Editing Time standard (section)

==Time standards for planetary motion calculations==
[[Ephemeris time]] (ET) and its successor time scales described below have all been intended for astronomical use, e.g. in planetary motion calculations, with aims including uniformity, in particular, freedom from irregularities of Earth rotation. Some of these standards are examples of [[dynamical time scale]]s and/or of [[coordinate time]] scales. Ephemeris Time was from 1952 to 1976 an official time scale standard of the [[International Astronomical Union]]; it was a [[dynamical time scale]] based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year. This ephemeris second was the standard for the [[SI]] second from 1956 to 1967, and it was also the source for calibration of the [[atomic clock|caesium atomic clock]];  its length has been closely duplicated, to within 1 part in 10<sup>10</sup>, in the size of the current SI second referred to atomic time.<ref>{{cite journal | last1=Markowitz | first1=W. |author-link1=William Markowitz| last2=Hall | first2=R. Glenn | last3=Essen | first3=L. |author-link3=Louis Essen| last4=Parry | first4=J. V. L. | title=Frequency of Cesium in Terms of Ephemeris Time | journal=Physical Review Letters | volume=1 | issue=3 | date=1958-08-01 | issn=0031-9007 | doi=10.1103/PhysRevLett.1.105 | pages=105–107| bibcode=1958PhRvL...1..105M }}</ref><ref>[[William Markowitz|Wm Markowitz]] (1988) 'Comparisons of ET(Solar), ET(Lunar), UT and TDT', in (eds.) A K Babcock & G A Wilkins, 'The Earth's Rotation and Reference Frames for Geodesy and Geophysics', IAU Symposia #128 (1988), at pp 413-418.</ref>  This Ephemeris Time standard was non-relativistic and did not fulfil growing needs for relativistic [[coordinate time]] scales.  It was in use for the official almanacs and planetary ephemerides from 1960 to 1983, and was replaced in official almanacs for 1984 and after, by numerically integrated [[Jet Propulsion Laboratory Development Ephemeris]] DE200 (based on the JPL relativistic coordinate time scale [[Ephemeris time#JPL ephemeris time argument Teph|T<sub>eph</sub>]]).

For applications at the Earth's surface, ET's official replacement was [[Terrestrial Dynamical Time]] (TDT), which maintained continuity with it. TDT is a uniform atomic time scale, whose unit is the SI second. TDT is tied in its rate to the SI second, as is International Atomic Time (TAI), but because TAI was somewhat arbitrarily defined at its inception in 1958 to be initially equal to a refined version of UT, TDT was offset from TAI, by a constant 32.184 seconds. The offset provided a continuity from Ephemeris Time to TDT. TDT has since been redefined as [[Terrestrial Time]] (TT).

For the calculation of ephemerides, [[Barycentric Dynamical Time]] (TDB) was officially recommended to replace ET. TDB is similar to TDT but includes relativistic corrections that move the origin to the barycenter, hence it is a dynamical time at the barycenter.<ref>{{cite journal | last1=Brumberg | first1=V. A. |author-link1=Victor A. Brumberg| last2=Kopejkin | first2=S. M. |author-link2=Sergei Kopeikin| title=Relativistic time scales in the solar system | journal=Celestial Mechanics and Dynamical Astronomy | volume=48 | issue=1 | date=1990 | issn=0923-2958 | doi=10.1007/BF00050674 | pages=23–44| bibcode=1990CeMDA..48...23B }}</ref> TDB differs from TT only in periodic terms. The difference is at most 2 milliseconds. Deficiencies were found in the definition of TDB (though not affecting T<sub>eph</sub>), and TDB has been replaced by [[Barycentric Coordinate Time]] (TCB) and [[Geocentric Coordinate Time]] (TCG), and redefined to be [[Ephemeris time#JPL ephemeris time argument Teph|JPL ephemeris time argument T<sub>eph</sub>]], a specific fixed linear transformation of TCB. As defined, TCB (as observed from the Earth's surface) is of divergent rate relative to all of ET, T<sub>eph</sub> and TDT/TT;<ref name=why1992>P K Seidelmann & T Fukushima (1992), [http://articles.adsabs.harvard.edu/full/1992A%26A...265..833S "Why new time scales?"], ''Astronomy & Astrophysics'' vol.265 (1992), pages 833-838, including [http://www.ucolick.org/~sla/leapsecs/deltat.png Fig. 1 at p.835, a graph giving an overview of the rate differences and offsets between various standard time scales], present and past, defined by the IAU.</ref> and the same is true, to a lesser extent, of TCG. The ephemerides of Sun, Moon and planets in current widespread and official use continue to be those calculated at the [[Jet Propulsion Laboratory]] (updated as from 2003 to [[DE405]]) using as argument T<sub>eph</sub>.