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The sky considered as the inside of a hollow sphere in order to describe the positions and motions of astronomical objects. Any particular observer is located at the centre of his own celestial sphere. It is easy to imagine the sky as a hemispherical dome. Half the sky is always hidden from an observer on the Earth's surface; which half of the celestial sphere can be seen by an observer depends on his latitude on the Earth and on the date and time. Measurements on the celestial sphere are made in angular measure (degrees) and do not depend on how far away the objects actually are.
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