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Celestial Sphere

It is often helpful to imagine that celestial objects are attached to a vast sphere centered on the earth. This fictitious construction is known as the celestial sphere. The earth's dimensions are assumed to be infinitesimally small compared to those of the sphere (since the distance of a typical celestial object from the earth is very much larger than the earth's radius). It follows that only half of the sphere is visible from any particular observation site on the earth's surface. Furthermore, the angular position of a given celestial object (relative to some fixed celestial reference) is the same at all such sites. In other words, there is negligible parallax associated with viewing the same celestial object from different observation sites on the surface of the earth.[*]

Richard Fitzpatrick 2010-07-21