The True rotation of the Earth measured by two successive transits of a fixed star over the observer's Meridian. The mean length of one sidereal day is 23h56m04.09054s (86164.09054s) of Civil Time. Note that this interval slightly shorter (0.0084s) than the mean time required for a fixed star to twice transit the Upper Meridian, due to the Precession which occurs in a single day.
Comments: Sidereal Day