sun takes to return to the same position in the sky –  as viewed from the Earth or another celestial body of the Solar System –  thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.
+The tropical year is one type of astronomical year and a particular orbital period. Another type is the sidereal year (or sidereal orbital period), which is the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars, resulting in a duration of 20 minutes and 24.7 seconds longer than the tropical year due to the precession of the equinoxes.
+Since antiquity, astronomers have progressively refined the definition of the tropical year. The entry for "year, tropical" in the Astronomical Almanac Online Glossary states:
+
+the period of time for the ecliptic longitude of the Sun to increase 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of seasons, and its length is approximated in the long term by the civil (Gregorian) calendar. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds.
+An equivalent, more descriptive definition is "The natural basis for computing passing tropical years is the mean longitude of the Sun reckoned from the precessionally moving equinox (the dynamical equinox or equinox of date). Whenever the longitude reaches a multiple of 360 degrees, the mean Sun crosses the vernal equinox and a new tropical year begins".
+The mean tropical year in 2000 was 365.24219 ephemeris days, each ephemeris day lasting 86,400 SI seconds. This is 365.24217 mean solar days. For this reason, the calendar year is an approximation of the solar year: the Gregorian calendar (with its rules for catch-up leap days) is designed to resynchronize the calendar year with the solar year at regular intervals.
+
+== History ==
+
+=== Origin ===
+The word "tropical" comes from the Greek tropikos meaning "turn". Thus, the tropics of Cancer and Capricorn mark the extreme north and south latitudes where the Sun can appear directly overhead, and where it appears to "turn" in its annual seasonal motion. Because of this connection between the tropics and the seasonal cycle of the apparent position of the Sun, the word "tropical" was lent to the period of the seasonal cycle. The early Chinese, Hindus, Greeks, and others made approximate measures of the tropical year.
+
+=== Early value, precession discovery ===
+In the 2nd century BC, Hipparchus measured the time required for the Sun to travel from an equinox to the same equinox again. He reckoned the length of the year to be 1/300 of a day less than 365.25 days (365 days, 5 hours, 55 minutes, 12 seconds, or 365.24667 days). Hipparchus used this method because he was better able to detect the time of the equinoxes, compared to that of the solstices.
+Hipparchus also discovered that the equinoctial points moved along the ecliptic (plane of the Earth's orbit, or what Hipparchus would have thought of as the plane of the Sun's orbit about the Earth) in a direction opposite that of the movement of the Sun, a phenomenon that came to be named "precession of the equinoxes". He reckoned the value as 1° per century, a value that was not improved upon until about 1000 years later, by Islamic astronomers. Since this discovery, a distinction has been made between the tropical year and the sidereal year.
+
+=== Middle Ages and the Renaissance ===
+During the Middle Ages and Renaissance, several progressively better tables were published that allowed computation of the positions of the Sun, Moon and planets relative to the fixed stars. An important application of these tables was the reform of the calendar.
+The Alfonsine Tables, published in 1252, were based on the theories of Ptolemy and were revised and updated after the original publication. The length of the tropical year was given as 365 solar days, 5 hours, 49 minutes, 16 seconds (≈ 365.24255 days). This length was used in devising the Gregorian calendar of 1582.
+In Uzbekistan, Ulugh Beg's Zij-i Sultani was published in 1437 and gave an estimate of 365 solar days, 5 hours, 49 minutes, 15 seconds (365.242535 days).
+In the 16th century, Copernicus put forward a heliocentric cosmology. Erasmus Reinhold used Copernicus' theory to compute the Prutenic Tables in 1551, and gave a tropical year length of 365 solar days, 5 hours, 55 minutes, 58 seconds (365.24720 days), based on the length of a sidereal year and the presumed rate of precession. This was actually less accurate than the earlier value of the Alfonsine Tables.
+Major advances in the 17th century were made by Johannes Kepler and Isaac Newton. In 1609 and 1619, Kepler published his three laws of planetary motion. In 1627, Kepler used the observations of Tycho Brahe and Waltherus to produce the most accurate tables up to that time, the Rudolphine Tables. He evaluated the mean tropical year as 365 solar days, 5 hours, 48 minutes, 45 seconds (365.24219 days).
+Newton's three laws of dynamics and theory of gravity were published in his Philosophiæ Naturalis Principia Mathematica in 1687. Newton's theoretical and mathematical advances influenced tables by Edmond Halley published in 1693 and 1749 and provided the underpinnings of all solar system models until Albert Einstein's theory of General relativity in the 20th century.

