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.
The tropical year is a unit of time that is a multiple of an SI unit. We have used the symbol at.
The average tropical year up to the year 2000 was 365.2422 days of 86400 seconds.
The phrase “tropical” comes from the Greek for “turn”; tropikos. Following this, and where the “turn” referred to is the same as that defining the tropics of Capricorn and Cancer (the extreme latitudes North and South where the sun can still appear directly overhead or perpendicular to the Earth’s surface); a tropical year is known as the time it takes the sun to complete a full cycle of seasons and return to the same relative position to the observer on Earth.
In 1627, Johannes Kepler used the findings of Tycho Brahe and Waltherus (the Rudolphine Tables) to establish the mean tropical year as 365 solar days, 5 hours, 48 minutes and 45 seconds.