text: Following this, an astronomical conference held in Paris in 1950 recommended "that in all cases where the mean solar second is unsatisfactory as a unit of time by reason of its variability, the unit adopted should be the sidereal year at 1900.0, that the time reckoned in this unit be designated ephemeris time", and gave Clemence's formula (see Definition of ephemeris time (1952)) for translating mean solar time to ephemeris time.
  The International Astronomical Union approved this recommendation at its 1952 general assembly.  Practical introduction took some time (see Use of ephemeris time in official almanacs and ephemerides);  ephemeris time (ET) remained a standard until superseded in the 1970s by further time scales (see Revision).
  During the currency of ephemeris time as a standard, the details were revised a little. The unit was redefined in terms of the tropical year at 1900.0 instead of the sidereal year; and the standard second was defined first as 1/31556925.975 of the tropical year at 1900.0, and then as the slightly modified fraction 1/31556925.9747 instead, finally being redefined in 1967/8 in terms of the cesium atomic clock standard (see below).
  Although ET is no longer directly in use, it leaves a continuing legacy.  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".  ET was used for the calibration of atomic clocks in the 1950s.  Close equality between the ET second with the later SI second (as defined with reference to the cesium atomic clock) has been verified to within 1 part in 1010.