data/en.wikipedia.org/wiki/Tropical_year-1.md

@@ -0,0 +1,31 @@
+---
+title: "Tropical year"
+chunk: 2/4
+source: "https://en.wikipedia.org/wiki/Tropical_year"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:34.155727+00:00"
+instance: "kb-cron"
+---
+
+=== 18th and 19th century ===
+From the time of Hipparchus and Ptolemy, the year was based on two equinoxes (or two solstices) some years apart, to average out both observational errors and periodic variations (caused by the gravitational pull of the planets, and the small effect of nutation on the equinox). These effects did not begin to be understood until Newton's time. To model short-term variations of the time between equinoxes (and prevent them from confounding efforts to measure long-term variations) requires precise observations and an elaborate theory of the apparent motion of the Sun. The necessary theories and mathematical tools came together in the 18th century due to the work of Pierre-Simon de Laplace, Joseph Louis Lagrange, and other specialists in celestial mechanics. They were able to compute periodic variations and separate them from the gradual mean motion. They could express the mean longitude of the Sun in a polynomial such as:
+
+L0 = A0 + A1T + A2T2 days
+where T is the time in Julian centuries. The derivative of this formula is an expression of the mean angular velocity, and the inverse of this gives an expression for the length of the tropical year as a linear function of T.
+Two equations are given in the table. Both equations estimate that the tropical year gets roughly a half-second shorter each century.
+
+Newcomb's tables were sufficiently accurate that they were used by the joint American-British Astronomical Almanac for the Sun, Mercury, Venus, and Mars through 1983.
+
+=== 20th and 21st centuries ===
+The length of the mean tropical year is derived from a model of the Solar System, so any advance that improves the solar system model potentially improves the accuracy of the mean tropical year. Many new observing instruments became available, including
+
+artificial satellites
+tracking of deep space probes such as Pioneer 4 beginning in 1959
+radars able to measure the distance to other planets beginning in 1961
+lunar laser ranging since the 1969 Apollo 11 left the first of a series of retroreflectors which allow greater accuracy than reflectorless measurements
+artificial satellites such as LAGEOS (1976) and the Global Positioning System (initial operation in 1993)
+very long baseline interferometry which finds precise directions to quasars in distant galaxies, and allows determination of the Earth's orientation with respect to these objects whose distance is so great they can be considered to show minimal space motion.
+The complexity of the model used for the Solar System must be limited to the available computational facilities. In the 1920s, punched card equipment came into use by L. J. Comrie in Britain. For the American Ephemeris an electromagnetic computer, the IBM Selective Sequence Electronic Calculator was used since 1948. When modern computers became available, it was possible to compute ephemerides using numerical integration rather than general theories; numerical integration came into use in 1984 for the joint US-UK almanacs.
+Albert Einstein's General Theory of Relativity provided a more accurate theory, but the accuracy of theories and observations did not require the refinement provided by this theory (except for the advance of the perihelion of Mercury) until 1984. Time scales incorporated general relativity beginning in the 1970s.
+A key development in understanding the tropical year over long periods of time is the discovery that the rate of rotation of the Earth, or equivalently, the length of the mean solar day, is not constant. William Ferrel in 1864 and Charles-Eugène Delaunay in 1865 predicted that the rotation of the Earth is being retarded by tides. This could be verified by observation only in the 1920s with the very accurate Shortt–Synchronome clock and later in the 1930s when quartz clocks began to replace pendulum clocks as time standards.

data/en.wikipedia.org/wiki/Tropical_year-2.md

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+---
+title: "Tropical year"
+chunk: 3/4
+source: "https://en.wikipedia.org/wiki/Tropical_year"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:34.155727+00:00"
+instance: "kb-cron"
+---
+
+== Time scales and calendar ==
+Apparent solar time is the time indicated by a sundial, and is determined by the apparent motion of the Sun caused by the rotation of the Earth around its axis as well as the revolution of the Earth around the Sun. Mean solar time is corrected for the periodic variations in the apparent velocity of the Sun as the Earth revolves in its orbit. The most important such time scale is Universal Time, which is the mean solar time at 0° longitude (the IERS Reference Meridian). Civil time is based on UT (actually UTC), and civil calendars count mean solar days.
+However, the rotation of the Earth itself is irregular and is slowing down, with respect to more stable time indicators: specifically, the motion of planets and atomic clocks.
