The length of time which passes between a given fixed star in the sky crossing a given projected meridian (line of longitude). The sidereal day is $23 h 56 m 4.1 s$, slightly shorter than the solar day because the Earth 's orbital motion about the Sun means the Earth has to rotate slightly more than one turn with respect to the "fixed" stars in order to reach the same Earth-Sun orientation. Another way of thinking about the difference is that it amounts to $1/365.2425^{th}$ of a day per day, since even if the Earth did not spin on its axis at all, the Sun would appear to make one rotation around the Earth as the Earth completed a single orbit (which takes one year).
Sidereal Day