Terrestrial Dynamical Time

Terrestrial Dynamical Time (TDT) is dynamical time for geocentric phenomena which replaced Ephemeris Time when the IAU 1976 System of Astronomical Constants was implemented in the Astronomical Almanac in 1984.

Terrestrial Dynamical Time
TDT The independent argument for apparent geocentric ephemerides. At 1977 January 1d00h00m00s TAI, the value of TDT was exactly 1977 January 1d.0003725. The unit of TDT is 86400 SI seconds at mean sea level.

Terrestrial Dynamical Time (TDT or TT). Time scale used in orbital computations; TDT is tied to atomic clocks (International Atomic Time, TAI), whereas Universal Time is tied to observations.

dynamical ~: [or Terrestrial Dynamical Time, TDT] "scientific time" - it superseded ephemeris time in 1984, and is based on a uniform scale of time derived from atomic clocks (i.e.

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A family of timescales results from the transformation by various theories and metrics of relativistic theories of Terrestrial Dynamical Time (TDT). TDB differs from TDT only by periodic variations.

As the theoretical basis for Ephemeris Time is wholly non-relativistic, in 1976 the IAU resolved that beginning in 1984 ET would be replaced by the two relativistic timescales Barycentric Dynamical Time (TDB) and Terrestrial Dynamical Time (TDT).

DT has two expressions, Terrestrial Time, TT, (or Terrestrial Dynamical Time, TDT), and Barycentric Dynamical Time, TDB. More information on these, and still more timekeeping expressions, may be found at the U.S. Naval Observatory website.

In 1984 ephemeris time was renamed terrestrial dynamical time (TDT or TT); also created was barycentric dynamical time (TDB), which is based on the orbital motion of the sun, moon, and planets.

See also: Time, Earth, Rotation, Planet, Orbit