Talk:Ephemeris time

message: Using the information that ET and TAI were already 32.184 seconds divergent when TAI was adopted in 1958, and a rough guesstimate of average increasing divergence of 0.7 seconds per year, it looks as if the divergence between UT and ET was first recognized around 1912.  Could we add information to this article about when ET was first adopted, and when its divergence from UT was first recognized? arkuat (talk) 15:32, 18 May 2008 (UTC)
replies: text: Ephemeris Time (ET) was so named because it was the independent time scale used in government ephemerides from 1900 through 1983, in the American Ephemeris and Nautical Almanac, the British Nautical Almanac and Astronomical Ephemeris, the French Connaissance des Temps, the German Astronomisches Jahrbuch, and the Spanish almanac. ET was defined (but not named) by Simon Newcomb in 1895/98 as the weighted average of mean solar time referred to the Royal Observatory, Greenwich between 1750 and 1892. It was implemented at Greenwich mean noon on 31 December 1899 (0 January 1900), meaning that from that date on the stated positions of the planets in the ephemerides would be calculated in terms of ET, not in terms of Greenwich mean solar time (named UT in 1928) which had been used before 1900. ET differs from UT quadratically, not linearally, that is, the difference is generally a parabola. See ΔT. However, this divergence was not recognized in 1895, instead its implemention included a "great empirical term", a sinusoid with a period of 257 years. — Joe Kress (talk) 08:09, 19 May 2008 (UTC)
thread_title:  When adopted? 

message: Much additional information (citation-sourced) has been added, and the article rearranged.  The 'Reference Desk' info about start date 1900 and definition by Newcomb appears to be incompatible with the sources, but owing to inexperience I'm not quite sure where the 'Reference Desk' is or what can be done about it.  (Newcomb published in terms of GMT, as was conventional for his time, and additional sources can be cited if required to show that he regarded the evidence for the rotational fluctuations as not yet conclusive.)
Terry0051 (talk) 01:41, 28 February 2009 (UTC)
thread_title: Notes about revision of article

message: Is 'earth's orbit around the sun' defined in relation to the 'fixed stars' (aka sidereal year - which corresponds to earth's actual position in space relative to the sun and 'fixed stars'), or to the 'tropical year' (which is the modern standard definition of 'year' but corresponds to the length of time between vernal equinoxes, and is slightly shorter than the period of a full orbit around the sun due to the direction of earth's axial tilt rotating counterclockwise over a 26,580 year period? this should be clarified in the article Firejuggler86 (talk) 00:39, 29 October 2020 (UTC)
replies: text: This is explained on page 82 of the Explanatory Supplement to the Astronomical Almanac 3rd ed. It's based on an abstraction of the tropical year. The mean motion of the Sun was calculated by Newcomb at the end of the 19th century. If one takes the derivative of the motion, that is, δL/δT at January 0, 1900, 12 h UT, and calculate how long long it would take the mean longitude of the Sun as viewed from the Earth to increase 360°, the answer is {{val}31556925.9747}} seconds, which implicitly defines the second. The modern definition of the second, in terms of a particular kind of atomic clock, was set up to be as close to the ephemeris second as possible. Jc3s5h (talk) 13:10, 29 October 2020 (UTC)
thread_title:  definition of 'earth's orbit around the sun'