Talk:Tropical year

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Naming conventions[edit]

Karl, lets settle on a convention about Latin, Greek, English words. I think we should stick to whatever is current in English. If you start to Romanize or Hellenize English words, there is no end - anglosaxons completely screw up foreign words; e.g.:

equinox would be aequinox (equus = horse, aequus = equal (sic!))

Homer would be Homeros

Now the English word is perihelium, not perihelion; like the stuff is helium, not helion. -- Tompeters

This would be a good argument, except for the fact that it's "perihelion" in English. What dictionary are you using that says otherwise? --Zundark, 2001 Oct 25

— Preceding unsigned comment added by Zundark (talkcontribs) 11:43, 25 October 2001 UTC

OK, I screwed up -- Tompeters — Preceding unsigned comment added by Conversion script (talkcontribs) Revision as of 15:51, 25 February 2002 UTC
Bad example: Helium ends in -ium because it was first found spectroscopically in the sun and they thought it was a metal: and metals get -ium or -um on the end e.g. Thorium, Hafnium, Aluminium, Neodymium, Molybdenum. If the naming convention for noble gases was followed strictly, Helium actually should be called Helion, though no-ones going to rename it at this late date - Malcolm Farmer

— Preceding unsigned comment added by Malcom Farmer (talkcontribs) 12:10, 25 October 2001

Thanx for pointing that out, I never noticed. Good to see someone writing Aluminium, americans usually say aluminum.— Preceding unsigned comment added by Conversion script (talkcontribs) Revision as of 15:51, 25 February 2002 UTC

Ephemeris Time[edit]

The article now says:

The time scale is Terrestrial Time (formerly Ephemeris Time) which is based on atomic clocks

I don't understand this. One could understand this as "Terrestrial Time was formerly called Ephemeris Time". This is not the case. Ephemeris Time is different from Terrestrial Time; and is not based on atomic clocks. Terrestrial Time is now used a lot where Ephemeris Time used to be used. But wouldn't the formulae be different in the past if they used Ephemeris Time in the past? Until someone who really understands this, sorts this out, I will delete the text in parentheses "(formerly Ephemeris Time)" -- Adhemar — Preceding unsigned comment added by (talkcontribs) 09:34, 29 August 2005

You have some valid points. Although Terrestrial Time (TT) is based on atomic clocks, it is a uniform time just like Ephemeris Time (ET), which was defined relative to the motion of solar system bodies, especially the Sun and Moon. TT (1991) is the new name for Terrestrial Dynamical Time (TDT, defined 1976). It was renamed because TDT was not dynamical, i.e., it was not based on the motion of the solar system. ET was the time base used in all national ephemerides until 1983. The offset of TT from International Atomic Time (TAI) was intentionally chosen to be 32.184 s so that it would equal ET, and thus ET can be directly substituted for TT in most astronomical equations. This is indeed done by Jean Meeus in his "Astronomical Algorithms". I am replacing the objectionable phrase by "(formerly, Ephemeris Time was used instead)". — Joe Kress 03:20, August 30, 2005 (UTC)

Which year?[edit]

The Gregorian average year-length is not designed to approximate mean tropical year.

The Gregorian Calendar's leapyear-system was designed to keep the calendar as stable as possible with respect to the March equinox. that the March equinox's calendar-date would vary as little as possible.

Twice, the article says that the Gregorian Leapyear system was designed so that the calendar's mean year would approximate the mean tropical year. That's incorrect.

Michael Ossipoff--MichaelOssipoff (talk) 02:38, 11 January 2017 (UTC)

I agree that the aim of the reform was to maintain the position of the March equinox in the calendar. What is not clear is whether the reformers were aware that the vernal equinox year and the mean tropical year are not currently the same. There are editors (one of them banned for sockpuppetry) who refuse to accept the concept of a mean time between vernal equinoxes. It would be useful if we could find more sources (other than Jean Meeus) that discuss this difference. Dbfirs 06:49, 11 January 2017 (UTC)
If you read the cited reference (page 123 in the Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its 400th Anniversary) it is apparent that the 16th century decision makers were considering the mean tropical year (I believe that at that time they would not have had the computational or theoretical wherewithal to define it as we do today, so would have to rely on observations over many years, or preferably, centuries). The extent to which they could distinguish this from the mean vernal equinox tropical year is not clear. The way I interpret this, until presented with even better sources, is that the overall goal was to follow the established religious convention that the calculation of the date of Easter called for the vernal equinox to fall as nearly as possible on the calendar date March 21, and the means to this end was to make the mean length of the Gregorian calendar year very close to the length of the mean tropical year. Jc3s5h (talk) 10:35, 11 January 2017 (UTC)