sidereal time

sidereal time (ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian . Because the earth moves in its orbit about the sun, the sidereal day is about 4 min shorter than the solar day (see solar time ). Thus, a given star will appear to rise 4 min earlier each night, so that different stars are visible at different times of the year. The local sidereal time of an observer is equal to the hour angle of the vernal equinox.

sidereal time Time as measured by reference to the stars; technically, it is the hour angle of the vernal equinox. The sidereal time is the same as the right ascension of stars currently on the observer's meridian. More generally, the sidereal time is the sum of the right ascension and the hour angle of any celestial object, and hence links these two coordinates. Depending on whether the true equinox or the mean equinox is used as the reference point, the resulting form of sidereal time is known as either apparent sidereal time or mean sidereal time, respectively. Their difference, which seldom exceeds a second of time, is called the equation of the equinoxes. See also greenwich sidereal time; local sidereal time.