Definition
Sidereal time is the hour angle of the vernal equinox. It measures how far Earth has rotated since the equinox crossed the local meridian. One sidereal day — the interval between two successive upper transits of the same star — lasts about 23 h 56 m 4 s of mean solar time.
Why astronomers use it
Right ascension is measured along the celestial equator from the vernal equinox. When local sidereal time equals an object's right ascension, that object is on the meridian — highest in the sky. This makes sidereal time essential for telescope pointing and observatory scheduling.
Relation to solar time
Because Earth moves about 1° per day along its orbit, a solar day is roughly 4 minutes longer than a sidereal day. Over a year the difference accumulates to one full rotation: 24 sidereal hours correspond to 23 h 56 m of solar time.