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9 - Dynamical and Coordinate Timescales

Published online by Cambridge University Press:  01 October 2018

Dennis D. McCarthy
Affiliation:
United States Naval Observatory
P. Kenneth Seidelmann
Affiliation:
University of Virginia
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Summary

With the recognition of the problems with Ephemeris Time and the need to make changes in the celestial reference system in 1976, improved dynamical timescales continuous with Ephemeris Time and consistent with the theory of relativity were developed. Dynamical time is understood as the time-like argument of dynamical theories and the independent variable of the equations of motion of solar system bodies. In 1976, Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB) were introduced. Problems with the definition of TDT and TDB and the need for a new reference system based on accurate observations of distant radio sources were recognized. So Terrestrial Time (TT), Geocentric Coordinate Time (TCG), and Barycentric Coordinate Time (TCB) were introduced, and TDB was redefined. Barycentric Ephemeris Time (Teph) was officially recognized. Ephemeris Time Revised is still necessary for timescales prior to 1956. Relativistic equations specify the relationships between the different timescales.
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Publisher: Cambridge University Press
Print publication year: 2018

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