Time as given by direct observation of the stars; strictly, the hour angle of the true equinox. The vernal equinox (the First Point of Aries) is not a completely fixed sidereal direction but varies due to general precession and nutation. Nutation is a periodic variation and imposes irregularities of about a second on sidereal time. The principal period of this variation is 18.6 years, which is the period of regression of nodes of the Moon’s mean orbit around the ecliptic. Apparent sidereal time corrected for this effect gives mean sidereal time.