ephemeris time

noun Astronomy.
time measured by the orbital movements of the earth, the moon, and the planets.

Origin:
1945–50

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Collins
World English Dictionary
ephemeris time
 
n
time that is based on the orbit of the earth around the sun rather than the axial rotation of the earth, one ephemeris second being 1/31 556 925.9747 of the tropical year 1900. It was used from 1960 to 1983 as an astronomical timescale but has been replaced by terrestrial dynamical time and barycentric dynamic time

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American Heritage
Science Dictionary
ephemeris time  
A highly accurate astronomical system for the measurement of time based in theory on the period of Earth's orbit, but in practice relying on lunar observations and an accurate lunar ephemeris so as to not be subject to the irregularities of Earth's rotation. The year 1900 CE is considered a standard year, containing 31,556,925.9747 seconds of ephemeris time.
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Example sentences
The program will then compute the other coordinates and the ephemeris time and pixel dn value.
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