ephemeris time

ephemeris time (ET), astronomical time 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 AD 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) The time-scale that was used in calculating orbits within the Solar System from 1960 to 1984. Its fundamental unit was the ephemeris second which was defined by the statement that the tropical year at the epoch 1900.0 was precisely 31 556 925.9747 ephemeris seconds. Ephemeris Time was in many ways inconvenient, and in 1984 it was replaced by Terrestrial Dynamical Time (now known as Terrestrial Time) which has the SI second as its fundamental unit.