Sidereal time (ST), time measured relative to the fixed stars; thus, the sidereal day is the period during which the earth completes one rotation on its axis so that some chosen star appears twice on the observer's celestial meridian. Because the earth moves in its orbit about the sun, the sidereal day is about 4 min shorter than the solar day (see solar time). Thus, a given star will appear to rise 4 min earlier each night, so that different stars are visible at different times of the year. The local sidereal time of an observer is equal to the hour angle of the vernal equinox.
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