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
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