celestial sphere Imaginary sphere of infinite radius used to define the positions of celestial bodies as seen from Earth, the centre of the sphere. The sphere rotates, once in 24 hours, about a line that is an extension of the Earth's axis. The position of a celestial body is the point at which a radial line through it meets the surface of the sphere. The position is defined in terms of coordinates, such as declination and right ascension or altitude and azimuth, which refer to great circles on the sphere, such as the celestial equator or the ecliptic.

ce·les·tial sphere • n. an imaginary sphere of which the observer is the center and on which all celestial objects are considered to lie.