# kb

id: jens-olsen-s-world-clock-20260505-1
file: data/en.wikipedia.org/wiki/Jens_Olsen's_World_Clock-1.md
category: reference
title: Jens Olsen's World Clock
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Jens_Olsen's_World_Clock
date_saved: 2026-05-05T11:12:52.768969+00:00
instance: kb-cron
parent: jens-olsen-s-world-clock-20260505-0
summary:
==== Sunrise/Sunset ==== The Sunrise/Sunset movement is the rightmost movement of the left section, and includes a part of shutters that move throughout the year to indicate the time of sunrise and sunset. These can be read from an inner stationary 24-hour solar time dial or an outer 24-hour mean time dial (compensating for the EoT). ==== Gregorian Calendar ==== The Gregorian Calendar is located at the bottom of the left section. It displays the year, month, day of month, and day of week. These are changed discontinuously at mean midnight, and are otherwise not moving. ==== Stellar Heavens ==== The Stellar Heavens movement is the top dial of the right section, and shows the current overhead celestial sphere. This is displayed using a stereoscopic projection (similar to an astrolabe), with fixed threads showing reference lines for meridian and zenith in the local frame and tropics, equator, and circumpolar circle in the celestial frame. The polar precession circle is also marked, which is notably the slowest motion in the clock. ==== Heliocentric Revolution ==== The Heliocentric Revolution movement is located on the right side of the right section and is functionally an orrery, showing the 8 planets rotating about a


id: jens-olsen-s-world-clock-20260505-2
file: data/en.wikipedia.org/wiki/Jens_Olsen's_World_Clock-2.md
category: reference
title: Jens Olsen's World Clock
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Jens_Olsen's_World_Clock
date_saved: 2026-05-05T11:12:52.768969+00:00
instance: kb-cron
parent: jens-olsen-s-world-clock-20260505-0
summary:
Rotational information is transmitted via sets of bevels gears at both the source and destination module, with a shaft between them. These are continuously rotating (unbounded), although not always constant speeds. The gearing at both modules is always configured such that the shaft rotates at a meaningful rate (such as 1 rotation per mean day). Note that this is not the simplest design but helps observers understand more readily, likely by intent. Linear translations are transmitted via steel ribbons. These are analog signals of limited range (bounded), such as the current value of the Equation of Time or various solar and lunar anomalies. The ribbons generally go upwards to the top of the case, where there is a bell-crank system to transfer them horizontally to above the relevant destination movement, from which they can descend. Impulses are transferred via rods. These are discretely occurring signals that trigger discontinuous events, such as midnight causing the date to change. These also use the bell-crank system in the top of the case == See also == Clock of the Long Now Rasmus Sørnes == Further reading == Otto Mortensen Jens Olsen's Clock: A Technical Description Technological Institute, Copenhagen, 1957. == References == ==


id: julian-day-20260505-0
file: data/en.wikipedia.org/wiki/Julian_day-0.md
category: reference
title: Julian day
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Julian_day
date_saved: 2026-05-05T11:12:53.938677+00:00
instance: kb-cron
parent: julian-day-20260505-0
summary:
The Julian day is a continuous count of days from the beginning of the Julian period; it is used primarily by astronomers, and in software for easily calculating elapsed days between two events (e.g., food production date and expiration date). The Julian period is a chronological interval of 7980 years, derived from three multi-year cycles: the Indiction, Solar, and Lunar cycles. The last year that was simultaneously the beginning of all three cycles was 4713 BC (−4712), so that is year 1 of the current Julian period, making AD 2026 year 6739 of that Period. The next Julian Period begins in the year AD 3268. Historians used the period to identify Julian calendar years within which an event occurred when no such year was given in the historical record, or when the year given by previous historians was incorrect. The Julian day number (JDN) has the same epoch as the Julian period, but counts the number of days since the epoch rather than the number of years since then. Specifically, Julian day number 0 is assigned to the day starting at noon Universal Time on Monday, January 1, 4713 BC, proleptic Julian calendar (November 24, 4714 BC, in the proleptic


id: julian-day-20260505-1
file: data/en.wikipedia.org/wiki/Julian_day-1.md
category: reference
title: Julian day
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Julian_day
date_saved: 2026-05-05T11:12:53.938677+00:00
instance: kb-cron
parent: julian-day-20260505-0
summary:
The Modified Julian Date (MJD) was introduced by the Smithsonian Astrophysical Observatory in 1957 to record the orbit of Sputnik via an IBM 704 (36-bit machine) and using only 18 bits until August 7, 2576. MJD is the epoch of VAX/VMS and its successor OpenVMS, using 63-bit date/time, which allows times to be stored up to July 31, 31086, 02:48:05.47. The MJD has a starting point of midnight on November 17, 1858, and is computed by MJD = JD − 2400000.5 The Truncated Julian Day (TJD) was introduced by NASA/Goddard in 1979 as part of a parallel grouped binary time code (PB-5) "designed specifically, although not exclusively, for spacecraft applications". TJD was a 4-digit day count from MJD 40000, which was May 24, 1968, represented as a 14-bit binary number. Since this code was limited to four digits, TJD recycled to zero on MJD 50000, or October 10, 1995, "which gives a long ambiguity period of 27.4 years". (NASA codes PB-1–PB-4 used a 3-digit day-of-year count.) Only whole days are represented. Time of day is expressed by a count of seconds of a day, plus optional milliseconds, microseconds and nanoseconds in separate fields. Later PB-5J was introduced which increased the


