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Barycentric Dynamical Time
Barycentric Dynamical Time (TDB) was defined by the International Astronomical Union (IAU) in 1976 to be used as the relativistic replacement for the non-relativistic Ephemeris Time which had been used in the ephemerides starting in 1960. Beginning with the ephemeris for 1984, TDB has been, effectively, the independent variable of time used for calculating the motions of bodies in the solar system. TDB is a timescale that increments uniformly in a reference frame co-moving with the barycenter of the solar system. On the long term average the rate of TDB matches the rate of atomic clocks ticking SI seconds on the surface of the earth, though there are notable differences during the year as the Earth moves closer to the Sun and speeds up in its orbit (northern winter) or moves farther and slows down (northern summer).
TDB is the direct successor of Ephemeris Time in that the values of physical constants, notably the Gaussian gravitational constant, match the traditional values from pre-relativistic days. Terrestrial Dynamical Time (TDT) and TDB were defined in a series of resolutions at the same meeting of the International Astronomical Union.
It was soon 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. Furthermore, because the length of the TDB second is determined by clocks on earth (as opposed to the barycentric reference frame itself) it disagrees with the SI second that would be determined by a clock at rest in the frame. As a result, in 1991 the IAU refined the notions of timescales by creating Barycentric Coordinate Time (TCB) and Geocentric Coordinate Time (TCG) as replacements for TDB.
Despite IAU recommendations that TCB be used for all further calculations of solar system ephemerides, as of 2002 TDB and Ephemeris Time continue to be used, the latter by the producer of the important DE200 ephemeris and its successors at the Jet Propulsion Laboratory. This somewhat controversial approach is taken because the timescale is fitted to observed data for the planets, and to a lesser extent some of their satellites. To adopt TDB or TCB would be to force a timestream based on terrestrial clocks, albeit "corrected" for (some) general relativistic effects, on a data set with which it might not be quite compatible. That said, the differences between Ephemeris Time and TDB appear to be immeasurably small as of 2005 AD. Nevertheless, as greater accuracy is attained with International Atomic Time and Ephemeris Time differences may appear; thus it seems worthwhile to retain the two timestreams, Ephemeris Time and TDB or TCB, in hopes that we can learn from any measured differences. For practical purposes the only difference between TDB and TCB is that TCB ticks faster. This rate difference means, according to some scientists, that physical constants have different values in TCB than they do in TDB. Changing software from the traditional TDB values to the recommended TCB values would require considerable effort, but please note the considerations in the next paragraph.
Relativists accept Einstein's Principle of Equivalence (see general relativity), so that fundamental physical constants are the same in all inertial coordinate systems, and most do not use alternate definitions of the second as, for example, set up in TDB or TCB, these seeming to be fossils of early attempts to define absolute time. Relativists who are familiar with general relativity insist that there is no unambiguous way to compare the rates of clocks separated from each other in space or in time, or in relative motion to one another, nor to so compare measures of length and so on. Attempts to set up such comparisons are bound to fail when pursued to higher and higher accuracy. These comparisons and equations that model them may, however, be useful in limited contexts, though they are not normally regarded as a basis for defining different units.
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