Ephemeris Time (ET) is the time scale used in ephemerides of celestial bodies, in particular the Sun (as observed from the Earth), Moon, planets, and other members of the solar system. This is distinct from Universal Time (UT): the time scale based on the rotation of the Earth around its axis.
In the late 19th century it was found that the rotation of the Earth (i.e. the length of the day) was both irregular on short time scales, and was slowing down on longer time scales. In fact, observing the position of the Moon, Sun and planets and comparing this with their ephemerides was a better way to determine the time.
At the time, very accurate and stable atomic (cesium) clocks had become available. Comparing the clock rates with astronomical observations allowed to match atomic clocks with ephemeris time, and a new definition of the second based on cesium clocks was accepted in 1972.
The difference between ET and UT is called Delta-T; it increases irregularly with about half a second per year. The atomic clock time TAI was set equal to UT2 at 1 January 1958 0:00:00 . At that time, Delta-T was already about 32.18 seconds. The difference between TT (the successor to ephemeris time) and atomic time was later defined as follows:
1977 January 1.0003725 TT = 1977 January 1.0000000 TAI, i.e.
ET - TAI = 32.184 seconds
This difference may be assumed constant, the rates of TT and TAI are designed to be identical.
P.K.Seidelmann (ed.): Explanatory Supplement to the Astronomical Almanac. University Science Books, CA, 1992 ; ISBN 0-935702-68-7