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Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth.〔The 1991 definition refers to the scale agreeing with the SI second "on the geoid", i.e. close to mean sea level on Earth's surface, see (IAU 1991 XXIst General Assembly (Buenos Aires) Resolutions ), Resolution A.4 (Recommendation IV). A redefinition by (resolution of the IAU 2000 24th General Assembly (Manchester) ), at Resolution B1.9, is in different terms intended for continuity and to come very close to the same standard.〕 For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time (TDT),〔TT is equivalent to TDT, see IAU conference 1991, Resolution A4, recommendation IV, note 4.〕 which in turn succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth. The unit of TT is the SI second, the definition of which is currently based on the caesium atomic clock,〔IAU conference 1991, Resolution A4, recommendation IV, part 2 states that the unit for TT is to agree with the SI second 'on the geoid'.〕 but TT is not itself defined by atomic clocks. It is a theoretical ideal, which real clocks can only approximate. TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT indirectly underlies UTC, via International Atomic Time (TAI). Each leap second that is introduced into UTC causes UTC to diverge a little further from TT. ==Approximation== Sometimes times described in TT must be handled in situations where TT's detailed theoretical properties are not significant. Where millisecond accuracy is enough (or more than enough), TT can be summarized in the following ways: * To millisecond accuracy, TT runs 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 history.〔The 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),〔((Steve Allen's Time Scales page ) at Lick Observatory).〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Terrestrial Time」の詳細全文を読む スポンサード リンク
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