Editing Barycentric Dynamical Time (section)

==History==
{{seealso|Ephemeris time#History of ephemeris time (1952 standard)}}

From the 17th century to the late 19th century, planetary ephemerides were calculated using time scales based on the Earth's rotation: usually the [[mean solar time]] of one of the principal observatories, such as Paris or Greenwich. After 1884, mean solar time at Greenwich became a standard, later named [[Universal Time]] (UT). But in the later 19th and early 20th centuries, with the increasing precision of astronomical measurements, it began to be suspected, and was eventually established, that the rotation of the Earth (i.e. the length of the day) showed irregularities on short time scales, and was slowing down on longer time scales. [[Ephemeris time]] was consequently developed as a standard that was free from the irregularities of Earth rotation, by defining the time "as the independent variable of the equations of celestial mechanics", and it was at first measured astronomically, relying on the existing gravitational theories of the motions of the Earth about the Sun and of the Moon about the Earth.

After the caesium [[atomic clock]] was invented, such clocks were used increasingly from the late 1950s as [[Ephemeris time#Secondary realizations by atomic clocks|secondary realizations of ephemeris time (ET)]]. These secondary realizations improved on the original ET standard by the improved uniformity of the atomic clocks, and (e.g. in the late 1960s) they were used to provide standard time for planetary ephemeris calculations and in astrodynamics.

But ET in principle did not yet take account of relativity theory. The size of the periodic part of the variations due to [[time dilation]] between earth-based atomic clocks and the [[coordinate time]] of the Solar-System barycentric reference frame had been estimated at under 2 milliseconds,<ref name=clem67/> but in spite of this small size, it was increasingly considered in the early 1970s that time standards should be made suitable for applications in which differences due to relativistic time dilation could no longer be neglected.

In 1976, two new time scales were defined<ref>They were defined in substance in 1976 but given their names in 1979.</ref> to replace ET (in the ephemerides for 1984 and afterwards) to take account of relativity. ET's direct successor for measuring time on a geocentric basis was [[Terrestrial Dynamical Time]] (TDT). The new time scale to supersede ET for planetary ephemerides was to be Barycentric Dynamical Time (TDB). TDB was to tick uniformly in a reference frame comoving with the [[barycenter]] of the Solar System. (As with any [[coordinate time]], a corresponding clock, to coincide in rate, would need not only to be at rest in that reference frame, but also (an unattainable hypothetical condition) to be located outside all of the relevant [[gravity well]]s.) In addition, TDB was to have (as observed/evaluated at the Earth's surface), over the long term average, the same rate as TDT (now [[Terrestrial Time|TT]]). TDT and TDB were defined in a series of resolutions at the same 1976 meeting of the [[International Astronomical Union]].

It was eventually realized that TDB was not well defined because it was not accompanied by a general relativistic metric and because the exact relationship between TDB and TDT had not been specified. (It was also later criticized as being not physically possible in exact accordance with its original definition: among other things the 1976 definition excluded a necessary small offset for the initial epoch of 1977.)<ref>E M Standish (1998), [http://articles.adsabs.harvard.edu/full/1998A%26A...336..381S "Time scales in the JPL and CfA ephemerides"], ''Astronomy and Astrophysics'', v.336 (1998), p.381-384.</ref> After the difficulties were appreciated, in 1991 the IAU refined the official definitions of timescales by creating additional new time scales: [[Barycentric Coordinate Time]] (TCB) and [[Geocentric Coordinate Time]] (TCG). TCB was intended as a replacement for TDB, and TCG was its equivalent for use in near-Earth space. TDT was also renamed to [[Terrestrial Time]] (TT), because of doubts raised about the appropriateness of the word "dynamical" in that connection.

In 2006 TDB was redefined by IAU 2006 resolution 3; the 'new' TDB was expressly acknowledged as equivalent for practical purposes to [[Ephemeris time#JPL ephemeris time argument Teph|JPL ephemeris time argument T<sub>eph</sub>]]; the difference between TDB according to the 2006 standard and [[Terrestrial Time|TT]] (both as observed from the surface of the Earth), remains under 2 ms for several millennia around the present epoch.<ref>[http://www.iau.org/static/resolutions/IAU2006_Resol3.pdf IAU 2006 resolution 3], see especially footnotes 3 and 4.</ref>