Editing Ephemeris time (section)

==Redefinition of the second==
Successive definitions of the unit of ephemeris time are mentioned above ([[#History (1952 standard)|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&times;60&times;60&times;36 525&times;86 400).

[[Caesium]] [[atomic clocks]] became operational in 1955, and quickly confirmed the evidence that the rotation of the Earth fluctuated irregularly.<ref name=McCarthySeidelmann2009>[[#McCarthySeidelmann2009|McCarthy & Seidelmann (2009) Ch. 4, "Variable Earth Rotation"]]</ref>  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, [[William Markowitz|Markowitz]] et al. (1958) determined that the ephemeris second corresponded to 9 192 631 770 ± 20 cycles of the chosen cesium resonance.<ref name=Mark1958>[[#refMark1958|W Markowitz, R G Hall, L Essen, J V L Parry (1958)]]</ref>

Following this, in 1967/68, the General Conference on Weights and Measures (CGPM) replaced the definition of the [[Second#International second|SI second]] by the following:
<blockquote>The second is the duration of 9&thinsp;192&thinsp;631&thinsp;770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
</blockquote>
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 [[Second#International second|SI second referred to atomic time]] was later verified by Markowitz (1988) to be in agreement, within 1 part in 10<sup>10</sup>, with the second of ephemeris time as determined from lunar observations.<ref name="refMark1988"/>

For practical purposes the length of the ephemeris second can be taken as equal to the length of the second of [[Barycentric Dynamical Time|Barycentric Dynamical Time (TDB)]] or [[Terrestrial Time|Terrestrial Time (TT)]] or its predecessor TDT.

The difference between ET and UT is called [[ΔT (timekeeping)|ΔT]]; it changes irregularly, but the long-term trend is [[parabola|parabolic]], decreasing from ancient times until the nineteenth century,<ref name=morr3 /> and increasing since then at a rate corresponding to an increase in the solar day length of 1.7&nbsp;ms per century (see [[leap second]]s).

[[International Atomic Time]] (TAI) was set equal to [[Universal Time|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.