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.

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