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An imaginary sphere, of large but indefinite dimension, used as a basis to define the position coordinates of celestial bodies. The center can be the Earth, the observer, or any other point which plays the role of origin for a given system of coordinates. Seen from the Earth, the celestial sphere rotates around the celestial axis every 23h 56m 04s (the sidereal day), as a result of the Earth's rotation. Two important circles on the celestial sphere are the celestial equator and the ecliptic. The angle between them, about 23.40 degrees, is known as the obliquity of the ecliptic. The celestial equator and the ecliptic intersect at two points, vernal equinox and autumnal equinox. The positions of the celestial poles and therefore that of the celestial equator move gradually on the celestial sphere, due to precession.


Celestial sphere



