Editing Barycentric Dynamical Time (section)

{{Short description|Linear scaling of Barycentric Coordinate Time}}
'''Barycentric Dynamical Time''' ('''TDB''', from the [[French language|French]] {{lang|fr|Temps Dynamique Barycentrique}}) is a relativistic [[coordinate time]] scale, intended for astronomical use as a [[time standard]] to take account of [[time dilation]]<ref>Explanations given with (a) [http://www.iau.org/static/resolutions/IAU1991_French.pdf IAU resolutions 1991], under Resolution A.4, at 'Notes for recommendation III', and [http://www.iau.org/static/resolutions/IAU2006_Resol3.pdf IAU 2006 resolution 3], and its footnotes; and (b) explanations and references cited at "[[Time dilation#Time dilation due to gravitation and motion together|Time dilation -- due to gravitation and motion together]]".</ref> when calculating orbits and [[Ephemeris|astronomical ephemerides]] of [[planet]]s, [[asteroid]]s, [[comet]]s and interplanetary [[spacecraft]] in the [[Solar System]]. TDB is now (since 2006) defined as a linear scaling of [[Barycentric Coordinate Time]] (TCB). A feature that distinguishes TDB from TCB is that TDB, when observed from the Earth's surface, has a difference from [[Terrestrial Time|Terrestrial Time (TT)]] that is about as small as can be practically arranged with consistent definition: the differences are mainly periodic,<ref name=clem67>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, [http://articles.adsabs.harvard.edu/abs/1967AJ.....72.1324C "Annual variation of an atomic clock"], Astronomical Journal, Vol.72 (1967), p.1324-6.</ref> and overall will remain at less than 2 milliseconds for several millennia.<ref>[http://www.iau.org/static/resolutions/IAU2006_Resol3.pdf IAU 2006 resolution 3], see Recommendation and footnotes, note 3.</ref>

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1976 as a successor to the (non-relativistic) former standard of [[ephemeris time]] (adopted by the [[IAU]] in 1952 and superseded 1976).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 deprecations since the 1970s,<ref>(a)P K Seidelmann & T Fukushima (1992), [http://articles.adsabs.harvard.edu/full/1992A%26A...265..833S "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".</ref> 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 [[Ephemeris time#JPL ephemeris time argument Teph|JPL ephemeris time argument T<sub>eph</sub>]], as implemented in [[Jet Propulsion Laboratory Development Ephemeris|JPL Development Ephemeris]] [[DE405]], "is for practical purposes the same as TDB defined in this Resolution".<ref>[http://www.iau.org/static/resolutions/IAU2006_Resol3.pdf IAU 2006 resolution 3], see footnotes, note 4.</ref> (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 was superseded by DE430.<ref>See US Naval Observatory (Naval Oceanography Portal), [http://aa.usno.navy.mil/publications/docs/asa_history.php "History of the Astronomical Almanac"] {{Webarchive|url=https://web.archive.org/web/20090305231056/http://aa.usno.navy.mil/publications/docs/asa_history.php |date=2009-03-05 }} (accessed October 2015); also, for details of DE405: E. M. Standish (1998), [https://web.archive.org/web/20200208202819/ftp://ssd.jpl.nasa.gov/pub/eph/planets/ioms/de405.iom.pdf 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 [https://books.google.com/books?id=Pr0H9aSEfKYC&pg=SL12-PA4 Astronomical Almanac for 2015, page L-4] (accessed October 2015).</ref>)