id: julian-day-20260505-2
file: data/en.wikipedia.org/wiki/Julian_day-2.md
category: reference
title: Julian day
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Julian_day
date_saved: 2026-05-05T11:12:53.938677+00:00
instance: kb-cron
parent: julian-day-20260505-0
summary:
where a is the year of the indiction cycle, b of the lunar cycle, and c of the solar cycle. John Collins described the details of how these three numbers were calculated in 1666, using many trials. A summary of Collin's description is in a footnote. Reese, Everett and Craun reduced the dividends in the Try column from 285, 420, 532 to 5, 2, 7 and changed remainder to modulo, but apparently still required many trials. The specific cycles used by Scaliger to form his tricyclic Julian Period were, first, the indiction cycle with a first year of 313. Then he chose the dominant 19-year Alexandrian lunar cycle with a first year of 285, the Era of Martyrs and the Diocletian Era epoch, or a first year of 532 according to Dionysius Exiguus. Finally, Scaliger chose the post-Bedan solar cycle with a first year of 776, when its first quadrennium of concurrents, 1 2 3 4, began in sequence. Although not their intended use, the equations of de Billy or Gauss can be used to determined the first year of any 15-, 19-, and 28-year tricyclic period given any first years of their cycles. For those of the Julian Period,


id: julian-day-20260505-3
file: data/en.wikipedia.org/wiki/Julian_day-3.md
category: reference
title: Julian day
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Julian_day
date_saved: 2026-05-05T11:12:53.938677+00:00
instance: kb-cron
parent: julian-day-20260505-0
summary:
The period thus arising of 7980 Julian years, is called the Julian period, and it has been found so useful, that the most competent authorities have not hesitated to declare that, through its employment, light and order were first introduced into chronology. We owe its invention or revival to Joseph Scaliger, who is said to have received it from the Greeks of Constantinople. The first year of the current Julian period, or that of which the number in each of the three subordinate cycles is 1, was the year 4713 BC, and the noon of January 1 of that year, for the meridian of Alexandria, is the chronological epoch, to which all historical eras are most readily and intelligibly referred, by computing the number of integer days intervening between that epoch and the noon (for Alexandria) of the day, which is reckoned to be the first of the particular era in question. The meridian of Alexandria is chosen as that to which Ptolemy refers the commencement of the era of Nabonassar, the basis of all his calculations. At least one mathematical astronomer adopted Herschel's "days of the Julian period" immediately. Benjamin Peirce of Harvard University used over 2,800 Julian days


id: julian-day-20260505-4
file: data/en.wikipedia.org/wiki/Julian_day-4.md
category: reference
title: Julian day
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Julian_day
date_saved: 2026-05-05T11:12:53.938677+00:00
instance: kb-cron
parent: julian-day-20260505-0
summary:
== Julian day number calculation == The Julian day number can be calculated using the following formulas (integer division rounding towards zero is used exclusively, that is, positive values are rounded down and negative values are rounded up): The months January to December are numbered 1 to 12. For the year, astronomical year numbering is used, thus 1 BC is 0, 2 BC is −1, and 4713 BC is −4712. JDN is the Julian Day Number. Use the previous day of the month if trying to find the JDN of an instant before midday UT. === Converting Gregorian calendar date to Julian day number === The algorithm is valid for all (possibly proleptic) Gregorian calendar dates after November 23, −4713. Divisions are integer divisions towards zero; fractional parts are ignored. JDN = 1461 ⋅ ( Y + 4800 + M − 14 12 ) 4 + 367 ⋅ ( M − 2 − 12 ⋅ M − 14 12 ) 12 − 3 ⋅ Y + 4900 + M − 14 12 100 4 + D − 32075 {\displaystyle {\text{JDN}}={\frac {1461\cdot \left({\text{Y}}+4800+{\frac {{\text{M}}-14}{12}}\right)}{4}}+{\frac {367\cdot \left({\text{M}}-2-12\cdot {\frac {{\text{M}}-14}{12}}\right)}{12}}-{\frac {3\cdot {\frac {{\text{Y}}+4900+{\frac {{\text{M}}-14}{12}}}{100}}}{4}}+{\text{D}}-32075} === Converting Julian calendar date to Julian day number


