# ephemeris time

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## ephemeris time

(ET), astronomical timetime,
sequential arrangement of all events, or the interval between two events in such a sequence. The concept of time may be discussed on several different levels: physical, psychological, philosophical and scientific, and biological.
defined by the orbital motions of the earth, moon, and planets. The earth does not rotate with uniform speed, so the solar day is an imprecise unit of time. Ephemeris time is calculated from the positions of the sun and moon relative to the earth, assuming that Newton's laws are perfectly obeyed. It is used to calculate the future positions of the sun and the planets. By convention, the standard seasonal year is taken to be A.D. 1900 and to contain 31,556,925.9747 sec of ephemeris time. 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. For most purposes they can be considered identical, since they differ by only milliseconds, and often therefore are referred to simply as dynamical time.

## ephemeris time

(ET) A measure of time for which a constant rate was defined and that was used from 1958 to the end of 1983 as the independent variable in gravitational theories of the Solar System. It was superceded in 1984 by dynamical time. ET was reckoned from the instant, close to the beginning of 1900, when the Sun's mean geometric longitude was 279° 41′ 48″.04, at which instant the measure of ephemeris time was 1900 Jan. 0d 12h precisely. The primary unit of ET was the tropical year at this epoch of 1900 Jan. 0.5 ET, which contains a calculable number of seconds: the ephemeris second is the fraction 1/31 556 925.9747 of the tropical year 1900 Jan. 0.5 ET. The ephemeris second was adopted in 1957 as the fundamental invariable unit of time until replaced in 1967 by the second of atomic time. This SI-adopted second was defined so that it was equal to the ephemeris second within the error of measurement. The ephemeris day contains 86 400 ephemeris seconds. ET could be found by adding a correction, delta T, to the universal time UT1.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006

## ephemeris time

[ə′fem·ə·rəs ′tīm]
(astronomy)
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. Abbreviated E.T.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## ephemeris 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.