celestial sphere

celestial sphere An imaginary sphere of indefinite size, used as a background for assigning positional coordinates to celestial objects (see diagram). The sphere may be centred on the Earth, the observer, or any other point which acts as the origin of the chosen system of coordinates. As seen from Earth, the celestial sphere appears to rotate about the axis joining the celestial poles every 23 h 56 m 04 s (a sidereal day), actually a result of the axial spin of the Earth. Two important circles on the celestial sphere are the celestial equator and the ecliptic. The angle between them is about 23°.4, and is known as the obliquity of the ecliptic. The celestial equator and the ecliptic intersect at the vernal and autumnal equinoxes. The positions of the celestial poles, and hence of the celestial equator, are gradually moving on the celestial sphere because of a slow wobbling of the Earth in space termed precession. See also celestial coordinates.