id: julian-year-astronomy-20260505-0
file: data/en.wikipedia.org/wiki/Julian_year_(astronomy)-0.md
category: reference
title: Julian year (astronomy)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Julian_year_(astronomy)
date_saved: 2026-05-05T11:12:55.113997+00:00
instance: kb-cron
parent: julian-year-astronomy-20260505-0
summary:
In astronomy, a Julian year (symbol: a or aj) is a unit of measurement of time defined as exactly 365.25 days of 86400 SI seconds each. The length of the Julian year is the average length of the year in the Julian calendar that was used in Western societies until the adoption of the Gregorian Calendar, and from which the unit is named. Nevertheless, because astronomical Julian years are measuring duration rather than designating dates, this Julian year does not correspond to years in the Julian calendar or any other calendar, nor does it correspond to the many other ways of defining a year. == Usage == The Julian year is not a unit of measurement in the International System of Units (SI), but it is recognized by the International Astronomical Union (IAU) as a non-SI unit for use in astronomy. Before 1984, both the Julian year and the mean tropical year were used by astronomers. In 1898, Simon Newcomb used both in his Tables of the Sun in the form of the Julian century (36 525 days) and the "solar century" (36524.22 days), a rounded form of 100 mean tropical years of 365.24219879 days each according to Newcomb. However,


id: local-mean-time-20260505-0
file: data/en.wikipedia.org/wiki/Local_mean_time-0.md
category: reference
title: Local mean time
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Local_mean_time
date_saved: 2026-05-05T11:12:56.287689+00:00
instance: kb-cron
parent: local-mean-time-20260505-0
summary:
Local mean time (LMT) is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude. This measurement of time was used for everyday use during the 19th century before time zones were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation. The difference between local mean time and local apparent time is the equation of time. == Past use == Local solar time or sundial time was used until the early 19th century when accurate mechanical clocks became commonplace. Local mean time continued in use until various countries adopted a national standard time. Each town or city kept its own meridian, so locations one degree of longitude apart had times four minutes apart. This became a problem in the mid 19th century when railways needed clocks for railway time that were synchronized between stations, while local people needed to match their clock (or the church clock) to the time tables. Standard time means that the same time is used throughout some regional time zone—usually, it is at an offset from Greenwich Mean Time or the local mean time of the capital


id: lunar-theory-20260505-0
file: data/en.wikipedia.org/wiki/Lunar_theory-0.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
Lunar theory attempts to account for the motions of the Moon. There are many small variations (or perturbations) in the Moon's motion, and many attempts have been made to account for them. After centuries of being problematic, lunar motion can now be modeled to a very high degree of accuracy (see section Modern developments). Lunar theory includes: the background of general theory; including mathematical techniques used to analyze the Moon's motion and to generate formulae and algorithms for predicting its movements; and also quantitative formulae, algorithms, and geometrical diagrams that may be used to compute the Moon's position for a given time; often by the help of tables based on the algorithms. Lunar theory has a history of over 2000 years of investigation. Its more modern developments have been used over the last three centuries for fundamental scientific and technological purposes, and are still being used in that way. == Applications == Applications of lunar theory have included the following: In the eighteenth century, comparison between lunar theory and observation was used to test Newton's law of universal gravitation by the motion of the lunar apogee. In the eighteenth and nineteenth centuries, navigational tables based on lunar theory, initially in


id: lunar-theory-20260505-1
file: data/en.wikipedia.org/wiki/Lunar_theory-1.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
The synodic month, i.e. the mean period for the phases of the Moon. Now called "System B", it reckons the synodic month as 29 days and (sexagesimally) 3,11;0,50 "time degrees", where each time degree is one degree of the apparent motion of the stars, or 4 minutes of time, and the sexagesimal values after the semicolon are fractions of a time degree. This converts to 29.530594 days = 29d 12h 44m 3.33s, to compare with a modern value (as at 1900 Jan 0) of 29.530589 days, or 29d 12h 44m 2.9s. This same value was used by Hipparchos and Ptolemy, was used throughout the Middle Ages, and still forms the basis of the Hebrew calendar. The mean lunar velocity relative to the stars they estimated at 13° 10′ 35″ per day, giving a corresponding month of 27.321598 days, to compare with modern values of 13° 10′ 35.0275″ and 27.321582 days. The anomalistic month, i.e. the mean period for the Moon's approximately monthly accelerations and decelerations in its rate of movement against the stars, had a Babylonian estimate of 27.5545833 days, to compare with a modern value 27.554551 days. The draconitic month, i.e. the mean period with which the path of


id: lunar-theory-20260505-2
file: data/en.wikipedia.org/wiki/Lunar_theory-2.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
The nodes and the inclination of the lunar orbital plane both appear to librate, with a monthly (according to Tycho) or semi-annual period (according to Kepler). The lunar longitude has a twice-monthly Variation, by which the Moon moves faster than expected at new and full moon, and slower than expected at the quarters. There is also an annual effect, by which the lunar motion slows down a little in January and speeds up a little in July: the annual equation. The refinements of Brahe and Kepler were recognized by their immediate successors as improvements, but their seventeenth-century successors tried numerous alternative geometrical configurations for the lunar motions to improve matters further. A notable success was achieved by Jeremiah Horrocks, who proposed a scheme involving an approximate 6 monthly libration in the position of the lunar apogee and also in the size of the elliptical eccentricity. This scheme had the great merit of giving a more realistic description of the changes in distance, diameter and parallax of the Moon. === Newton === A first gravitational period for lunar theory started with the work of Newton. He was the first to define the problem of the perturbed motion of the Moon in


