Time measured relative to the "fixed stars" instead of the Sun. Sidereal time is useful because the offset of sidereal time from the right ascension of a celestial object, known as the hour angle, gives the time before or after transit. The most useful form of sidereal time is local sidereal time, which gives the right ascension of a transiting celestial object at a given location.

Hour Angle, Local Sidereal Time, Right Ascension