celestial sphere

celestial sphere imaginary sphere of infinite radius with the earth at its center. It is used for describing the positions and motions of stars and other objects. For these purposes, any astronomical object can be thought of as being located at the point where the line of sight from the earth through the object intersects the surface of the celestial sphere. In astronomical coordinate systems , the coordinate axes are great circles on the celestial sphere. In most systems of this type, the reference points are fixed on the sphere, so the two coordinates needed to locate a body are relatively constant.

celestial sphere, the imaginary sphere on to which the heavenly bodies appear to be projected. For astronomical work the radius of the celestial sphere is considered to be infinite. For some purposes the sun is regarded as occupying the central position, but for others the earth's centre or the observer's eye is considered to lie at the centre. Systems of great circles on the celestial sphere, particularly the horizon system, the equinoctial system, and the ecliptic system, have been devised to facilitate the solution of mathematical astronomical problems, especially those related to nautical astronomy. For illus. see ecliptic.