id: lunar-theory-20260505-3
file: data/en.wikipedia.org/wiki/Lunar_theory-3.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
Newton's diagrammatic scheme, since his time, has been represented in other and perhaps visually clearer ways. Shown here is a vector presentation indicating, for two different positions, P1 and P2, of the Moon in its orbit around the Earth, the respective vectors LS1 and LS2 for the perturbing acceleration due to the Sun. The Moon's position at P1 is fairly close to what it was at P in Newton's diagram; corresponding perturbation LS1 is like Newton's LS in size and direction. At another position P2, the Moon is farther away from the Sun than the Earth is, the Sun's attraction LQ2 on the Moon is weaker than the Sun's attraction SQ=SQ2 on the Earth, and then the resulting perturbation LS2 points obliquely away from the Sun. Constructions like those in Newton's diagram can be repeated for many different positions of the Moon in its orbit. For each position, the result is a perturbation vector like LS1 or LS2 in the second diagram. Shown here is an often-presented form of the diagram that summarises sizes and directions of the perturbation vectors for many different positions of the Moon in its orbit. Each small arrow is a perturbation vector like LS, applicable


id: lunar-theory-20260505-4
file: data/en.wikipedia.org/wiki/Lunar_theory-4.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
the mean motions or positions of the Moon and the Sun, together with three coefficients and three angular positions, which together define the shape and location of their apparent orbits: the two eccentricities ( e {\displaystyle e} , about 0.0549, and e ′ {\displaystyle e'} , about 0.01675) of the ellipses that approximate to the apparent orbits of the Moon and the Sun; the angular direction of the perigees ( Γ {\displaystyle \Gamma } and Γ ′ {\displaystyle \Gamma '} ) (or their opposite points the apogees) of the two orbits; and the angle of inclination ( i {\displaystyle i} , mean value about 18523") between the planes of the two orbits, together with the direction ( Ω {\displaystyle \Omega } ) of the line of nodes in which those two planes intersect. The ascending node ( Ω {\displaystyle \Omega } ) is the node passed by the Moon when it is tending northwards relative to the ecliptic. From these basic parameters, just four basic differential angular arguments are enough to express, in their different combinations, nearly all of the most significant perturbations of the lunar motions. They are given here with their conventional symbols due to Delaunay; they are


id: lunar-theory-20260505-5
file: data/en.wikipedia.org/wiki/Lunar_theory-5.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
==== Reduction to the ecliptic ==== The reduction to the ecliptic represents the geometric effect of expressing the Moon's motion in terms of a longitude in the plane of the ecliptic, although its motion is really taking place in a plane that is inclined by about 5 degrees. Its principal term is − 412 ″ sin ⁡ ( 2 F ) {\displaystyle -412''\sin(2F)} . The analysts of the mid-18th century expressed the perturbations of the Moon's position in longitude using about 25-30 trigonometrical terms. However, work in the nineteenth and twentieth century led to very different formulations of the theory so these terms are no longer current. The number of terms needed to express the Moon's position with the accuracy sought at the beginning of the twentieth century was over 1400; and the number of terms needed to emulate the accuracy of modern numerical integrations based on laser-ranging observations is in the tens of thousands: there is no limit to the increase in number of terms needed as requirements of accuracy increase. == Modern developments == === Digital computers and lunar laser ranging === Since the Second World War and especially since the 1960s, lunar theory has been further developed


id: lunar-theory-20260505-6
file: data/en.wikipedia.org/wiki/Lunar_theory-6.md
category: reference
title: Lunar theory
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_theory
date_saved: 2026-05-05T11:12:57.491453+00:00
instance: kb-cron
parent: lunar-theory-20260505-0
summary:
== Bibliography == 'AE 1871': "Nautical Almanac & Astronomical Ephemeris" for 1871, (London, 1867). E W Brown (1896). An Introductory Treatise on the Lunar Theory, Cambridge University Press. E W Brown. "Theory of the Motion of the Moon", Memoirs of the Royal Astronomical Society, 53 (1897), 39–116. E W Brown. "Theory of the Motion of the Moon", Memoirs of the Royal Astronomical Society, 53 (1899), 163–202. E W Brown. "Theory of the Motion of the Moon", Memoirs of the Royal Astronomical Society, 54 (1900), 1–63. E W Brown. "On the verification of the Newtonian law", Monthly Notes of the Royal Astronomical Society 63 (1903), 396–397. E W Brown. "Theory of the Motion of the Moon", Memoirs of the Royal Astronomical Society, 57 (1905), 51–145. E W Brown. "Theory of the Motion of the Moon", Memoirs of the Royal Astronomical Society, 59 (1908), 1–103. E W Brown (1919). Tables of the Motion of the Moon, New Haven. M Chapront-Touzé & J Chapront. "The lunar ephemeris ELP-2000", Astronomy & Astrophysics 124 (1983), 50–62. M Chapront-Touzé & J Chapront: "ELP2000-85: a semi-analytical lunar ephemeris adequate for historical times", Astronomy & Astrophysics 190 (1988), 342–352. M Chapront-Touzé & J Chapront, Analytical Ephemerides of the


