Terrestrial Time

chronology

Also known as: TDT, TT, Terrestrial Dynamical Time

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July 14, 2026

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Terrestrial Time (TT) is a specialized timescale used to describe the motion of objects in space. It was formerly known as Terrestrial Dynamical Time (TDT) and is the successor to Ephemeris Time (ET). TT is defined as exactly 32.184 seconds offset from International Atomic Time (TAI) as of January 1, 1977. For times before 1955, TT is extrapolated backward using ET, which was based on the length of the tropical year in 1900. TT is crucial for geocentric ephemerides, which are tables of the predicted positions of celestial bodies.1

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eclipse
Prediction and calculation of solar and lunar eclipses
…made some years ahead in Terrestrial Time (TT), which is defined by the orbital motion of Earth and the other planets. At the time of the eclipse, the correction is made to Universal Time (UT), which is defined by the rotation of Earth and is not rigorously uniform.
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…made some years ahead in Terrestrial Time (TT), which is defined by the orbital motion of Earth and the other planets. At the time of the eclipse, the correction is made to Universal Time (UT), which is defined by the rotation of Earth and is not rigorously uniform.
Read More