Sidereal Time

Sidereal Time is a unique time that astronomers use during observations. Sidereal time has no relation whatsoever to real time. Sidereal clocks are based on the location of the vernal equinox, which is the location from which the right ascension is measured. Midnight sidereal time is defined as the time when the vernal equinox crosses your upper meridian.

Sidereal Day
A sidereal day is the time between two successive upper meridian passages of the vernal equinox. By contrast, an apparent solar day is the time between two successive upper meridian crossings of the Sun. Both times of day are not equal. Each sidereal day is 4 minutes shorter, to be precise, of a solar day. This accounts for a sidereal day to be 23hours, 56minutes and 4.091seconds long.