id: metonic-cycle-20260505-0
file: data/en.wikipedia.org/wiki/Metonic_cycle-0.md
category: reference
title: Metonic cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Metonic_cycle
date_saved: 2026-05-05T11:12:58.748160+00:00
instance: kb-cron
parent: metonic-cycle-20260505-0
summary:
The Metonic cycle or enneadecaeteris (from Ancient Greek: ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these whole numbers facilitates the construction of a lunisolar calendar. A tropical year (about 365.24 days) is longer than 12 lunar months (about 354.36 days) and shorter than 13 of them (about 383.90 days). In a Metonic calendar (a type of lunisolar calendar), there are twelve years of 12 lunar months and seven years of 13 lunar months. == Application in traditional calendars == In the Babylonian and Hebrew lunisolar calendars, the years 3, 6, 8, 11, 14, 17, and 19 are the long (13-month) years of the Metonic cycle. This cycle forms the basis of the Greek and Hebrew calendars. A 19-year cycle is used for the computation of the date of Easter each year.


id: morning-20260505-0
file: data/en.wikipedia.org/wiki/Morning-0.md
category: reference
title: Morning
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Morning
date_saved: 2026-05-05T11:12:59.945120+00:00
instance: kb-cron
parent: morning-20260505-0
summary:
Morning is either the period from sunrise to noon, or the period from midnight to noon. In the first definition it is preceded by the twilight period of dawn, and there are no exact times for when morning begins (also true of evening and night) because it can vary according to one's latitude, and the hours of daytime at each time of year. However, morning strictly ends at noon, when afternoon starts. Morning precedes afternoon, evening, and night in the sequence of a day. Originally, the term referred to sunrise. == Etymology == The Late Modern English words morning and tomorrow began in Early Middle English as morwening, developing into morwen, then morwe, and eventually morrow. English, unlike some other languages, has separate terms for morning and tomorrow, despite their common root. Other languages, like Dutch, Scots and German, may use a single word – morgen – to signify both morning and tomorrow. == Significance == === Cultural implications === Morning prayer is a common practice in several religions. The morning period includes specific phases of the Liturgy of the Hours of Christianity. Some languages that use the time of day in greeting have a special greeting for morning, such


id: night-20260505-0
file: data/en.wikipedia.org/wiki/Night-0.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
Night, or nighttime, is the period of darkness when the Sun is below the horizon. Daylight illuminates one side of the Earth, leaving the other in darkness. The opposite of nighttime is daytime. Earth's rotation causes the appearance of sunrise and sunset. Moonlight, airglow, starlight, and light pollution dimly illuminate night. The duration of day, night, and twilight varies depending on the time of year and the latitude. Night on other celestial bodies is affected by their rotation and orbital periods. The planets Mercury and Venus have much longer nights than Earth. On Venus, night lasts about 58 Earth days. The Moon's rotation is tidally locked, rotating so that one of the sides of the Moon always faces Earth. Nightfall across portions of the near side of the Moon results in lunar phases visible from Earth. Organisms respond to the changes brought by nightfall: darkness, increased humidity, and lower temperatures. Their responses include direct reactions and adjustments to circadian rhythms governed by an internal biological clock. These circadian rhythms, regulated by exposure to light and darkness, affect an organism's behavior and physiology. Animals more active at night are called nocturnal and have adaptations for low light, including different forms of


id: night-20260505-1
file: data/en.wikipedia.org/wiki/Night-1.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
The length of night on Earth varies depending on the time of year. Longer nights occur in winter, with the winter solstice being the longest. Nights are shorter in the summer, with the summer solstice being the shortest. Earth orbits the Sun on an axis tilted 23.44 degrees. Nights are longer when a hemisphere is tilted away from the Sun and shorter when a hemisphere is tilted toward the Sun. As a result, the longest night of the year for the Northern Hemisphere will be the shortest night of the year for the Southern Hemisphere. Night's duration varies least near the equator. The difference between the shortest and longest night increases approaching the poles. At the equator, night lasts roughly 12 hours throughout the year. The tropics have little difference in the length of day and night. At the 45th parallel, the longest winter night is roughly twice as long as the shortest summer night. Within the polar circles, night will last the full 24 hours of the winter solstice. The length of this polar night increases closer to the poles. Utqiagvik, Alaska, the northernmost point in the United States, experiences 65 days of polar night. At the pole itself,


