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*in ‘astronomy’ + ‘navigation’, the celestial sphere is an ‘abstract sphere’, with an arbitrarily large ‘radius’, that is ‘con-centric’ to ‘earth’*
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(all objects in the observer’s sky can be conceived as projected upon the inner surface of the celestial sphere, as if it were the underside of a dome or a hemispherical screen)
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(the ‘celestial sphere’ is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or trivial)