Earth and the Moon

The Night Sky

What is the celestial sphere?

When viewed from Earth’s surface, the stars in the night sky appear to surround our planet and spin around while we stay in place. Ancient astronomers like Claudius Ptolemy (90–168) thought that stars were all on a sphere at the same distance away from the sun. Today, we know the stars are at different distances. Even so, when viewed from Earth, it is still useful to think of a map of the night sky as if it were drawn on the inside of a distant sphere. This celestial sphere has a line straddling across it—the celestial equator—that is simply an extension of Earth’s equator outward into the night sky, and the sphere is crisscrossed by lines of declination and right ascension which are analogous to lines of latitude and longitude on Earth’s surface.


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