text: Redefinition of the second Successive definitions of the unit of ephemeris time are mentioned above (History).  The value adopted for the 1956/1960 standard second:
  the fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time.
  was obtained from the linear time-coefficient in Newcomb's expression for the solar mean longitude (above), taken and applied with the same meaning for the time as in formula (3) above.  The relation with Newcomb's coefficient can be seen from:
  1/31 556 925.9747 = 129 602 768.13 / (360×60×60×36 525×86 400).
  Caesium atomic clocks became operational in 1955, and quickly confirmed the evidence that the rotation of the Earth fluctuated irregularly.  This confirmed the unsuitability of the mean solar second of Universal Time as a measure of time interval for the most precise purposes.  After three years of comparisons with lunar observations, Markowitz et al. (1958) determined that the ephemeris second corresponded to 9 192 631 770 ± 20 cycles of the chosen cesium resonance.
  Following this, in 1967/68, the General Conference on Weights and Measures (CGPM) replaced the definition of the SI second by the following:
  The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.