Time standards for planetary motion calculations Ephemeris time (ET) and its successor time scales described below have all been intended for astronomical use, e.g. in planetary motion calculations, with aims including uniformity, in particular, freedom from irregularities of Earth rotation. Some of these standards are examples of dynamical time scales and/or of coordinate time scales. Ephemeris Time was from 1952 to 1976 an official time scale standard of the International Astronomical Union; it was a dynamical time scale based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year. This ephemeris second was the standard for the SI second from 1956 to 1967, and it was also the source for calibration of the caesium atomic clock; its length has been closely duplicated, to within 1 part in 1010, in the size of the current SI second referred to atomic time.Wm Markowitz (1988) 'Comparisons of ET(Solar), ET(Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413-418. This Ephemeris Time standard was non-relativistic and did not fulfil growing needs for relativistic coordinate time scales. It was in use for the official almanacs and planetary ephemerides from 1960 to 1983, and was replaced in official almanacs for 1984 and after, by numerically integrated Jet Propulsion Laboratory Development Ephemeris DE200 (based on the JPL relativistic coordinate time scale Teph).
time standards for planetary motion calculations ephemeris time (et) and its successor time scales described below have all been intended for astronomical use eg in planetary motion calculations with aims including uniformity in particular freedom from irregularities of earth rotation some of these standards are examples of dynamical time scales and/or of coordinate time scales ephemeris time was from 1952 to 1976 an official time scale standard of the international astronomical union; it was a dynamical time scale based on the orbital motion of the earth around the sun from which the ephemeris second was derived as a defined fraction of the tropical year this ephemeris second was the standard for the si second from 1956 to 1967 and it was also the source for calibration of the caesium atomic clock; its length has been closely duplicated to within 1 part in 1010 in the size of the current si second referred to atomic timewm markowitz (1988) 'comparisons of et(solar) et(lunar) ut and tdt' in (eds) a k babcock & g a wilkins 'the earth's rotation and reference frames for geodesy and geophysics' iau symposia #128 (1988) at pp 413-418 this ephemeris time standard was non-relativistic and did not fulfil growing needs for relativistic coordinate time scales it was in use for the official almanacs and planetary ephemerides from 1960 to 1983 and was replaced in official almanacs for 1984 and after by numerically integrated jet propulsion laboratory development ephemeris de200 (based on the jpl relativistic coordinate time scale teph)