Sidereal timeSidereal time is time reckoned by sidereal days and
is the time elapsed since a transit of the vernal equinox, this
is, strictly, apparent sidereal time. Mean sidereal time being
reckoned, not from the actual transit of the vernal equinox, but
rather from the transit of the mean vernal equinox (see: mean
sun). Generally, time is based off the apparent motion of the
stars. Normal clocks follow the motion of the Sun: whenever the
clock says 1 p.m. the Sun is high in the sky. The Sun seems to
circle the Earth in about 24 hours. This is not true with
sidereal time. If, on a certain night, at 23:25 you see a star
over a hill, next night it will be seen at the same place at
23:21. A sidereal clock is a clock which gains four minutes a
day, so if a star is seen at 13:22 sidereal time, it will appear
at 13:22, sidereal time, the next night. To make a sidereal
clock, imagine that in a year (a mean tropical year) the Sun
turns 365.2422 times around the Earth, while the stars turn
exactly once more, 366.242