Editing Ephemeris time (section)

==History (1952 standard)==
'''Ephemeris time''' ('''ET'''), adopted as standard in 1952, was originally designed as an approach to a uniform time scale, to be freed from the effects of irregularity in the rotation of the Earth, "for the convenience of astronomers and other scientists", for example for use in [[ephemeris|ephemerides]] of the Sun (as observed from the Earth), the Moon, and the planets.  It was proposed in 1948 by [[Gerald Maurice Clemence|G M Clemence]].<ref name=Clemence1948>[[#refClem1948|G M Clemence (1948)]].</ref>

From the time of [[John Flamsteed]] (1646–1719) it had been believed that the Earth's daily rotation was uniform.  But in the later nineteenth and early twentieth centuries, with increasing precision of astronomical measurements, it began to be suspected, and was eventually established, that the rotation of the Earth (''i.e.'' the length of the [[day]]) showed irregularities on short time scales, and was slowing down on longer time scales.  The evidence was compiled by [[Willem de Sitter|W de Sitter]] (1927)<ref>[[#refSitter1927|W de Sitter (1927)]].</ref> who wrote "If we accept this hypothesis, then the 'astronomical time', given by the Earth's rotation, and used in all practical astronomical computations, differs from the 'uniform' or 'Newtonian' time, which is defined as the independent variable of the equations of celestial mechanics".  De Sitter offered a correction to be applied to the mean solar time given by the Earth's rotation to get uniform time.

Other astronomers of the period also made suggestions for obtaining uniform time, including [[Andre Danjon|A Danjon]] (1929), who suggested in effect that observed positions of the Moon, Sun and planets, when compared with their well-established gravitational ephemerides, could better and more uniformly define and determine time.<ref name=Clemence1971>[[#refClem1971|G M Clemence (1971)]].</ref>

Thus the aim developed, to provide a new time scale for astronomical and scientific purposes, to avoid the unpredictable irregularities of the [[Solar time#Mean solar time|mean solar time]] scale, and to replace for these purposes [[Universal Time]] (UT) and any other time scale based on the rotation of the Earth around its axis, such as [[sidereal time]].

The American astronomer [[Gerald Maurice Clemence|G M Clemence]] (1948)<ref name="Clemence1948" /> made a detailed proposal of this type based on the results of the English [[Astronomer Royal]] [[Harold Spencer Jones|H Spencer Jones]] (1939).<ref name=HSJ1939>[[#refHSJ1939|H Spencer Jones (1939)]].</ref>  Clemence (1948) made it clear that his proposal was intended "for the convenience of astronomers and other scientists only" and that it was "logical to continue the use of mean solar time for civil purposes".<ref>[[#refClem1948|Clemence (1948)]], at p. 171.</ref>

De Sitter and Clemence both referred to the proposal as 'Newtonian' or 'uniform' time. [[Dirk Brouwer|D Brouwer]] suggested the name 'ephemeris time'.<ref name=ESAAp79>[[#refESAA|ESAA (1992)]], see [https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA79 page 79].</ref>

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 (1952)|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.<ref name=ESAAp79 /><ref>At the [[International Astronomical Union|IAU]] meeting in Rome 1952: see [[#refESAE|ESAE (1961)]] at sect.1C, p. 9;  also [[#refClem1971|Clemence (1971)]].</ref>  Practical introduction took some time (see [[#Use of ephemeris time in official almanacs and ephemerides|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 of time scales|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;<ref name=ESAAp79 /> and the standard second was defined first as 1/31556925.975 of the tropical year at 1900.0,<ref name=ESAAp79 /><ref>ESAA 1992, p. 79: citing decision of [[International Committee for Weights and Measures]] (CIPM), Sept 1954.</ref> and then as the slightly modified fraction 1/31556925.9747 instead,<ref>[[#refESAA|ESAA (1992)]], see [https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA80 page 80], citing CIPM recommendation Oct 1956, adopted 1960 by the [[General Conference on Weights and Measures]] (CGPM).</ref> 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 [[International Atomic Time|IAT (TAI)]], were designed with a relationship that "provides continuity with ephemeris time".<ref name=ESAAp42>[[#refESAA|ESAA (1992)]], at [https://books.google.com/books?id=uJ4JhGJANb4C&pg=PA42 page 42].</ref>  ET was used for the calibration of atomic clocks in the 1950s.<ref name="Mark1958" />  Close equality between the ET second with the later [[Second#International second|SI second]] (as defined with reference to the cesium atomic clock) has been verified to within 1 part in 10<sup>10</sup>.<ref name="refMark1988">[[#refMark1988|Wm Markowitz (1988)]].</ref>

In this way, decisions made by the original designers of ephemeris time influenced the length of today's [[Second#International second|standard SI second]], and in turn, this has a continuing influence on the number of [[leap second]]s which have been needed for insertion into current broadcast time scales, to keep them approximately in step with [[Solar time#Mean solar time|mean solar time]].