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6 - Ephemeris Time

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

The discovery of the variability of the Earth's rotation rate called for a uniform timescale for ephemerides and other scientific purposes. André Danjon and Gerald Clemence proposed a timescale based on the orbital motion of the Earth that was adopted in 1952, and called Ephemeris Time (ET). The working definition was based on a mathematical expression for the longitude of the Sun as a function of time. However, observations of the Sun are difficult and limited in accuracy, so observations of the Moon were used in practice to determine Ephemeris Time. This time depended on the analysis of astronomical observations. Consequently, it was not available immediately. Fortunately, atomic clocks started to become available in 1956, providing a timescale accurately representing ET. The availability of atomic timescales led to the introduction of International Atomic Time (TAI) and dynamical timescales.
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Publisher: Cambridge University Press
Print publication year: 2018

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