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Barycentric Dynamical Time (TDB) is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation〔Explanations given with (a) (IAU resolutions 1991 ), under Resolution A.4, at 'Notes for recommendation III', and (IAU 2006 resolution 3 ), and its footnotes; and (b) explanations and references cited at "Time dilation -- due to gravitation and motion together".〕 when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar System. TDB is now (since 2006) defined as a linear scaling of Barycentric Coordinate Time (TCB), and a feature that distinguishes TDB from TCB is that TDB, when observed from the Earth's surface, has a difference from Terrestrial Time (TT) that is about as small as can be practically arranged with consistent definition: the differences are mainly periodic,〔The periodic differences, due to relativistic effects, between a coordinate time scale applicable to the Solar-System barycenter, and time measured at the Earth's surface, were first estimated and are explained in: G M Clemence & V Szebehely, ("Annual variation of an atomic clock" ), Astronomical Journal, Vol.72 (1967), p.1324-6.〕 and overall will remain at less than 2 milliseconds for several millennia.〔(IAU 2006 resolution 3 ), see Recommendation and footnotes, note 3.〕 TDB applies to the Solar-System-barycentric reference frame, and was first defined in 1976 as a successor to the (non-relativistic) former standard of ephemeris time (adopted by the IAU in 1952 and superseded 1976). In 2006, after a history of multiple time-scale definitions and deprecation since the 1970s,〔(a)P K Seidelmann & T Fukushima (1992), ("Why new time scales?" ), ''Astronomy & Astrophysics'' vol.265 (1992), pages 833-838: and (b) IAU resolution (1991) A.4(recommendation V), which recommended limiting the use of TDB (previously defined 1976-79) to cases "where discontinuity with previous work is deemed to be undesirable".〕 a redefinition of TDB was approved by the IAU. The 2006 IAU redefinition of TDB as an international standard expressly acknowledged that the long-established JPL ephemeris time argument Teph, as implemented in JPL Development Ephemeris DE405, "is for practical purposes the same as TDB defined in this Resolution"〔(IAU 2006 resolution 3 ), see footnotes, note 4.〕 (By 2006, ephemeris DE405 had already been in use for a few years as the official basis for planetary and lunar ephemerides in the Astronomical Almanac; it was the basis for editions for 2003 through 2014; in the edition for 2015 it is superseded by DE430).〔See US Naval Observatory (Naval Oceanography Portal), ("History of the Astronomical Almanac" ) (accessed October 2015); also, for details of DE405:- E M Standish (1998), (JPL Planetary and Lunar Ephemerides, DE405/LE405 ), Jet Propulsion Laboratory Interoffice Memorandum 312F-98-48, August 26, 1998; also, the Astronomical Almanac for 2015 commences use of the more recent JPL ephemeris version DE430, which is now based expressly on TDB, see section L, especially page L-4 (Astronomical Almanac for 2015, page L-4 ) (accessed October 2015).〕 ==Definition== IAU resolution 3 of 2006〔(IAU 2006 resolution 3 )〕 defines TDB as a linear transformation of TCB. TCB diverges from both TDB and TT. TCB progresses faster at a differential rate of about 0.5 second/year, while TDB and TT remain close.〔(Fig. 1 at p.835, a graph giving an overview of the rate differences and offsets between various standard time scales ), present and past, defined by the IAU: for description see P K Seidelmann & T Fukushima (1992), ("Why new time scales?" ), ''Astronomy & Astrophysics'' vol.265 (1992), pages 833-838.〕 As of the beginning of 2011, the difference between TDB and TCB is about 16.6 seconds. :TDB = TCB − L×(JD − T)×86400 + TDB where L = 1.550519768, TDB = −6.55 s, T = 2443144.5003725, and JD is the TCB Julian date (that is, a quantity which was equal to T on 1977 January 1 00:00:00 TAI at the geocenter and which increases by one every 86400 seconds of TCB). 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「Barycentric Dynamical Time」の詳細全文を読む スポンサード リンク
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