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 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.