id: night-20260505-2
file: data/en.wikipedia.org/wiki/Night-2.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
Living organisms react directly to the darkness of night. Light and darkness also affect circadian rhythms, the physical and mental changes that occur in a 24-hour cycle. This daily cycle is regulated by an internal "biological clock" that is adjusted by exposure to light. The length and timing of nighttime depend on location and time of year. Organisms that are more active at night possess adaptations to the night's dimmer light, increased humidity, and lower temperatures. === Animals === Animals that are active primarily at night are called nocturnal and usually possess adaptations for night vision. In vertebrates' eyes, two types of photoreceptor cells sense light. Cone cells sense color but are ineffective in low light; rod cells sense only brightness but remain effective in very dim light. The eyes of nocturnal animals have a greater percentage of rod cells. In most mammals, rod cells contain densely packed DNA near the edge of the nucleus. For nocturnal mammals, this is reversed with the densely packed DNA in the center of the nucleus, which reduces the scattering of light. Some nocturnal animals have a mirror, the tapetum lucidum, behind the retina. This doubles the amount of light their eyes can process.


id: night-20260505-3
file: data/en.wikipedia.org/wiki/Night-3.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
During the day, plants engage in photosynthesis and release oxygen. By night, plants engage in respiration, consuming oxygen and releasing carbon dioxide. Plants can draw up more water after sunset, which facilitates new leaf growth. As plants cannot create energy through photosynthesis after sunset, they use energy stored in the plant, typically as starch granules. Plants use this stored energy at a steady rate, depleting their reserves almost right at dawn. Plants will adjust their rate of consumption to match the expected time until sunrise. This avoids prematurely running out of starch reserves, and it allows the plant to adjust for longer nights in the winter. If a plant is subjected to artificially early darkness, it will ration its energy consumption to last until dawn. Succulent plants, including cacti, have adapted to the limited water availability in arid environments like deserts. The stomata of cacti do not open until night. When the temperature drops, the pores open to allow the cacti to store carbon dioxide for photosynthesis the next day, a process known as crassulacean acid metabolism (CAM). Cacti and night-blooming plants use CAM to store up to 99% of the carbon dioxide they use in daily photosynthesis. Ceroid cacti


id: night-20260505-4
file: data/en.wikipedia.org/wiki/Night-4.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
Artificial lighting expanded the scope of acceptable work and leisure after dark. In the 1600s, the major European cities introduced streetlights. These were lit by lamplighters each evening outside of the summer months. Early streetlights were metal and glass enclosures housing candles or oil lamps. They were suspended above streets or mounted on posts. The use of artificial lighting led to an increase in acceptable nightlife. In more rural areas, night remained a period of rest and nocturnal labor. Young adults, the urban poor, prostitutes, and thieves benefited from the anonymity of darkness and frequently smashed the new lanterns. Gas lighting was invented in the 1800s. A gas mantle was over ten times brighter than an oil lamp. Gas lighting was associated with the creation of regular police forces. In England, police departments were tasked with maintaining the gas lights, which became known as "police lamps". Daytime routines were further pushed back into the night by the electric light bulb—invented in the late 19th century—and the widespread usage of newer timekeeping devices like watches. Electric lights created night shifts for traditionally daytime fields, like India's cotton industry, and created opportunities for working adults to attend night school. === Fear of


id: night-20260505-5
file: data/en.wikipedia.org/wiki/Night-5.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
Many times in the night season, there have been certain spirits heard softly going or spitting or groaning, who being asked what they were have made answer that they were the souls of this or that man and that they now endure extreme torments. In folklore, nocturnal preternatural beings like goblins, fairies, werewolves, pucks, brownies, banshees, and boggarts have overlapping but non-synonymous definitions. The werewolf—and its francophone variations, the loup-garou and rougarou—were believed to be people who transformed into beasts at night. In West Africa and among the African diaspora, there is a widespread tradition of a type of vampire who removes their human skin at night and travels as a blood-sucking ball of light. Variation includes the feu-follet, the Surinamese asema, the Caribbean sukuyan, the Ashanti obayifo, and the Ghanaian asanbosam. The medieval fear of night-flying European witches was influenced by the Roman strix. The Romans described the strix as capable of changing between a beautiful woman and an owl-shaped monster. Common themes among these mythical nocturnal entities include hypersexuality, predation, shapeshifting, deception, mischief, and malice. === Nightlife === Nightlife, sometimes referred to as "the night-time economy", is a range of entertainment available and generally more popular from the


