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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Astrolabe | 4/5 | https://en.wikipedia.org/wiki/Astrolabe | reference | science, encyclopedia | 2026-05-05T09:40:07.689542+00:00 | kb-cron |
The blue sphere represents the celestial sphere. The blue arrow indicates the direction of true north (the North Star). The central blue point represents Earth (the observer's location). The geographic south of the celestial sphere acts as the projection pole. The celestial equatorial plane serves as the projection plane. Three parallel circles represent the projection on the celestial sphere of Earth's main circles of latitude: In orange, the celestial Tropic of Cancer. In purple, the celestial equator. In green, the celestial Tropic of Capricorn. When projecting onto the celestial equatorial plane, three concentric circles correspond to the celestial sphere's three circles of latitude (left side of the image). The largest of these, the projection on the celestial equatorial plane of the celestial Tropic of Capricorn, defines the size of the astrolabe's tympanum. The center of the tympanum (and the center of the three circles) is actually the north-south axis around which Earth rotates, and therefore, the rete of the astrolabe will rotate around this point as the hours of the day pass (due to Earth's rotational motion). The three concentric circles on the tympanum are useful for determining the exact moments of solstices and equinoxes throughout the year: if the sun's altitude at noon on the rete is known and coincides with the outer circle of the tympanum (Tropic of Capricorn), it signifies the winter solstice (the sun will be at the zenith for an observer at the Tropic of Capricorn, meaning summer in the southern hemisphere and winter in the northern hemisphere). If, on the other hand, its altitude coincides with the inner circle (Tropic of Cancer), it indicates the summer solstice. If its altitude is on the middle circle (equator), it corresponds to one of the two equinoxes.
==== The horizon and the measurement of altitude ====
On the right side of the image above:
The blue arrow indicates the direction of true north (the North Star). The central blue point represents Earth (the observer's location). The black arrow represents the zenith direction for the observer (which would vary depending on the observer's latitude). The two black circles represent the horizon surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the celestial sphere visible to the observer, and its projection on the celestial equatorial plane. The geographic south of the celestial sphere acts as the projection pole. The celestial equatorial plane serves as the projection plane. When projecting the horizon onto the celestial equatorial plane, it transforms into an ellipse upward-shifted relatively to the center of the tympanum (both the observer and the projection of the north-south axis). This implies that a portion of the celestial sphere will fall outside the outer circle of the tympanum (the projection of the celestial Tropic of Capricorn) and, therefore, won't be represented.
Additionally, when drawing circles parallel to the horizon up to the zenith (almucantar), and projecting them on the celestial equatorial plane, as in the image above, a grid of consecutive ellipses is constructed, allowing for the determination of a star's altitude when its rete overlaps with the designed tympanum.
==== The meridians and the measurement of azimuth ====
On the right side of the image above:
The blue arrow indicates the direction of true north (the North Star). The central blue point represents Earth (the observer's location). The black arrow represents the zenith direction for the observer (which would vary depending on the observer's latitude). The two black circles represent the horizon surrounding the observer, which is perpendicular to the zenith vector and defines the portion of the celestial sphere visible to the observer, and its projection on the celestial equatorial plane. The five red dots represent the zenith, the nadir (the point on the celestial sphere opposite the zenith with respect to the observer), their projections on the celestial equatorial plane, and the center (with no physical meaning attached) of the circle obtained by projecting the secondary meridian (see below) on the celestial equatorial plane. The orange circle represents the celestial meridian (or meridian that goes, for the observer, from the north of the horizon to the south of the horizon passing through the zenith). The two red circles represent a secondary meridian with an azimuth of 40° East relative to the observer's horizon (which, like all secondary meridians, intersects the principal meridian at the zenith and nadir), and its projection on the celestial equatorial plane. The geographic south of the celestial sphere acts as the projection pole. The celestial equatorial plane serves as the projection plane. When projecting the celestial meridian, it results in a straight line that overlaps with the vertical axis of the tympanum, where the zenith and nadir are located. However, when projecting the 40° E meridian, another circle is obtained that passes through both the zenith and nadir projections, so its center is located on the perpendicular bisection of the segment connecting both points. Indeed, the projection of the celestial meridian can be considered as a circle with an infinite radius (a straight line) whose center is on this bisection and at an infinite distance from these two points. If successive meridians that divide the celestial sphere into equal sectors (like "orange slices" radiating from the zenith) are projected, a family of curves passing through the zenith projection on the tympanum is obtained. These curves, once overlaid with the rete containing the major stars, allow for determining the azimuth of a star located on the rete and rotated for a specific time of day.
== See also ==
== Footnotes ==
== References ==