Sidereal Time

The Hour Angle of the Vernal Point. Mean Sidereal Time is measured by the diurnal motion of the mean Vernal Point ("mean equinox of date"), which is affected the secular quantities of Precession; Apparent Sidereal Time is measured by the position of the true Vernal Point ("true equinox of date"), which is affected by Nutation. The difference (Apparent minus Mean) is called the Equation of the Equinoxes (called Nutation in Right Ascension, before 1960). Apparent Sidereal Time on the observer's meridian is called LST (Local Sidereal Time); on the Greenwich meridian, it is called GST (Greenwich Sidereal Time). The difference (LST-GST) is equal to the geographic longitude of the observer. The value of GST is tabulated in an ephemeris under the heading "ST" or "Sidereal Time." The argument used to compute the GST is the UT (Universal Time), which differs from the ET (Ephemeris Time) used to compute the planets. Sidereal Time is a direct measure of the diurnal rotation of the Earth, and is, therefore, independent of the value of Delta-T.