id: night-20260505-6
file: data/en.wikipedia.org/wiki/Night-6.md
category: reference
title: Night
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Night
date_saved: 2026-05-05T11:13:01.139082+00:00
instance: kb-cron
parent: night-20260505-0
summary:
Night photography can capture the natural colors of night by increasing the exposure time, or length of time captured in the photography. Longer exposures open the possibility for photographers to use light painting to selectively illuminate a scene. Digital photography can also make use of high-ISO settings, which increase the sensitivity to light, to take shorter exposure shots. This makes it possible to capture moving subjects without turning their movements into a blur. === Painting === Dating back to prehistoric cave paintings, artists have used a range of symbols to denote and depict the night sky. The first widely accepted portrayal of the night sky is the Nebra sky disc created c. 1600 BC. In medieval art, astrological signs gave meaning to paintings of night scenes. Adam Elsheimer's paintings on copper plates were some of the earliest realistic depictions of the night sky. Baroque paintings typically used a darker color scheme than previous painting styles in Europe. From the 17th century, darkness took up larger areas of paintings on average. Changes in the chemical composition of the paint itself and the development of new techniques for representing light led to the tenebrism style of painting. Tenebrism used stark, realistic depictions


id: noon-20260505-0
file: data/en.wikipedia.org/wiki/Noon-0.md
category: reference
title: Noon
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Noon
date_saved: 2026-05-05T11:13:02.309491+00:00
instance: kb-cron
parent: noon-20260505-0
summary:
Noon (also known as noontime or midday) is 12 o'clock in the daytime. Solar noon is the time when the Sun appears to contact the local celestial meridian. This is when the Sun reaches its apparent highest point in the sky, at 12 noon apparent solar time and can be observed using a sundial. The local or clock time of solar noon depends on the date, longitude, and time zone, with Daylight Saving Time tending to place solar noon closer to 1:00pm. == Etymology == The word noon is derived from Latin nona hora, the ninth canonical hour of the day, in reference to the Western Christian liturgical term Nones, (number nine), one of the seven fixed prayer times in traditional Christian denominations. The Roman and Western European medieval monastic day began at 6:00 a.m. (06:00) at the equinox by modern timekeeping, so the ninth hour ended at what is now 3:00 p.m. (15:00) at the equinox. In English, the meaning of the word shifted to midday and the time gradually moved back to 12:00 local time – that is, not taking into account the modern invention of time zones. The change began in the 12th century and was fixed


id: octaeteris-20260505-0
file: data/en.wikipedia.org/wiki/Octaeteris-0.md
category: reference
title: Octaeteris
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Octaeteris
date_saved: 2026-05-05T11:13:03.509120+00:00
instance: kb-cron
parent: octaeteris-20260505-0
summary:
In astronomy, an octaeteris (Greek: ὀκταετηρίς, plural: octaeterides) is the period of eight solar years after which the moon phase occurs on the same day of the year plus one or two days. This period is also in a very good synchronicity with five Venusian visibility cycles (the Venusian synodic period) and thirteen Venusian revolutions around the Sun (Venusian sidereal period). This means, that if Venus is visible beside the Moon, after eight years the two will be again close together near the same date of the calendar. The octaeteris, also known as oktaeteris, was noted by Cleostratus in ancient Greece as a ⁠2 923+ 1 /2⁠ day cycle. The octaeteris is the calendar used for the Olympic games; if one Olympiad was 50 months long, the next would be 49 lunar months long. This octaeteris calendar is used for the Olympic dial of the Antikythera mechanism, to determine the time of the Olympic games and other Greek festivities. == See also == Metonic cycle Eclipse cycle == References == Meeus, Jean (1997). Mathematical Astronomy Morsels. Willmann-Bell. Chapter 9, p. 51, Table 9.A "Some eclipse Periodicities".


id: orbital-period-20260505-0
file: data/en.wikipedia.org/wiki/Orbital_period-0.md
category: reference
title: Orbital period
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbital_period
date_saved: 2026-05-05T11:13:04.738684+00:00
instance: kb-cron
parent: orbital-period-20260505-0
summary:
The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets, exoplanets orbiting other stars, or binary stars. It may also refer to the time it takes a satellite orbiting a planet or moon to complete one orbit. For celestial objects in general, the orbital period is determined by a 360° revolution of one body around its primary, e.g. Earth around the Sun. Periods in astronomy are expressed in units of time, usually hours, days, or years. Its reciprocal is the orbital frequency, a kind of revolution frequency, in units of hertz. == Small body orbiting a central body == According to Kepler's Third Law, the orbital period T of two point masses orbiting each other in a circular or elliptic orbit is: T = 2 π a 3 G M {\displaystyle T=2\pi {\sqrt {\frac {a^{3}}{GM}}}} where: a is the orbit's semi-major axis G is the gravitational constant, M is the mass of the more massive body. For all ellipses with a given semi-major axis the orbital period is the same, regardless of


id: orbital-period-20260505-1
file: data/en.wikipedia.org/wiki/Orbital_period-1.md
category: reference
title: Orbital period
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbital_period
date_saved: 2026-05-05T11:13:04.738684+00:00
instance: kb-cron
parent: orbital-period-20260505-0
summary:
For celestial objects in general, the orbital period typically refers to the sidereal period, determined by a 360° revolution of one body around its primary relative to the fixed stars projected in the sky. For the case of the Earth orbiting around the Sun, this period is referred to as the sidereal year. This is the orbital period in an inertial (non-rotating) frame of reference. Orbital periods can be defined in several ways. The tropical period is more particularly about the position of the parent star. It is the basis for the solar year, and respectively the calendar year. The synodic period refers not to the orbital relation to the parent star, but to other celestial objects, making it not a merely different approach to the orbit of an object around its parent, but a period of orbital relations with other objects, normally Earth, and their orbits around the Sun. It applies to the elapsed time where planets return to the same kind of phenomenon or location, such as when any planet returns between its consecutive observed conjunctions with or oppositions to the Sun. For example, Jupiter has a synodic period of 398.8 days from Earth; thus, Jupiter's opposition occurs


