terrestrial dynamical time
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terrestrial dynamical time(TDT) See dynamical time.
The uniform measure of time defined by the laws of dynamics and determined in principle from the orbital motions of the planets, specifically the orbital motion of the earth as represented by Newcomb's Tables of the Sun. An ephemeris day equals 84,600 ephemeris seconds. An ephemeris second was the fundamental unit of time of the International System of Units of 1960; it equals ⅓1556925.9747 of the tropical year defined by the mean motion of the sun in longitude at the epoch 1900 January 0 day 12 h. Also known as terrestrial dynamical time.