Editing Ephemeris time (section)

==Implementations==

===Secondary realizations by lunar observations===
Although ephemeris time was defined in principle by the orbital motion of the Earth around the Sun,<ref>[[#refClem1948|Clemence (1948)]], at pp. 171—3.</ref> it was usually measured in practice by the orbital motion of the Moon around the Earth.<ref>[[#refMark1955|W Markowitz & others (1955)]]; [[#refMark1959|W Markowitz (1959)]]; also [[#refMark1958|W Markowitz, R G Hall, L Essen, J V L Parry (1958)]].</ref>  These measurements can be considered as secondary realizations (in a [[metrology|metrological]] sense) of the primary definition of ET in terms of the solar motion, after a calibration of the mean motion of the Moon with respect to the mean motion of the Sun.<ref name=guin88>
[[#refGuin1988|B Guinot & P K Seidelmann (1988)]], at p. 305.</ref>

Reasons for the use of lunar measurements were practically based: the Moon moves against the background of stars about 13 times as fast as the Sun's corresponding rate of motion, and the accuracy of time determinations from lunar measurements is correspondingly greater.

When ephemeris time was first adopted, time scales were still based on astronomical observation, as they always had been.  The accuracy was limited by the accuracy of optical observation, and corrections of clocks and time signals were published in arrear.

===Secondary realizations by atomic clocks===
A few years later, with the invention of the [[atomic clock|cesium atomic clock]], an alternative offered itself.  Increasingly, after the calibration in 1958 of the cesium atomic clock by reference to ephemeris time,<ref name="Mark1958" /> cesium atomic clocks running on the basis of ephemeris seconds began to be used and kept in step with ephemeris time.  The atomic clocks offered a further secondary realization of ET, on a quasi-real time basis<ref name=guin88 /> that soon proved to be more useful than the primary ET standard: not only more convenient, but also more precisely uniform than the primary standard itself. Such secondary realizations were used and described as 'ET', with an awareness that the time scales based on the atomic clocks were not identical to that defined by the primary ephemeris time standard, but rather, an improvement over it on account of their closer approximation to uniformity.<ref>[[#tr32-1306|W G Melbourne & others, 1968]], section II.E.4-5, pages 15—16, including footnote 7, noted that the Jet Propulsion Laboratory spacecraft tracking and data reduction programs of that time (including the Single Precision Orbit Determination Program) used, as ET, the current US atomic clock time A.1 offset by 32.25 seconds. The discussion also noted that the usage was "inaccurate" (the quantity indicated was not identical with any of the other realizations of ET such as ET0, ET1), and that while A.1 gave "certainly a closer approximation to uniform time than ET1" there were no grounds for considering either the atomic clocks or any other measures of ET as (perfectly) uniform. Section II.F, pages 18—19, indicates that an improved time measure of (A.1 + 32.15 seconds), applied in the JPL Double Precision Orbit Determination Program, was also designated ET.</ref> The atomic clocks gave rise to the [[International Atomic Time|atomic time scale]], and to what was first called Terrestrial Dynamical Time and is now [[Terrestrial Time]], defined to provide continuity with ET.<ref name=ESAAp42 />

The availability of atomic clocks, together with the increasing accuracy of astronomical observations (which meant that relativistic corrections were at least in the foreseeable future no longer going to be small enough to be neglected),<ref>[[#refWink1977|G M R Winkler and T C van Flandern (1977)]].</ref> led to the eventual replacement of the ephemeris time standard by more refined time scales including [[terrestrial time]] and [[barycentric dynamical time]], to which ET can be seen as an approximation.