What is Terrestrial Time (TT)?
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Terrestrial Time (TT) is a time scale established by the International Astronomical Union (IAU) to serve as the independent argument for apparent geocentric ephemerides. That is, TT is used for the prediction or recording of the positions of celestial bodies as measured by an observer on Earth. It is the successor to Ephemeris Time (ET), but is based on the SI second.

Terrestrial Time is effectively equal to International Atomic Time (TAI) plus 32.184 seconds exactly: TT = TAI + 32.184

The epoch designated "J2000.0" is specified as Julian date 2451545.0 TT, or 2000 January 1, 12h TT. This epoch can also be expressed as 2000 January 1, 11:59:27.816 TAI or 2000 January 1, 11:58:55.816 UTC.