The sidereal year is a unit of time that is a multiple of an SI unit. We have used the symbol a-sr.
From the Latin ‘sidus’ for ‘star’, sidereal is a term used to describe the orbital time of the earth or an object to complete one full cycle with respect to a fixed, distant star or constellation pattern.
The sidereal year is defined as 365.242 190 402 days of 86,400 SI seconds. This is 20 minutes and 24.5 seconds longer than a tropical year.
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.