[1] For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth.
TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.
TT is indirectly the basis of UTC, via International Atomic Time (TAI).
In 1991, in Recommendation IV of the XXI General Assembly, the IAU redefined TDT, also renaming it "Terrestrial Time".
TT was defined to be a linear scaling of TCG, such that the unit of TT is the "SI second on the geoid",[4] i.e. the rate approximately matched the rate of proper time on the Earth's surface at mean sea level.
was the gravitational potential at the geoid surface, a value measured by physical geodesy.
In 2000, the IAU very slightly altered the definition of TT by adopting an exact value, LG = 6.969290134×10−10.
[5] TT differs from Geocentric Coordinate Time (TCG) by a constant rate.
The Julian Date is a linear transformation of the raw count of seconds represented by the variable TCG, so this form of the equation is not simplified.
The above equation is often given with the Julian Date 2443144.5 for the epoch, but that is inexact (though inappreciably so, because of the small size of the multiplier
Time coordinates on the TT and TCG scales are specified conventionally using traditional means of specifying days, inherited from non-uniform time standards based on the rotation of Earth.
For continuity with their predecessor Ephemeris Time (ET), TT and TCG were set to match ET at around Julian Date 2443144.5 (1977-01-01T00Z).
TT and TCG expressed as Julian Dates can be related precisely and most simply by the equation
For practical use, physical clocks must be measured and their readings processed to estimate TT.
The BIPM TAI service, performed since 1958, estimates TT using measurements from an ensemble of atomic clocks spread over the surface and low orbital space of Earth.
TAI is canonically defined retrospectively, in monthly bulletins, in relation to the readings shown by that particular group of atomic clocks at the time.
Estimates of TAI are also provided in real time by the institutions that operate the participating clocks.
The atomic time scale A1 (a predecessor of TAI) was set equal to UT2 at its conventional starting date of 1 January 1958,[8] when ΔT (ET − UT) was about 32 seconds.
[12] Approximately annually since 1992, the International Bureau of Weights and Measures (BIPM) has produced better realizations of TT based on reanalysis of historical TAI data.
The researchers observed that their scale was within 0.5 microseconds of TT(BIPM17), with significantly lower errors since 2003.
The data used was insufficient to analyze long-term stability, and contained several anomalies, but as more data is collected and analyzed, this realization may eventually be useful to identify defects in TAI and TT(BIPM).
ΔT is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future.
In fine detail, ΔT is somewhat unpredictable, with 10-year extrapolations diverging by 2-3 seconds from the actual value.
As a result, TT (even as a theoretical ideal) does not match the proper time of all observers.
In relativistic terms, TT is described as the proper time of a clock located on the geoid (essentially mean sea level).
[20] The redefinition did not quantitatively change TT, but rather made the existing definition more precise.
In effect it defined the geoid (mean sea level) in terms of a particular level of gravitational time dilation relative to a notional observer located at infinitely high altitude.