Sidereal Days

Wikipedia Sidereal Day
http://en.wikipedia.org/wiki/Sidereal_day

Wikipedia Sidereal Time
http://en.wikipedia.org/wiki/Sidereal_time

Wolfram Research - Astronomy World
http://scienceworld.wolfram.com/astronomy/SiderealDay.html

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A sidereal (pronounced sigh-dear'-real) day refers to the rotation of the Earth measured relative to the stars. It is the time it takes the Earth to rotate 360 degrees and is equal to 23 hours, 56 minutes and 4 seconds.

A solar day is 24 hours long. This is the time it takes from one noon (sun overhead) to the next noon.

The difference in the two "days" arises from the fact that during a day the Earth also travels nearly a degree further on its yearly trek around the Sun.

Time - MSN Encarta
http://encarta.msn.com/encyclopedia_761561386/Time.html