Terrestrial Time

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.^[1] For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT or TD),^[2] which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

The unit of TT is the SI second, the definition of which is based currently on the caesium atomic clock,^[3] but TT is not itself defined by atomic clocks. It is a theoretical ideal, and real clocks can only approximate it.

TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT is indirectly the basis of UTC, via International Atomic Time (TAI). Because of the historical difference between TAI and ET when TT was introduced, TT is 32.184 s ahead of TAI.

^ The 1991 definition refers to the scale agreeing with the SI second "on the geoid", i.e. close to mean sea level on Earth's surface, see IAU 1991 XXIst General Assembly (Buenos Aires) Resolutions, Resolution A.4 (Recommendation IV). A redefinition by resolution of the IAU 2000 24th General Assembly (Manchester), at Resolution B1.9, is in different terms intended for continuity and to come very close to the same standard.
^ TT is equivalent to TDT, see IAU conference 1991, Resolution A4, recommendation IV, note 4.
^ IAU conference 1991, Resolution A4, recommendation IV, part 2 states that the unit for TT is to agree with the SI second 'on the geoid'.

[1] The 1991 definition refers to the scale agreeing with the SI second "on the geoid", i.e. close to mean sea level on Earth's surface, see IAU 1991 XXIst General Assembly (Buenos Aires) Resolutions, Resolution A.4 (Recommendation IV). A redefinition by resolution of the IAU 2000 24th General Assembly (Manchester), at Resolution B1.9, is in different terms intended for continuity and to come very close to the same standard.

[iau1991-4-n4-2] TT is equivalent to TDT, see IAU conference 1991, Resolution A4, recommendation IV, note 4.

[iau1991-4-2-3] IAU conference 1991, Resolution A4, recommendation IV, part 2 states that the unit for TT is to agree with the SI second 'on the geoid'.

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