What is a sidereal day?
A sidereal day is one 360 degree rotation of Earth. This is about 23
hours 56 minutes. Since our clocks are set to the position of the
sun, we must turn just a bit more than this to have the sun once
again line up with the meridian at noon. This rotation time, called
synodic, for the sun to align to the same point in the sky takes 24 hours.
---Nathan A. Unterman
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Update: June 2012