Talk:Tropical year

Naming conventions

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 (talk • contribs) 11:43, 25 October 2001 UTC

OK, I screwed up -- Tompeters — Preceding unsigned comment added by Conversion script (talk • contribs) 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 (talk • contribs) 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 (talk • contribs) Revision as of 15:51, 25 February 2002 UTC

Ephemeris Time

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 212.65.42.127 (talk • contribs) 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)

That’s not the mean tropical year.

No. That’s not the mean tropical year.The time from one vernal-Equinox to the next is the Vernal-Equinox year.

A tropical year can be defined for any point on the ecliptic. It’s the time for the Sun to return to that ecliptic-point again after passing it.

That tropical year is different for each point on the ecliptic.

The mean tropical year is what it says: The (arithmetical) mean of the tropical year averaged over all of the ecliptic.

The tropical years differ due to the precessions of the equinoxes & of the apsides. 2600:6C55:7900:2B8:536:126B:20CB:9B49 (talk) 10:53, 7 March 2025 (UTC)

Prove it by citing reliable sources. Jc3s5h (talk) 14:30, 7 March 2025 (UTC)

Will do. Meanwhile, do you have a reliable-source for the claim that the mean tropical year & the Vernal-Equinox tropical year are the same? :-) 2600:6C55:7900:2B8:536:126B:20CB:9B49 (talk) 23:45, 7 March 2025 (UTC)

The passage in the article that gives the current definition of the tropical year is

Since antiquity, astronomers have progressively refined the definition of the tropical year. The entry for "year, tropical" in the Astronomical Almanac Online Glossary states:

the period of time for the ecliptic longitude of the Sun to increase 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of seasons, and its length is approximated in the long term by the civil (Gregorian) calendar. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds.

An equivalent, more descriptive, definition is "The natural basis for computing passing tropical years is the mean longitude of the Sun reckoned from the precessionally moving equinox (the dynamical equinox or equinox of date). Whenever the longitude reaches a multiple of 360 degrees the mean Sun crosses the vernal equinox and a new tropical year begins".

See the article for the citations, since citations don't work very well in talk pages. The article does not say that the modern understanding of tropical year is the vernal equinox tropical year. Jc3s5h (talk) 02:03, 8 March 2025 (UTC)

Thanks for fixing the error in the article.

The Astronomical Almanac quote correctly defines a tropical year as the duration of a 360 degree increase in ecliptic longitude.

 :I’ll keep my promise to refer you to a reliable source. (You’ve probably already found one.)
 :A tropical year starting with Earth near aphelion:

1. The equinoxes, & every point of ecliptic-longitude reckoned from them, precess westward at a nearly constant rate.

2. The tropical year is shortened a bit when the

ecliptic longitude point from which we’re reckoning moves west to meet the eastward moving Sun.

 :It’s shortened because obviously the distance that that ecliptic-point moved would have taken some time to be covered by the Solar ecliptic travel.
 :3. At aphelion, it would have taken longer for the Earth to move far enough for the Sun to appear to cover that distance.

The tropical year is shorter than it would have been at other orbital points, such as perihelion.

 :North solstice is near aphelion. Our Summer-Solstice tropical year is about about a minute & a half shorter than the Winter-Solstice tropical year.

Thank you for fixing that article error.

2600:6C55:7900:2B8:2C73:5E67:FE38:70 (talk) 02:15, 9 March 2025 (UTC)

2600:6C55:7900:2B8:2C73:5E67:FE38:70 wrote "your last sentence of your post is incorrect. The Wikipedia article explicitly defines “Mean Tropical Year” as the duration between Vernal-Equinox’s." Please quote the exact passage in the article. I suspect either you have misunderstood whatever you read, or are taking it out of context. Jc3s5h (talk) 02:58, 9 March 2025 (UTC)

No. The article previously defined “Mean Tropical Year” as duration between Vernal Equinoxes. Thank you for fixing it. 2600:6C55:7900:2B8:2C73:5E67:FE38:70 (talk) 03:27, 9

2600:6C55:7900:2B8:2C73:5E67:FE38:70 (talk) 06:44, 9 March 2025 (UTC)

MTY length determination. Cardinal-points’ calendar-drift & TY lengths

Find the time at which the Sun arrives at each degree of ecliptic longitude, over a 2-year period. From that, determine the TY length at each degree.

Numerically-integrate that over the whole ecliptic. Divide result by 360.

Could be Simpson’s rule or Romberg-integration for those uniform intervals. They say Gauss-integration is the most accurate for a given number of data-points, but it would require its own specific choice of datapoints instead of the uniformly-spaced ones.

Tychos.info says, for TY-length:

365.24+ Northward: .002374 North: 1626 Southward: 2018 South: 2189

max difference about 1.5 minutes.

Northern solstice TY-length is currently much more constant than the others. Some alternative calendar proposals therefore suggest that as the calendrical year length, to minimize that solstice’s calendrical-drift. Appealing idea.

Someone posted a graph he’d made via celestial mechanics software, showing that the North solstice has been the most constant since around the Norman Conquest. He said: “Welcome to the 1st millennium of the Age of the North-Solstice!”

The constancies of the 4 cardinal TYs varies with the Apsidal-precession.

Apsides & equinoxes precess in opposite directions, with periods of 112,000 & 25,772 years. …resulting in a 20,951 period for precession with respect to each other.

Currently the Southward equinox has the least Gregorian-drift, in the Gregorian 400-year cycle. The South Solstice has the least drift by the current 365.25 day mid-century average calendar year. . Minimizing that was, of course was the Gregorian design-purpose, but things change. Northern solstice has the most Gregorian-drift. 2600:6C55:7900:2B8:2C73:5E67:FE38:70 (talk) 07:23, 9 March 2025 (UTC)

Oops!! My post was correct, except that the last paragraph, about Gregorian-drift was all wrong, because I was looking at TY-lengths for the year 0. (That’s 1 B.C.). Actually our northern Spring equinox has by far the least Gregorian-drift. …both by the overall 400-year mean Gregorian-year, & also by our mid-century 365.25 Gregorian-year. 2600:6C55:7900:2B8:2C73:5E67:FE38:70 (talk) 21:01, 9 March 2025 (UTC)