The year is a unit of time that is a multiple of an SI unit and uses the symbol a.
The Julian year is made up of exactly 365.25 days – each with 60 x 60 x 24 seconds (86,400 seconds). The .25 days is worked into the system by counting 366 days once every 4 cycles. This is known as a “leap year” and the “leap day” happens at the end of February.
The term year represents the length of time it takes for the earth to complete one full cycle around the sun. Each planet therefore has a different year length.
To track years, humanity has assigned an incremental numbering system. Depending on which culture, religion or part of the world you are from or follow, this number varies. The most common numbering system suggests we are in the 21st century – i.e. in the 2000’s. This system started 0 AD (Anno Domini – which translates from Latin as “In the year of our Lord”). Time before this is referred to as BC (before Christ) and the number increases as you go further into history (like a negative number would).
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.