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The Question

(Submitted February 28, 1997)

Can somebody please tell me what sidereal time is?

The Answer

Sidereal time is based on how long it takes for a celestial object, like a star, to return to the same position in the sky after the Earth rotates. This is not quite the same as the typical 24 hour day because the Earth moves in its orbit around the Sun during the 24 hour day, and this motion causes an apparent shift in the positions of stars, etc. (from the Earth's point of view). It turns out that a sidereal day is about 4 minutes shorter than a 24 hour day.

Andy Ptak
for the Ask an Astrophysicist Team

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