ephemeris second

ephemeris second
the unit of ephemeris time, defined as a precise fraction of the length of the tropical year 1900.

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ephemeris second,
Astronomy. the fundamental unit of ephemeris time, adopted in 1956 by the International Bureau of Weights and Measures, based upon a particular fraction of the solar year 1900: »

The…ephemeris second is accurate within a few parts per billion (Scientific American).

Useful english dictionary. 2012.

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