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What is Terrestrial Time?


If you’ve ever heard of Terrestrial Time you might be confused as to why it exists and what it is used for.


What Is Terrestrial Time Used For?


Terrestrial Time (TT) or Terrestrial Dynamical Time – as it used to be called – has been adopted by astronomers to calculate the location or planetary position of other objects as they relate to the center of the Earth and its rotation. 


It is measured in something called the SI second, which is equal to a single second in International Atomic Time (TAI).  


It was first established in 1976 as Terrestrial Dynamical Time. In 1991, it was redefined and renamed to Terrestrial Time. 


Before that astronomers used a different form of measurement called Ephemeris Time (ET). That’s because Ephemeris Time was used to calculate the motion of planets before the existence of atomic clocks. Since ET was nothing more than an approximation, astronomers swapped to Terrestrial Time after atomic clocks came into the picture. Terrestrial Time is still an approximation, however, astronomers consider it to be much more accurate than ET. 


Terrestrial Time actually differs from International Atomic Time (TAI) as it is slightly ahead by 32.184 seconds. This also results in a difference between Coordinated Universal Time (UTC) – the current time standard used across the world – and TT. Every time a leap second is added to UTC the separation between UTC and TT grows even more.


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