definition 
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.

