Editing Terrestrial Time (section)

==History==

A definition of a terrestrial time standard was adopted by the [[International Astronomical Union]] (IAU) in 1976 at its XVI General Assembly and later named ''Terrestrial Dynamical Time'' (TDT). It was the counterpart to [[Barycentric Dynamical Time]] (TDB), which was a time standard for Solar system [[ephemerides]], to be based on a [[dynamical time scale]]. Both of these time standards turned out to be imperfectly defined. Doubts were also expressed about the meaning of 'dynamical' in the name TDT.

In 1991, in Recommendation IV of the XXI General Assembly, the IAU redefined TDT, also renaming it "Terrestrial Time". TT was formally defined in terms of [[Geocentric Coordinate Time]] (TCG), defined by the IAU on the same occasion. TT was defined to be a linear scaling of TCG, such that the unit of TT is the "SI second on the [[geoid]]",<ref>{{cite web |title=IAU(1991) RECOMMENDATION IV |url=https://www.iers.org/IERS/EN/Science/Recommendations/recommendation4.html |website=IERS}}</ref> i.e. the rate approximately matched the rate of [[proper time]] on the Earth's surface at mean sea level. Thus the exact ratio between TT time and TCG time was <math>1-L_\mathrm{G}</math>, where <math>L_\mathrm{G} = U_\mathrm{G} / c^2</math> was a constant and <math>U_\mathrm{G}</math> was the [[gravitational potential]] at the geoid surface, a value measured by [[physical geodesy]]. In 1991 the best available estimate of <math>L_\mathrm{G}</math> was {{val|6.969291|e=−10}}.

In 2000, the IAU very slightly altered the definition of TT by adopting an exact value, {{math | 1=''L''<sub>G</sub> = {{val|6.969290134|e=−10}}}}.<ref>{{cite web| url = http://chiron.mtk.nao.ac.jp/~toshio/iaudiv1/IAU_resolutions/Resol-UAI.htm| title = Resolution B1.9 of the IAU XXIV General Assembly, 2000}}</ref>