Dynamical time scales

The astronomers stuck with ephemeris time until 1979, when they defined two new time scales that used the atomic second and that took into account relativity (velocity affects time). From 1 January 1984, these scales replaced ephemeris time in national ephemerides like the Nautical Almanac.

Terrestrial Dynamical Time (TDT) views time from the earth's position and motion. It was defined as being equal to TAI (Atomic time) plus 32.184 (atomic) seconds at the instant beginning 1 January 1977.

Barycentric Dynamical Time (TDB, from the French) is time at the center of mass of the solar system. TDB has various forms depending on the theory of relativity adopted.

By International Astronomical Union (IAU) Resolution A4 in 1991, Terrestrial Dynamical Time was renamed Terrestrial Time (TT). Recommendations III and V of the same resolution created Barycentric Coordinate Time (TCB) to take the place of Barycentric Dynamical Time, except in situations where maintaining continuity in ongoing work made retaining the old scale preferable.

In 2006 (Resolution B3)ยน, responding to the "multiple realizations of TDB" and other factors, the IAU defined TDB in terms of TCB. One result is that, within a few thousand years around the present, the difference between Terrestrial Time and Barycentric Dynamical Time on the surface of the Earth is less than 2 milliseconds. 

1. www.iau.org/static/resolutions/IAU2006_Resol3.pdf 

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