Sidereal time

Sidereal time is the hour angle of the equinox. Real sidereal time refers to the real equinox and mean sidereal time is referred to when it is the mean equinox on that date. At a given place and time, the sum of the right ascension of a celestial body and its hour angle is equal to its real sidereal time. At the time it is crossing the meridian of a body, its real right ascension is therefore equal to its real sidereal time.