Sidereal day
An apparent sidereal day is the time it takes for the earth to turn 360 degrees in its rotation; more precisely, is the time it takes the vernal equinox to make two successive upper meridian transits. This is slightly shorter than a solar day; there are 366.2564 sidereal days in a year, but 365.2564 solar days, resulting in a sidereal day of 86,164.10 s.Midnight, in sidereal time, is when the vernal equinox crosses the upper meridian.
A mean sidereal day is reckoned, not from the actual transit, but from the transit of the mean vernal equinox (see: mean sun).
See also: sidereal month, sidereal time, time
