Current Definition
TT differs from Geocentric Coordinate Time (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 seconds 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 epochs (see below). LG is defined as exactly 6.969 290 134 × 10−10. (In 1991 when TT was first defined, LG was to be determined by experiment, and the best available estimate was 6.969 291 × 10−10.)
The equation linking TT and TCG is more commonly seen in the form
- TT = TCG − LG × (JDTCG − 2443 144.500 3725) × 86 400
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 2443 144.5 for the epoch, but that is inexact (though inappreciably so, because of the small size of the multiplier LG). The value 2443 144.500 3725 is exactly in accord with the definition.
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 Gregorian calendar are used. For continuity with their predecessor Ephemeris Time (ET), TT and TCG were set to match ET at around Julian Date 2443 144.5 (1977-01-01T00Z). More precisely, it was defined that TT instant 1977-01-01T00:00:32.184 exactly and TCG instant 1977-01-01T00:00:32.184 exactly correspond to the International Atomic Time (TAI) instant 1977-01-01T00:00:00.000 exactly. This is also the instant at which TAI introduced corrections for gravitational time dilation.
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 2443 144.500 3725 exactly.
Read more about this topic: Terrestrial Time
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