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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 a typical star to make two successive upper meridian transits. This is slightly shorter than a solar day. There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days, resulting in a sidereal day of 86,164.09 seconds (or: 23 hours, 56 minutes, 4.09 seconds).
The reason there is one more sidereal day than "normal" days in a year is that the Earth's orbit around the Sun cancels one sidereal day out, giving observers on Earth 365 (and a quarter) days, even though the
planet itself rotated 366 (and a quarter) times.
Midnight, in sidereal
time, is when the First Point of Aries 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
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