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).
The sidereal hour is a unit of time used by astronomers and is derived from the SI unit system. We have used the symbol hr-sr.
1 sidereal hour is made up of 60 sidereal minutes. This is derived ultimately from the sidereal day which is the time taken (in solar seconds) for the Earth to complete one rotation with respect to a distant star or constellation.
The sidereal hour angle is used when calculating sidereal time which is actually the angle along the celestial equator; from where one stands to the great circle that travels through the March equinox and both celestial poles.