Sidereal time

Sidereal time, time as measured by the rotation of the earth relative to the fixed stars rather than relative to the sun. The sidereal day is the time it takes the earth to complete one rotation on its axis, such that a given star reappears on the observer's celestial meridian. The sidereal day (23 hr, 56 min, 4 sec) is slightly shorter than the solar day. The difference is caused by the earth's movement around the sun.