DYNAMICAL TIME

the time scale of dynamical astronomy (celestial mechanics). The orbital motions of celestial bodies subject only to Newton's laws of motion and law of gravitation, as modified by relativity, proceed as a function of dynamical time. In 1952 the International Astronomical Union (IAU) defined a dynamical time scale named Ephemeris Time (ET), which is based on the Earth's orbital motion as given by the American astronomer Simon Newcomb in his nonrelativistic tables of the Sun (1898). In practice, however, ET was determined more accurately by observing the Moon's motions and then using a lunar ephemeris. For this operation an empirical, nongravitational term was used to correct for the disturbing effect exerted by the EarthMoon tidal couple, as given in the nonrelativistic Improved Lunar Ephemeris of 1954. Highly accurate observations made since about 1967 by means of radar, lasers, radioastronomy, spacecraft, and atomic clocks have since made improved, relativistic ephemerides necessary. In 1984 ET was replaced in national almanacs by Barycentric Dynamical Time (TDB), whose equations of motion refer to the barycentre, or centre of mass, of the solar system and include relativistic terms. Different theories of relativity are used in the various forms of TDB, and the IAU did not specify a particular ephemeris with which to numerically define TDB. The IAU also defined an auxiliary scale, Terrestrial Dynamical Time (TDT), for use in apparent geocentric ephemerides (i.e., those using equations of motion referring to the centre of the Earth). By definition, TDT = TAI + 32.184 seconds, with TAI denoting International Atomic Time. See also atomic time.