The independent variable in the differential equations of motion of orbiting celestial bodies in a theory of relativity. The bodies move in accord with Newton's laws of motion and gravitation. Several types of dynamical time have been defined for different origins and theories of relativity. Integration of the equations, combined with observed positions, leads to an ephemeris, which lists coordinates as a function of dynamical time. See also Ephemeris.
Dynamical Time is a uniform scale of time, unlike the mean solar scale Universal Time (UT), based on the Earth's rotation, which has variations in rotational speed. In 1952 the International Astronomical Union (IAU) defined a dynamical scale and named it Ephemeris Time (ET). See also Earth rotation and orbital motion.
Also, in 1952 the U.S. Naval Observatory began a program to obtain Ephemeris Time rapidly by photographing the Moon and nearby stars of known position. In 1958 a joint program with the National Physical Laboratory at Teddington, England, gave 9,192,631,770 cycles per second of Ephemeris Time as the frequency of its cesium-beam atomic clock. See also Atomic time.
Comparisons between Ephemeris Time and Atomic Time (AT) showed no definite systematic difference other than a constant. Therefore, Ephemeris Time could be obtained from Atomic Time with very high accuracy, and immediately.
In 1976 the IAU defined two timelike arguments based on International Atomic Time (French acronym, TAI). Terrestrial Dynamical Time (TDT) was for use with geocentric ephemerides. In effect, TDT = TAI + 32.184 s. Barycentric Dynamical Time (TDB) was the argument to be used when the origin was at the barycenter of the solar system. TDT and TDB differ only by periodic relativistic terms. Thus, Dynamical Time, which was based formerly on motions of bodies in the solar system, was now derived from Atomic Time in the form of timelike arguments.
The use of the word dynamical for scales that do not involve dynamics caused some problems. In 1991 the IAU replaced TDT by Terrestrial Time (TT). Also, it introduced terminologies Geocentric Coordinate Time (TCG) and Barycentric Coordinate Time (TCB). See also Celestial mechanics;