+Ephemeris time (ET) is the independent variable in the equations of motion of the Solar System, in particular, the equations from Newcomb's work, and this ET was in use from 1960 to 1984. These ephemerides were based on observations made in solar time over a period of several centuries, and as a consequence represent the mean solar second over that period. The SI second, defined in atomic time, was intended to agree with the ephemeris second based on Newcomb's work, which in turn makes it agree with the mean solar second of the mid-19th century. ET as counted by atomic clocks was given a new name, Terrestrial Time (TT), and for most purposes ET = TT = TAI + 32.184 SI seconds. Since the era of the observations, the rotation of the Earth has slowed down and the mean solar second has grown somewhat longer than the SI second. As a result, the time scales of TT and UT1 build up a growing difference: the amount that TT is ahead of UT1 is known as ΔT, or Delta T. As of 5 July 2022, TT is ahead of UT1 by 69.28 seconds.
+As a consequence, the tropical year following the seasons on Earth, as counted in solar days of UT, is increasingly out of sync with expressions for equinoxes in ephemerides in TT.
+As explained below, long-term estimates of the length of the tropical year were used in connection with the reform of the Julian calendar, which resulted in the Gregorian calendar. Participants in that reform were unaware of the non-uniform rotation of the Earth, but now this can be taken into account to some degree. The table below gives Morrison and Stephenson's estimates and standard errors (σ) for ΔT at dates significant in the process of developing the Gregorian calendar.
+
+The low-precision extrapolations are computed with an expression provided by Morrison and Stephenson:
+
+ΔT in seconds = −20 + 32t2
+where t is measured in Julian centuries from 1820. The extrapolation is provided only to show that ΔT is not negligible when evaluating the calendar for long periods; Borkowski cautions that "many researchers have attempted to fit a parabola to the measured ΔT values to determine the magnitude of the deceleration of the Earth's rotation. The results, when taken together, are rather discouraging."
+
+== Length of tropical year ==
+
+One definition of the tropical year would be the time required for the Sun, beginning at a chosen ecliptic longitude, to make one complete cycle of the seasons and return to the same ecliptic longitude.
+
+=== Mean time interval between equinoxes ===
+
+Before considering an example, the equinox must be examined. There are two important planes in solar system calculations: the plane of the ecliptic (the Earth's orbit around the Sun), and the plane of the celestial equator (the Earth's equator projected into space). These two planes intersect in a line. One direction points to the so-called vernal, northward, or March equinox which is given the symbol ♈︎ (the symbol looks like the horns of a ram because it used to be toward the constellation Aries). The opposite direction is given the symbol ♎︎ (because it used to be toward Libra). Because of the precession of the equinoxes and nutation, these directions change, compared to the direction of distant stars and galaxies, whose directions have no measurable motion due to their great distance (see International Celestial Reference Frame).
+The ecliptic longitude of the Sun is the angle between ♈︎ and the Sun, measured eastward along the ecliptic. This creates a relative and not an absolute measurement, because as the Sun is moving, the direction from which the angle is measured is also moving. It is convenient to have a fixed (with respect to distant stars) direction to measure from; the direction of ♈︎ at noon January 1, 2000, fills this role and is given the symbol ♈︎0.
+There was an equinox on March 20, 2009, 11:44:43.6 TT. The 2010 March equinox was March 20, 17:33:18.1 TT, which gives an interval –  and a duration of the tropical year –  of 365 days, 5 hours, 48 minutes, 34.5 seconds. While the Sun moves, ♈︎ moves in the opposite direction. When the Sun and ♈︎ met at the 2010 March equinox, the Sun had moved east 359°59'09" while ♈︎ had moved west 51" for a total of 360° (all with respect to ♈︎0). This is why the tropical year is 20 minutes shorter than the sidereal year.
+When tropical year measurements from several successive years are compared, variations are found which are due to the perturbations by the Moon and planets acting on the Earth, and to nutation. Meeus and Savoie provided the following examples of intervals between March (northward) equinoxes:
+
+Until the beginning of the 19th century, the length of the tropical year was found by comparing equinox dates that were separated by many years; this approach yielded the mean tropical year.

