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 second is a unit of time used by astronomers and is derived from the SI unit system. We have used the symbol s-sr.
1 sidereal second is 1/60 of a sidereal minute. 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.
Like with normal seconds, minutes, hours and days, there are still 60 x 60 x 24 seconds in a day, but the day itself is approximately 23 h 56 min and 4.1 s in (normal) seconds.