In astronomy, a celestial coordinate system is a coordinate system for mapping positions in the sky.
There are different celestial coordinate systems each using a coordinate grid[?] projected on the celestial sphere, in analogy to the geographic coordinate system used on the surface of the Earth. The coordinate systems differ only in their choice of the fundamental plane[?], which divides the sky into two equal hemispheres along a great circle. (The fundamental plane of the geographic system is the Earth's equator). Each coordinate system is named for its choice of fundamental plane: