- the imaginary spherical shell formed by the sky, usually represented as an infinite sphere, the center of which is a given observer’s position.
- an imaginary sphere of infinitely large radius enclosing the universe so that all celestial bodies appear to be projected onto its surface
- An imaginary sphere with Earth at its center. The stars, planets, Sun, Moon, and other celestial bodies appear to be located on this sphere, and the sphere appears to rotate around the Earth’s extended axis once every 24 hours, carrying the celestial bodies with it overhead and giving them their diurnal motions. The celestial sphere is essentially a spherical map of the sky that provides the basis for the coordinate systems used in celestial navigation and in specifying the positions and motions of celestial objects. See more at altazimuth coordinate system ecliptic coordinate system equatorial coordinate system.