Definitions for "Ephemeris time"
the time scale used prior to 1984 as the independent variable in gravitational theories of the solar system. In 1984, ET was replaced by dynamical time.
Time kept by calculating the positions of the planets, and comparing the theoretical time used to make the calculation, to the time when the planets arrive at that position. Used for tables of planetary positions, such as in the "Astronomical Ephemeris". For a short time, the official time of astronomers, but now superseded by Atomic Time, as averaged in Coordinated Universal Time.
(astronomy) a measure of time defined by Earth's orbital motion; terrestrial time is mean solar time corrected for the irregularities of the Earth's motions