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HISTORY The approximate concept of TT was standardised by the International Astronomical Union (IAU) in 1976 at its XVIth General Assembly, under the name Terrestrial Dynamical Time ('''TDT'''). It was the counterpart to Barycentric Dynamical Time (TDB), which was a time standard for Solar system Ephemerides . Both of these time standards turned out to be poorly defined, and TDT was also misnamed, having nothing dynamical about it. In (TCG), which was defined by the same General Assembly. TT was defined to be a Linear transformation of TCG, such that TT agrees with Proper Time on the Geoid . This left the exact ratio between TT time and TCG time as something to be determined by experiment. The determination of the Gravitational Potential at the geoid is a task in Physical Geodesy . In in terms of a precise Gravitational Potential , thus removing the need for horologists to study sea levels. DEFINITION TT differs from TCG by a constant rate. Formally it is defined by the equation :TT = (1 - LG) TCG + E where TT and TCG are linear counts of SI Second s in Terrestrial Time and Geocentric Coordinate Time respectively, LG is the constant difference in the rates of the two time scales, and E is a constant to resolve the Epoch s (see below). LG is defined as exactly . (In 1991 when TT was first defined, LG was to be determined by experiment, and the best available estimate was .) The equation linking TT and TCG is more commonly seen in the form :TT = TCG - LG x (JDTCG - 2443144.5003725) x 86400 where JDTCG is the TCG time expressed as a Julian Date . This is just a transformation of the raw count of seconds represented by the variable TCG, so this form of the equation is needlessly complex. The use of a Julian Date does specify the epoch fully, however (see next paragraph). The above equation is often given with the Julian Date 2443144.5 for the epoch, but that is wrong, the value given above is exactly correct. Time coordinates on the TT and TCG scales are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the . TT and TCG expressed as Julian Dates can be related precisely and most simply by the equation :JDTT = EJD + (JDTCG - EJD) (1 - LG) where EJD is 2443144.5003725 exactly. REALISATION TT is a Platonic time scale: a theoretical ideal, not dependent on a particular realisation. For practical purposes, TT must be realised by actual clocks in the Earth system. The main realisation of TT is supplied by International Atomic Time (TAI). The TAI service, running since 1958 , attempts to match the rate of Proper Time on the Geoid , using an ensemble of Atomic Clock s spread over the surface and low orbital space of the Earth . TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings that particular groups of atomic clocks showed at the time. Estimates of TAI are also provided in Real Time by the institutions that operate the participating clocks. Because of the historical difference between TAI and ET when TT was introduced, the TAI realisation of TT is defined thus: :TT(TAI) = TAI + 32.184 s Because TAI is never revised once published, it is possible for errors in it to become known and remain uncorrected. It is thus possible to produce a better realisation of TT based on reanalysis of historical TAI data. The BIPM has done this approximately annually since 1992 . These realisations of TT are named in the form "TT(BIPM05)", with the digits indicate the year of publication. They are published in the form of table of differences from TT(TAI). The latest as of April 2006 is TT(BIPM05) . The international communities of precision timekeeping, Astronomy , and Radio broadcasts are preparing to create a new precision time scale based on observations of an ensemble of Pulsar s. This new pulsar time scale will serve as an independent means of computing TT, and it may eventually be useful to identify defects in TAI. SEE ALSO
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