id: ramesside-star-clocks-20260505-0
file: data/en.wikipedia.org/wiki/Ramesside_star_clocks-0.md
category: reference
title: Ramesside star clocks
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ramesside_star_clocks
date_saved: 2026-05-05T11:13:05.928376+00:00
instance: kb-cron
parent: ramesside-star-clocks-20260505-0
summary:
The Ramesside star clocks are ancient Egyptian star clocks appearing on the ceilings of several royal tombs of the Ramesside period. Two sets of star clocks appear on the tomb Ramesses VII and Ramesses IX. Although all of the sets are corrupt to some degree, a prototype clock can be reconstructed from them. == References ==


id: relative-hour-20260505-0
file: data/en.wikipedia.org/wiki/Relative_hour-0.md
category: reference
title: Relative hour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Relative_hour
date_saved: 2026-05-05T11:13:07.167936+00:00
instance: kb-cron
parent: relative-hour-20260505-0
summary:
Relative hour (Hebrew singular: shaʿah zǝmanit / שעה זמנית; plural: shaʿot - zǝmaniyot / שעות זמניות), sometimes called halachic hour, temporal hour, seasonal hour and variable hour, is a term used in rabbinic Jewish law that assigns 12 hours to each day and 12 hours to each night, all throughout the year. A relative hour has no fixed length in absolute time, but changes with the length of daylight each day - depending on summer (when the days are long and the nights are short), and in winter (when the days are short and the nights are long). Even so, in all seasons a day is always divided into 12 hours, and a night is always divided into 12 hours, which invariably makes for a longer hour or a shorter hour. At Mediterranean latitude, one hour can be about 45 minutes at the winter solstice, and 75 minutes at summer solstice. All of the hours mentioned by the Sages in either the Mishnah or Talmud, or in other rabbinic writings, refer strictly to relative hours. Another feature of this ancient practice is that, unlike the standard modern 12-hour clock that assigns 12 o'clock pm for noon time, in the ancient


id: relative-hour-20260505-1
file: data/en.wikipedia.org/wiki/Relative_hour-1.md
category: reference
title: Relative hour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Relative_hour
date_saved: 2026-05-05T11:13:07.167936+00:00
instance: kb-cron
parent: relative-hour-20260505-0
summary:
=== Disputations === In old times, the hour was detected by observation of the position of the sun, or when the first three stars appeared in the night sky. During the first six hours of the day, the sun is seen in the eastern sky. At the sixth hour, the sun is always at its zenith in the sky, meaning, it is either directly overhead, or parallel (depending on the hemisphere). Those persons living in the Northern Hemisphere, the sun at noon time will appear overhead slightly towards the south, whereas for those living in the Southern Hemisphere, the sun at noon time will appear overhead slightly towards the north (an exception being in the tropics, the sun can sometimes be directly overhead). From the 6th and a half hour to the 12th hour, the sun inclines towards the west, until it sets. The conclusion of a day at the end of twilight may slightly vary in minutes from place to place, depending on the elevation and the terrain. Typically, nightfall ushers in more quickly in the low-lying valleys, than it does on a high mountaintop. There are two major opinions how to calculate these times: The Magen Avraham (Shulhan


id: relative-hour-20260505-2
file: data/en.wikipedia.org/wiki/Relative_hour-2.md
category: reference
title: Relative hour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Relative_hour
date_saved: 2026-05-05T11:13:07.167936+00:00
instance: kb-cron
parent: relative-hour-20260505-0
summary:
== Practical bearing == In Jewish Halacha, the practical bearing of this teaching is reflected in many halachic practices. For example, according to Jewish law, the morning recitation of Kriyat Shema must be made between slightly before sunrise and the end of the third hour of the day, a time that actually fluctuates on the standard 12-hour clock, depending on the time of year. Its application is also used in determining the time of the Morning Prayer, which must be recited between sunrise until the end of the fourth hour, but post facto can be said until noon time, and which times will vary if one were to rely solely on the dials of the standard 12-hour clock, depending on the seasons. On the eve of Passover, chametz can only be eaten until the end of the fourth-hour of the day, and must be disposed of by the end of the fifth hour. In Jewish tradition, prayers were usually offered at the time of the daily whole-burnt offerings. The historian, Josephus, writing about the daily whole-burnt offering, says that it was offered twice each day, in the morning and about the ninth hour. The Mishnah, a compendium of Jewish oral


