Talk:Ephemeris time

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Should the correct title of this article be "Ephemeris time" or "Ephemeris Time"? The article is at the former title, but the text uses "Time". If someone knows, I can change or move it accordingly. -- Infrogmation 14:39, 24 August 2005 (UTC)

I believe it should be "Ephemeris time". There were articles under both names, and I merged them together here, so there may be a relic 'Time' left. Salsb 15:31, August 24, 2005 (UTC)

When adopted?

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)

WP:RD/Science gave me the hint that Simon Newcomb was probably involved. arkuat (talk) 02:59, 19 May 2008 (UTC)

Further info, copied from the Reference Desk:

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)

Notes about revision of article

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)

definition of 'earth's orbit around the sun'

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)

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)

Ephemeris time based on mean solar day

Yesterday I changed

"Although ephemeris time was defined in principle by the orbital motion of the Earth around the Sun"

to

"Although ephemeris time was defined in principle by the mean solar day"

with the comment "Ephemeris time was defined based on 36525 mean solar days, not on the actual orbital motion of the Earth around the Sun".

Now Jc3s5h has reverted that with the comment "Not true. Read the lead of this article. By the 1950s it was certain that the rotation of the Earth on it's axis was irregular, and the time required for the Earth to orbit the Sun was more nearly constant, so the official definition of time used in the calculation of ephemerides was changed."

Yes, I have read the lead, and I have read much of the article by G.M. Clemence. Yes, I know that it was known that the rotationi of the Earth on its axis was irregular. But the fact is that the ephemeris day was based on the length of 36525 mean solar days. It wasn't based on some figure for the length of a tropical year or of a sidereal year. That's why the number of ephemeris seconds in a day is very close to 86,400 -- it's more or less by definition.

Eric Kvaalen (talk) 05:51, 16 November 2022 (UTC)

I will quote some passages from Chapter 3, "Ephemeridies", by McCarthy & Seidelmann

The independent variable in ephemerides is uniform time From the time of the Hellenistic astronomer Ptolemy, the concept of mean solar time was used for this independent variable. Following the recognition of the variability of the Earth's rotation, a new time scale, Ephemeris Time, based on the Earth's orbital motion was introduced. Problems with real-time realization of Ephemeris Time soon became apparent, and atomic time became available as an alternative source of Ephemeris Time. [p. 35]

McCarthy & Seidelmann's Chapter 6 is titled "Ephemeris Time". The development of the definition is described on pages 80 to 83. They describe it as a recommendation at the International Colloquium on the Fundamental Constants of Astronomy in Paris in 1950, followed by a series of improvements until the CIPM put it in it's final form in 1956. When I apply the changes to the starting definition, I come up with

In all cases where the mean solar second is unsatisfactory as a unit of time by reason of its variability, the unit adopted should be the tropical year at 1900.0; that the time reckoned in these units be designated Ephemeris Time; that the change of mean solar time to ephemeris time be acomblished by the following correction

ΔT = 24.349s + 72.381s T + 2950s T² =1.82144 B.

where T is reckoned in Julian centuries from 1900.0 January 0 Greenwich Mean Noon and B has the meaning given by Spencer Jones in Monthly Notices R.A.S. (Vol 99, 1938, p. 541) and the second is the fraction 1/31556925.9747 of the length of the tropical year for 1900.0

A quote from page 86 helps to understand the situation:

Until 1960 the second was defined as 1/86400 of the mean solar day, ignoring the variability in the earth's rotation and assuming that the Earth's rotation was uniform. In 1960 the ephemeris second was introduced as the replacement for the second defined in terms of mean solar time.

My interpretation is that the difference between time observed with instruments like those described in the Transit instrument article, and time used in equations to calculate the position of celestial objects, was glossed over. Once ephemeris time was clearly defined, these two different kinds of time could be dealt with more rigorously. Jc3s5h (talk) 14:26, 16 November 2022 (UTC)

Hilton & McCarthy explain the lack of a clear modern definition of mean solar time on their page 231

Underlying the concept of mean solar time was the assumption that the rotation of the Earth was uniform. In the first half of the twentieth century, the lunar ephemeris demonstrated that this assumtion was incorrect. Thus the mean solar time was no longer used in precise timekeeping. It was replaced by two somewhat different concepts of time: Ephemeris Time, (ET), was introduced to satisfy the desire for a uniform measure of time, and Universal Time, UT, to measure the earth's rotation. Originally UT was introduced to specify Greenwich mean Time measured from noon instead of midnight. [References to other sections of the book omitted.]

Jc3s5h (talk) 14:50, 16 November 2022 (UTC)

References

• James L. Hilton & Dennis McCarthy (2013), "Precession, Nutation, Polar Motion, and Earth Rotation" in Sean E. Urban and P. Kenneth Seidelmann, eds., Explanatory Supplement to the Astronomical Almanac 3rd ed., University Science Books, Mill Valley, CA.
• Dennis McCarthy & P. Kenneth Seidelmann (2009), TIME From Earth Rotation to Atomic Physics, Wiley-VCH, Weinheim, ISBN 978-3-527-40780-4.