The imaginary sphere whose center is the center of the Earth and on which the fixed stars, planets, Sun, Moon, and the zodiac are located.
A huge sphere centered on the Earth, which provides a convenient way to fix a coordinate system on the sky. Every object in the sky can be found by knowing its declination and right ascension. The Celestial Sphere
Apparent sphere of the sky, a sphere of large radius centered on the observer. Directions of objects in the sky can be denoted by their position on the celestial sphere.