The apparent surface of the imaginary sphere on which celestial bodies appear to be projected; SYN. sphere, empyrean, firmament, heavens, vault of heaven, welkin.
Imaginary sphere surrounding the Earth, on which the celestial bodies seem to lie. The positions of bodies such as stars, planets, and galaxies are specified by their coordinates on the celestial sphere. The equivalents of latitude and longitude on the celestial sphere are called declination and right ascension (which is measured in hours from 0 to 24). The celestial poles lie directly above the Earth’s poles, and the celestial equator lies over the Earth’s equator. The celestial sphere appears to rotate once around the Earth each day, actually a result of the rotation of the Earth on its axis.