celestial sphere

The celestial sphere is the sky thought of as an imaginary sphere around an observer (basically, around the Earth, or for some purposes, around the Sun), representing the position of stars and astronomical bodies the observer sees when looking in different directions, upward and around. The portion of the sky hidden "under" the Earth is easily considered a hidden part of the same sphere. Celestial coordinate systems, such as equatorial coordinate system (with its latitude- and longitude-like coordinates, declination and right ascension), can be used to identify the position of stars and other objects on the celestial sphere.