The time since the mean equinox crossed the meridian, i.e. the hour angle of the mean equinox. The equinox is not fixed with respect to the stars, but changes its position due to precession and nutation. It is convenient to define a mean equinox with the effect of nutation removed. The mean sidereal time defined by this equinox will, unlike apparent sidereal time, proceed uniformly.
Subjects: Astronomy and Astrophysics.