TDB applies to the Solar-System-barycentric reference frame, and was first defined in 1976 as a successor to the ( non-relativistic ) former standard of ephemeris time ( adopted by the IAU in 1952 and superseded 1976 ).
32.
Time standards based on Earth rotation were replaced ( or initially supplemented ) for astronomical use from 1952 onwards by an " ephemeris time " standard based on the Earth's orbital period and in practice on the motion of the Moon.
33.
:Ephemeris Time ( ET ) was so named because it was the independent time scale used in government ephemerides from 1900 through 1983, in the American Ephemeris and Nautical Almanac, the British talk ) 08 : 09, 19 May 2008 ( UTC)
34.
Using data from Newcomb's Tables of the Sun ( based on the theory of the apparent motion of the Sun by Simon Newcomb, 1895, as retrospectively used in the definition of ephemeris time ), the SI second was defined in 1960 as:
35.
In 1967 / 8 the length of the SI second was redefined to be 9, 192, 631, 770 cycles of the caesium resonance, equal to the previous measurement result for the ephemeris second ( see Ephemeris time-redefinition of the second ).
36.
In this table, time is expressed in Ephemeris Time ( more precisely Terrestrial Time ) with days of 86, 400 SI seconds . " Y " is years since the epoch ( 2000 ), expressed in ? " T ".
37.
Since some event in the remote past, more days and hours have passed ( as measured in full rotations of Earth ) ( Universal Time ) than would be measured by stable clocks calibrated to the present, longer length of the day ( ephemeris time ).
38.
The small tidal effect cannot be observed in a short period, but the cumulative effect on Earth's rotation as measured with a stable clock ( ephemeris time, atomic time ) of a shortfall of even a few milliseconds every day becomes readily noticeable in a few centuries.
39.
Later work has shown this to be true, and astronomers now make a distinction between Universal Time, which is based on the Earth's rotation, and Terrestrial Time ( formerly Ephemeris time ), which is a uniform measure of the passage of time ( see also ?T ).
40.
I hope you don't mind that I've copied your entry here to Talk : Ephemeris time, because I think several of the points you make ought to be incorporated into the text of that article . ( talk ) 21 : 32, 20 May 2008 ( UTC)
