Time coordinates on the TT and TCG scales are conventionally specified using traditional means of specifying days, inherited from non-uniform time standards based on the rotation of 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 (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. 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 realization of TT based on reanalysis of historical TAI data. The BIPM has done this approximately annually since 1992. These realizations of TT are named in the form "TT(BIPM08)", with the digits indicating the year of publication. They are published in the form of table of differences from TT(TAI). The latest is TT(BIPM19). To millisecond accuracy, TT is parallel to the atomic timescale (International Atomic Time, TAI) maintained by the BIPM. TT is ahead of TAI, and can be approximated as TT ≅ TAI + 32.184 seconds.IAU conference 1991, Resolution A4, recommendation IV, note 9. (The offset 32.184 s arises from the historyThe atomic time scale A1 (a predecessor of TAI) was set equal to UT2 at its conventional starting date of 1 January 1958 (see L Essen, "Time Scales", Metrologia, vol.4 (1968), 161-165, at 163), when ΔT (ET-UT) was about 32 seconds. The offset 32.184 seconds was the 1976 estimate of the difference between Ephemeris Time (ET) and TAI, "to provide continuity with the current values and practice in the use of Ephemeris Time" (see IAU Commission 4 (Ephemerides), Recommendations to IAU General Assembly 1976, Notes on Recommendation 5, note 2)). TT also runs in parallel with the GPS time scale, which has a constant difference from atomic time (TAI − GPS time = +19 seconds), so that TT ≅ GPS time + 51.184 seconds. TT is in effect a continuation of (but is more precisely uniform than) the former Ephemeris Time (ET). It was designed for continuity with ET,P K Seidelmann (ed.) (1992), 'Explanatory Supplement to the Astronomical Almanac', at p.42; also IAU Commission 4 (Ephemerides), Recommendations to IAU General Assembly 1976, Notes on Recommendation 5, note 2. and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations). TT runs a little ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as ΔT = TT − UT1. ΔT was measured at +67.6439 seconds (TT ahead of UT1) at 0h UTC on 1 January 2015;US Naval Observatory (USNO) data file online at ftp://maia.usno.navy.mil/ser7/deltat.data (accessed 27 October 2015). and by retrospective calculation, ΔT was close to zero around the year 1900. The difference ΔT, though somewhat unpredictable in fine detail, is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future. The present definition of TT is a linear scaling of Geocentric Coordinate Time (TCG), which is the proper time of a notional observer who is infinitely far away (so not affected by gravitational time dilation) and at rest relative to Earth. TCG is used so far mainly for theoretical purposes in astronomy. From the point of view of an observer on Earth's surface the second of TCG passes in slightly less than the observer's SI second. The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster.
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. 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 (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. 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 realization of TT based on reanalysis of historical TAI data. The Bureau International of Weights and Measures (BIPM) has done this approximately annually since 1992. These realizations of TT are named in the form "TT(BIPM08)", with the digits indicating the year of publication. They are published in the form of table of differences from TT(TAI). The latest is TT(BIPM19). To millisecond accuracy, TT is parallel to the atomic timescale (International Atomic Time, TAI) maintained by the BIPM. TT is ahead of TAI, and can be approximated as TT ≅ TAI + 32.184 seconds.IAU conference 1991, Resolution A4, recommendation IV, note 9. (The offset 32.184 s arises from the historyThe atomic time scale A1 (a predecessor of TAI) was set equal to UT2 at its conventional starting date of 1 January 1958 (see L Essen, "Time Scales", Metrologia, vol.4 (1968), 161-165, at 163), when ΔT (ET-UT) was about 32 seconds. The offset 32.184 seconds was the 1976 estimate of the difference between Ephemeris Time (ET) and TAI, "to provide continuity with the current values and practice in the use of Ephemeris Time" (see IAU Commission 4 (Ephemerides), Recommendations to IAU General Assembly 1976, Notes on Recommendation 5, note 2)). TT is also parallel with the GPS time scale, which has a constant difference from atomic time (TAI − GPS time = +19 seconds), so that TT ≅ GPS time + 51.184 seconds. TT is in effect a continuation of (but is more precisely uniform than) the former Ephemeris Time (ET). It was designed for continuity with ET,P K Seidelmann (ed.) (1992), 'Explanatory Supplement to the Astronomical Almanac', at p.42; also IAU Commission 4 (Ephemerides), Recommendations to IAU General Assembly 1976, Notes on Recommendation 5, note 2. and it runs at the rate of the SI second, which was itself derived from a calibration using the second of ET (see, under Ephemeris time, Redefinition of the second and Implementations). TT is slightly ahead of UT1 (a refined measure of mean solar time at Greenwich) by an amount known as ΔT = TT − UT1. ΔT was measured at +67.6439 seconds (TT ahead of UT1) at 0h UTC on 1 January 2015;US Naval Observatory (USNO) data file online at ftp://maia.usno.navy.mil/ser7/deltat.data (accessed 27 October 2015). and by retrospective calculation, ΔT was close to zero about the year 1900. The difference ΔT, though somewhat unpredictable in fine detail, is expected to continue to increase, with UT1 becoming steadily (but irregularly) further behind TT in the future. The present definition of TT is a linear scaling of Geocentric Coordinate Time (TCG), which is the proper time of a notional observer who is infinitely far away (so not affected by gravitational time dilation) and at rest relative to Earth. TCG is used to date mainly for theoretical purposes in astronomy. From the point of view of an observer on Earth's surface the second of TCG passes in slightly less than the observer's SI second. The comparison of the observer's clock against TT depends on the observer's altitude: they will match on the geoid, and clocks at higher altitude tick slightly faster.
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conventionally specified
specified conventionally
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BIPM
Bureau International of Weights and Measures (BIPM)
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also runs in
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around
about
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