Time Standards For Planetary Motion Calculations
Ephemeris time and its successor time scales described below have all been intended for astronomical use, e.g. in planetary motion calculations, with aims including uniformity, in particular, freedom from irregularities of Earth rotation. Some of these standards are examples of dynamical time scales and/or of coordinate time scales.
- Ephemeris Time (ET) was from 1952 to 1976 an official time scale standard of the International Astronomical Union; it was a dynamical time scale based on the orbital motion of the Earth around the Sun, from which the ephemeris second was derived as a defined fraction of the tropical year. This ephemeris second was the standard for the SI second from 1956 to 1967, and it was also the source for calibration of the caesium atomic clock; its length has been closely duplicated, to within 1 part in 1010, in the size of the current SI second referred to atomic time. This Ephemeris Time standard was non-relativistic and did not fulfil growing needs for relativistic coordinate time scales. It was in use for the official almanacs and planetary ephemerides from 1960 to 1983, and was replaced in official almanacs for 1984 and after, by numerically integrated Jet Propulsion Laboratory Development Ephemeris DE200 (based on the JPL relativistic coordinate time scale Teph).
For applications at the Earth's surface, ET's official replacement was Terrestrial Dynamical Time (TDT), since redefined as Terrestrial Time (TT). For the calculation of ephemerides, TDB was officially recommended to replace ET, but deficiencies were found in the definition of TDB (though not affecting Teph), and these led to the IAU defining and recommending further time scales, Barycentric Coordinate Time (TCB) for use in the solar system as a whole, and Geocentric Coordinate Time (TCG) for use in the vicinity of the Earth. As defined, TCB (as observed from the Earth's surface) is of divergent rate relative to all of ET, Teph and TDT/TT; and the same is true, to a lesser extent, of TCG. The ephemerides of sun, moon and planets in current widespread and official use continue to be those calculated at the Jet Propulsion Laboratory (updated as from 2003 to DE405) using as argument Teph.
- Terrestrial Dynamic Time (TDT) replaced Ephemeris Time and maintained continuity with it. TDT is a uniform atomic time scale, whose unit is the SI second. TDT is tied in its rate to the SI second, as is International Atomic Time (TAI), but because TAI was somewhat arbitrarily defined at its inception in 1958 to be initially equal to a refined version of UT, TT is offset from TAI, by a constant 32.184 seconds. The offset provided a continuity from Ephemeris Time to TDT. TDT has since been redefined as Terrestrial Time (TT).
- Barycentric Dynamical Time (TDB) is similar to TDT but includes relativistic corrections that move the origin to the barycenter. TDB differs from TT only in periodic terms. The difference is at most 2 milliseconds.
In 1991, in order to clarify the relationships between space-time coordinates, new time scales were introduced, each with a different frame of reference. Terrestrial Time is time at Earth's surface. Geocentric Coordinate Time is a coordinate time scale at Earth's center. Barycentric Coordinate Time is a coordinate time scale at the center of mass of the solar system, which is called the barycenter. Barycentric Dynamical Time is a dynamical time at the barycenter.
- Terrestrial Time (TT) is the time scale which had formerly been called Terrestrial Dynamical Time. It is now defined as a coordinate time scale at Earth's surface.
- Geocentric Coordinate Time (TCG) is a coordinate time having its spatial origin at the center of Earth's mass. TCG is linearly related to TT as: TCG - TT = LG * (JD -2443144.5) * 86400 seconds, with the scale difference LG defined as 6.969290134e-10 exactly.
- Barycentric Coordinate Time (TCB) is a coordinate time having its spatial origin at the solar system barycenter. TCB differs from TT in rate and other mostly periodic terms. Neglecting the periodic terms, in the sense of an average over a long period of time the two are related by: TCB - TT = LB * (JD -2443144.5) * 86400 seconds. According to IAU the best estimate of the scale difference LB is 1.55051976772e-08.
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