The rotation period of the Earth relative to some external point. Several types of day are defined according to the reference point chosen. For most purposes the day is defined as 86 400 s (i.e. 24 hours exactly), which corresponds to the mean solar day, but the mean solar day depends on the rotation of the Earth, which is neither completely uniform nor completely predictable. The apparent solar day, the interval from one meridian transit of the Sun (apparent noon) to the next, has a considerable annual variation of about 30 s due to the changing value of the equation of time. This is averaged out to give the mean solar day. The mean sidereal day, on the other hand, is the Earth’s period of axial rotation relative to the stars, with a minor correction of 0.0084 s to allow for precession. It is the interval between successive transits of the mean equinox, and is nearly 4 min shorter than the mean solar day. Due to the Earth’s orbital motion around the Sun there is precisely one more sidereal day in the year than the number of solar days.