The 1961 official reference put it this way : " The origin and rate of ephemeris time are defined to make the Sun's mean longitude agree with Newcomb's expression"
22.
Time : A major change introduced with the 1960 issue of " The Astronomical Ephemeris " was the use of ephemeris time in place of mean solar time for the major ephemeris tabulations.
23.
Its successor time scales, such as TDT, as well as the atomic time scale IAT ( TAI ), were designed with a relationship that " provides continuity with ephemeris time ".
24.
The ephemeris time of the 1952 standard leaves a continuing legacy, through its ephemeris second which became closely duplicated in the length of the current standard SI second ( see below : Redefinition of the second ).
25.
However, the International Astronomical Union meeting in Rome in 1952 had adopted the ephemeris time scale, on a proposal by Gerald Clemence defining the time unit in terms of the Earth s motion round the sun.
26.
The length of the SI second was calibrated on the basis of the second of ephemeris time and can now be seen to have a relationship with the mean solar day observed between 1750 and 1892, analysed by Simon Newcomb.
27.
Thus the new TDB, like T eph, is essentially a more refined continuation of the older ephemeris time ET and ( apart from the periodic fluctuations ) has the same mean rate as that established for ET in the 1950s.
28.
In 1976, however, the IAU resolved that the theoretical basis for ephemeris time was wholly non-relativistic, and therefore, beginning in 1984 ephemeris time would be replaced by two further time scales with allowance for relativistic corrections.
29.
In 1976, however, the IAU resolved that the theoretical basis for ephemeris time was wholly non-relativistic, and therefore, beginning in 1984 ephemeris time would be replaced by two further time scales with allowance for relativistic corrections.
30.
Previous to the 1960 change, the'Improved Lunar Ephemeris'had already been made available in terms of ephemeris time for the years 1952-1959 ( computed by Brown's theory with modifications recommended by Clemence ( 1948 ) ).
