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  • $\begingroup$ This isn't fully accurate - ephemeris time is based on the year length using 1900 as the baseline epoch, not 1820. $\endgroup$ Commented Apr 23, 2018 at 8:06
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    $\begingroup$ @EmilioPisanty - It is fully accurate. It's just missing a piece or two. Ephemeris time was defined in 1948 based on Newcomb's late 19th century model of the Earth's orbit, which in turn was based on measurements of the solar day over a span of 140 years, centered on 1820. $\endgroup$ Commented Apr 23, 2018 at 20:49
  • $\begingroup$ @DavidHammen Is there a refernce to that analysis, and how they extrapolated to 1900 (apparently it is day 0.5 of January 1900, so really it's mid day of the last day of 1899, so they get a full night's observing without fiddling with the clocks IIRC) $\endgroup$ Commented Apr 24, 2018 at 7:45
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    $\begingroup$ @Philip-Oakley References: An influential analysis towards ephemeris seconds was Spencer Jones' (adsabs.harvard.edu/abs/1939MNRAS..99..541S), see also many other references in (en.wikipedia.org/wiki/Ephemeris_time). The ephemeris second was only proposed originally for scientific use, and would have left mean solar time in use for civil life that did not need such precision. This caveat was later ignored by standards bodies, which partly explains the perversity of adopting a standard second that has always been a bit too short. $\endgroup$ Commented Apr 24, 2018 at 9:03
  • $\begingroup$ @Philip-Oakley Starting the astronomical day at noon was a very old tradition, the official almanacs only converted to the civil-midnight day-start from 1925. $\endgroup$ Commented Apr 24, 2018 at 9:05