data/en.wikipedia.org/wiki/Tropical_year-3.md

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+---
+title: "Tropical year"
+chunk: 4/4
+source: "https://en.wikipedia.org/wiki/Tropical_year"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:34.155727+00:00"
+instance: "kb-cron"
+---
+
+=== Different tropical year definitions ===
+If a different starting longitude for the Sun is chosen than 0° (i.e. ♈︎), then the duration for the Sun to return to the same longitude will be different. This is a second-order effect of the circumstance that the speed of the Earth (and conversely the apparent speed of the Sun) varies in its elliptical orbit: faster in the perihelion, slower in the aphelion. The equinox moves with respect to the perihelion (and both move with respect to the fixed sidereal frame). From one equinox passage to the next, or from one solstice passage to the next, the Sun completes not quite a full elliptic orbit. The time saved depends on where it starts in the orbit. If the starting point is close to the perihelion (such as the December solstice), then the speed is higher than average, and the apparent Sun saves little time for not having to cover a full circle: the "tropical year" is comparatively long. If the starting point is near aphelion, then the speed is lower and the time saved for not having to run the same small arc that the equinox has precessed is longer: that tropical year is comparatively short.
+The "mean tropical year" is based on the mean sun, and is not exactly equal to any of the times taken to go from an equinox to the next or from a solstice to the next.
+The following values of time intervals between equinoxes and solstices were provided by Meeus and Savoie for the years 0 and 2000. These are smoothed values which take account of the Earth's orbit being elliptical, using well-known procedures (including solving Kepler's equation). They do not take into account periodic variations due to factors such as the gravitational force of the orbiting Moon and gravitational forces from the other planets. Such perturbations are minor compared to the positional difference resulting from the orbit being elliptical rather than circular.
+
+=== Mean tropical year current value ===
+The mean tropical year on January 1, 2000, was 365.2421897 or 365 ephemeris days, 5 hours, 48 minutes, 45.19 seconds. This changes slowly; an expression suitable for calculating the length of a tropical year in ephemeris days, between 8000 BC and 12000 AD, is
+
+  
+    
+      
+        365.2421896698
+        −
+        6.15359
+        ×
+        
+          10
+          
+            −
+            6
+          
+        
+        T
+        −
+        7.29
+        ×
+        
+          10
+          
+            −
+            10
+          
+        
+        
+          T
+          
+            2
+          
+        
+        +
+        2.64
+        ×
+        
+          10
+          
+            −
+            10
+          
+        
+        
+          T
+          
+            3
+          
+        
+      
+    
+    {\displaystyle 365.2421896698-6.15359\times 10^{-6}T-7.29\times 10^{-10}T^{2}+2.64\times 10^{-10}T^{3}}
+  
+
+where T is in Julian centuries of 36,525 days of 86,400 SI seconds measured from noon January 1, 2000, TT.
+Modern astronomers define the tropical year as the time for the Sun's mean longitude to increase by 360°. The process for finding an expression for the length of the tropical year is to first find an expression for the Sun's mean longitude (with respect to ♈︎), such as Newcomb's expression given above, or Laskar's expression. When viewed over one year, the mean longitude is very nearly a linear function of Terrestrial Time. To find the length of the tropical year, the mean longitude is differentiated to give the angular speed of the Sun as a function of Terrestrial Time, and this angular speed is used to compute how long it would take for the Sun to move 360°.
+The above formulae give the length of the tropical year in ephemeris days (equal to 86,400 SI seconds), not solar days. It is the number of solar days in a tropical year that is important for keeping the calendar synchronized with the seasons (see below).
+
+== Calendar year ==
+The Gregorian calendar, as used for civil and scientific purposes, is an international standard. It is a solar calendar that is designed to maintain synchrony with the mean tropical year. It has a cycle of 400 years (146,097 days). Each cycle repeats the months, dates, and weekdays. The average year length is 146,097/400 = 365+97⁄400 = 365.2425 days per year, a close approximation to the mean tropical year of 365.2422 days.
+The Gregorian calendar is a reformed version of the Julian calendar organized by the Catholic Church and enacted in 1582. By the time of the reform, the date of the vernal equinox had shifted about 10 days, from about March 21 at the time of the First Council of Nicaea in 325, to about March 11. The motivation for the change was the correct observance of Easter. The rules used to compute the date of Easter used a conventional date for the vernal equinox (March 21), and it was considered important to keep March 21 close to the actual equinox.
