ephemeris time definition of ephemeris time in the Free Online Encyclopedia.

ephemeris time (ET), astronomical time time, 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.
..... Click the link for more information. 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 [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.