text: Although this is an independent definition that does not refer to the older basis of ephemeris time, it uses the same quantity as the value of the ephemeris second measured by the cesium clock in 1958.  This SI second referred to atomic time was later verified by Markowitz (1988) to be in agreement, within 1 part in 1010, with the second of ephemeris time as determined from lunar observations.
  For practical purposes the length of the ephemeris second can be taken as equal to the length of the second of Barycentric Dynamical Time (TDB) or Terrestrial Time (TT) or its predecessor TDT.
  The difference between ET and UT is called ΔT; it changes irregularly, but the long-term trend is parabolic, decreasing from ancient times until the nineteenth century, and increasing since then at a rate corresponding to an increase in the solar day length of 1.7 ms per century (see leap seconds).
  International Atomic Time (TAI) was set equal to UT2 at 1 January 1958 0:00:00.  At that time, ΔT was already about 32.18 seconds.  The difference between Terrestrial Time (TT) (the successor to ephemeris time) and atomic time was later defined as follows:
  1977 January 1.000 3725 TT = 1977 January 1.000 0000 TAI, i.e.
  TT − TAI = 32.184 seconds
  This difference may be assumed constant—the rates of TT and TAI are designed to be identical.