- noun the region of space that seems to surround the earth and in which astronomical objects are seen
- The sphere which seems to surround the Earth, and upon which astronomical objects are seen. The night sky is in fact infinitely deep, but the celestial sphere is a powerful optical illusion of immense use. Treating it like the sphere of the Earth itself allows us to imagine lines of right ascension and declination upon it, analogous to longitude and latitude on the Earth, so that the positions of objects can be described.
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