The sidereal day is a unit of time used by astronomers and is derived from the SI unit system. We have used the symbol d-sr.
Sidereal literally means "of the stars". Otherwise known as the "orbital period", it defines the time period taken for two point masses to orbit each other.
On Earth, we used the solar day to regularise time and this represents the period taken to orbit the sun. The sidereal day (or stellar day) is the time taken for the earth to rotate with respect to a distant star and equals 23 hours, 56 minutes and 4.0905 seconds (or 86164.1 s, 23.934 h).