+If society in the future still attaches importance to the synchronization between the civil calendar and the seasons, another reform of the calendar will eventually be necessary. According to Blackburn and Holford-Strevens (who used Newcomb's value for the tropical year), if the tropical year remained at its 1900 value of 365.24219878125 days, the Gregorian calendar would be between three hours and four days behind the Sun after 10,000 years. Aggravating this error, the length of the tropical year (measured in Terrestrial Time) is decreasing at a rate of approximately 0.53 seconds per century, the mean solar day is getting longer at a rate of about 1.5 ms per century, and length of the "tropical millennium" is decreasing by about 0.06 solar days per millennium (neglecting the oscillatory changes in the real length of the tropical year). These effects will cause the calendar to be as much as one day behind the Sun in 3200. As a result, many have suggested that the number of leap days should decrease as time goes on. One possible reform that has been proposed involves omitting the leap day in 3200, keeping 3600 and 4000 as leap years, and making all centennial years common except 4500, 5000, 5500, 6000, etc. (i.e. making centennial leap years occur once every 500 years instead of 400 starting from the year 4000), but the quantity ΔT is not sufficiently predictable to form more precise proposals.
+
+== See also ==
+Anomalistic year
+Gregorian calendar
+Sidereal and tropical astrology
+
+== Notes ==
+
+== References ==
+
+== Further reading ==
+Dershowitz, N.; Reingold, E.M. (2008). Calendrical calculations (3rd ed.). Cambridge University Press. ISBN 978-0-521-70238-6.
+Meeus, Jean (August 10, 2009) [1998]. Astronomical Algorithms (2nd, with corrections as of August 10, 2009 ed.). Richmond, VA: Willmann-Bell. ISBN 978-0-943396-61-3.
+Meeus, Jean (2002). More astronomical astronomy morsels. Richmond, VA: Willmann-Bell. ISBN 0-943396-74-3. Contains updates to Meeus & Savoie 1992.
+Simon, J. L.; Bretagnon, P.; Chapront, J.; Chapront-Touze, M.; Francou, G.; Laskar, J. (February 1994). "Numerical expressions for precession formulae and mean elements for the Moon and the planets". Astronomy and Astrophysics. 282: 663–683. Bibcode:1994A&A...282..663S. ISSN 0004-6361. Referenced in Astronomical almanac for the year 2011 and contains expressions used to derive the length of the tropical year.
+
+== External links ==
+ Media related to Tropical year at Wikimedia Commons

data/en.wikipedia.org/wiki/Tzolkinex-0.md

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+---
+title: "Tzolkinex"
+chunk: 1/1
+source: "https://en.wikipedia.org/wiki/Tzolkinex"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:35.358456+00:00"
+instance: "kb-cron"
+---
+
+The tzolkinex is an eclipse cycle equal to a period of two saros (13,170.636 days) minus one inex (10,571.946 days).  As consecutive eclipses in an inex series belongs to the next consecutive saros series, each consecutive tzolkinex belongs to the previous saros series.
+The tzolkinex is equal to 2598.691 days (about 7 years, 1 month and 12 days). It is related to the tritos in that a period of one tritos plus one tzolkinex is exactly equal to one saros. It is also related to the inex in that a period of one inex plus one tzolkinex is exactly equal to two saros.
+It corresponds to:
+
+88 synodic months
+95.49723 draconic months
+7.49723 eclipse years (15 eclipse seasons)
+94.31081 anomalistic months.
+Because of the non-integer number of anomalistic month each eclipse varies in type, i.e. total vs. annular, and greatly varies in length. From remainder of 0.31081, being near 1⁄3, every third tzolkinex comes close to an even number of anomalistic months, but occurs during a different season, and in the opposite hemisphere, thus they may be of the same type (annular vs. total) but otherwise do not have a similar character.
+
+
+== Details ==
+It was first studied by George van den Bergh (1951). The name was suggested by Felix Verbelen (2001) because its length is nearly 10 Tzolk'ins (260-day periods).
+It alternates hemispheres with each cycle, occurring at alternating nodes, each successive occurrence is one saros less than the last.
+
+
+== See also ==
+Eclipse cycle
+
+
+== References ==

data/en.wikipedia.org/wiki/Universal_Time-0.md

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+---
+title: "Universal Time"
+chunk: 1/2
+source: "https://en.wikipedia.org/wiki/Universal_Time"
+category: "reference"
+tags: "science, encyclopedia"
+date_saved: "2026-05-05T11:13:36.566553+00:00"
+instance: "kb-cron"
+---
+
+Universal Time (UT or UT1) is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle (ERA, which serves as the replacement for Greenwich Mean Sidereal Time). UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
+
+where Tu = (Julian UT1 date − 2451545.0).
+
+== History ==
+Prior to the introduction of standard time, each municipality throughout the clock-using world set its official clock, if it had one, according to the local position of the Sun (see solar time). This served adequately until the introduction of rail travel in Britain, which made it possible to travel fast enough over sufficiently long distances as to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Starting in 1847, Britain established Greenwich Mean Time (GMT), the mean solar time at Greenwich, England, to solve this problem: all clocks in Great Britain were set to this time regardless of local solar noon. Using telescopes, GMT was calibrated to the mean solar time at the prime meridian through the Royal Observatory, Greenwich. Chronometers or telegraphy were used to synchronize these clocks.
+
+As international commerce increased, the need for an international standard of time measurement emerged. Several authors proposed a "universal" or "cosmic" time (see Time zone § Worldwide time zones). The development of Universal Time began at the International Meridian Conference. At the end of this conference, on 22 October 1884, the recommended base reference for world time, the "universal day", was announced to be the local mean solar time at the Royal Observatory in Greenwich, counted from 0 hours at Greenwich mean midnight. This agreed with the civil Greenwich Mean Time used on the island of Great Britain since 1847. In contrast, astronomical GMT began at mean noon, i.e. astronomical day X began at noon of civil day X. The purpose of this was to keep one night's observations under one date. The civil system was adopted as of 0 hours (civil) 1 January 1925. Nautical GMT began 24 hours before astronomical GMT, at least until 1805 in the Royal Navy, but persisted much later elsewhere because it was mentioned at the 1884 conference. Greenwich was chosen because by 1884 two-thirds of all nautical charts and maps already used it as their prime meridian.
+During the period between 1848 and 1972, all of the major countries adopted time zones based on the Greenwich meridian.
+In 1928, the term Universal Time (UT) was introduced by the International Astronomical Union to refer to GMT, with the day starting at midnight. The term was recommended as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. As the general public had always begun the day at midnight, the timescale continued to be presented to them as Greenwich Mean Time.
+When introduced, broadcast time signals were based on UT, and hence on the rotation of the Earth. In 1955 the BIH adopted a proposal by William Markowitz, effective 1 January 1956, dividing UT into UT0 (UT as formerly computed), UT1 (UT0 corrected for polar motion) and UT2 (UT0 corrected for polar motion and seasonal variation). UT1 was the version sufficient for "many astronomical and geodetic applications", while UT2 was to be broadcast over radio to the public.
+UT0 and UT2 soon became irrelevant due to the introduction of Coordinated Universal Time (UTC). Starting in 1956, WWV broadcast an atomic clock signal stepped by 20 ms increments to bring it into agreement with UT1. The error from UT1 of up to 20 ms is on the same order of magnitude as the differences between UT0, UT1, and UT2. By 1960, the U.S. Naval Observatory, the Royal Greenwich Observatory, and the UK National Physical Laboratory had developed UTC, with a similar stepping approach. The 1960 URSI meeting recommended that all time services should follow the lead of the UK and US and broadcast coordinated time using a frequency offset from cesium aimed to match the predicted progression of UT2 with occasional steps as needed. Starting 1 January 1972, UTC was defined to follow UT1 within 0.9 seconds rather than UT2, marking the decline of UT2.
+Modern civil time generally follows UTC. In some countries, the term Greenwich Mean Time persists in common usage to this day in reference to UT1, in civil timekeeping as well as in astronomical almanacs and other references. Whenever a level of accuracy better than one second is not required, UTC can be used as an approximation of UT1. The difference between UT1 and UTC is known as DUT1.
+
+=== Adoption in various countries ===
+The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.
+
+Apart from Nepal Standard Time (UTC+05:45), the Chatham Standard Time Zone (UTC+12:45) used in New Zealand's Chatham Islands and the officially unsanctioned Central Western Time Zone (UTC+8:45) used in Eucla, Western Australia and surrounding areas, all time zones in use are defined by an offset from UTC that is a multiple of half an hour, and in most cases a multiple of an hour.
+
+== Measurement ==
+
+Historically, Universal Time was computed from observing the position of the Sun in the sky. But astronomers found that it was more accurate to measure the rotation of the Earth by observing stars as they crossed the meridian each day. Nowadays, UT in relation to International Atomic Time (TAI) is determined by very-long-baseline interferometry (VLBI) observations of the positions of distant celestial objects (stars and quasars), a method which can determine UT1 to within 15 microseconds or better.