Talk:Gregorian calendar

Semi-inaccuracy in top section

The number 365.2422 is the current length of the "mean tropical year", but the Gregorian calendar reformers weren't trying to approximate that (I'm not sure that the concept of mean tropical year was known in 1582). Since one of the main motivations of the Gregorian reform was to correct the date of Easter, and Easter is defined in terms of the vernal equinox, they were trying to approximate the vernal equinox year, or time interval separating vernal equinoxes, which is not exactly the same (the equinox interval is mentioned prominently near the beginning of our tropical year article). According to Tropical year#Different tropical year definitions, the current length of the vernal equinox year is 365.242374 days, and this number is more relevant for judging the accuracy of the Gregorian calendar than 365.2422. The devisers of the Revised Julian Calendar ignored this when they made the average year length be 365.242 days (a disimprovement with respect to the vernal equinox year -- they were more desperate to show their independence from Catholics by having their calendar not be exactly the same, than thinking about the historical function of the calendar with respect to Christianity). AnonMoos (talk) 09:49, 14 October 2022 (UTC)

THIS IS IMPORTANT!

You are right, but it is real, not just semi, inaccuracy that spreads a common astronomical misconception that needs to be corrected. I was going to edit the page directly, and if this is ignored I probably will.

I am not sure what a "Solar Year" is, but the period that the Earth orbits the sun is close to 365.26 years. As a professional astronomer, I would call this the "Orbital Period of the Earth around the Sun."

I would also call a "Tropical Year" the time between Vernal Equinoxes. Since the Vernal Equinox precesses, this is a little different than the period of the Earth around the sun.

The beginning of this should reflect this change. I propose the following:

CHANGE THIS:

The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun.

TO THIS:

The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' year that is determined by the time from one 'vernal equinox' to the next. This is smaller than the period of the 'Earth's orbit', which is about 365.256 days. Jonathan Keohane (talk) 19:30, 9 February 2024 (UTC)

Please read our article "Tropical year" which explains that the modern definition of the tropical year is not the time between vernal equinoxes, nor the mean time between vernal equinoxes, but "the period of time for the ecliptic longitude of the Sun to increase 360 degrees" (quoting the Astronomical Almanac Online Glossary). Jc3s5h (talk) 20:20, 9 February 2024 (UTC)

I would oppose wording that talks about the "period of the Earth's orbit", since this is an ambiguous description. You seem to be using it as a synonym for "sidereal year", but "period of the Earth's orbit" could have other meanings (such as the sidereal year or the anomalistic year). If we want to refer to the sidereal year, we should use that terminology since it is unambiguous, but I don't really see that it's necessary to mention it in this context. The tropical year is different than a lot of different time periods; we don't need to mention all the things that it is not equal to. CodeTalker (talk) 00:58, 10 February 2024 (UTC)

:Time for a WP:rs then. How accurately could astronomers of the day measure the tropical year? And the reformers had to consider what would be acceptable and explicable to a substantially illiterate and inumerate "flock". The algorithm is good enough. IMO, the current statement in the lead is also good enough for our purposes: the "perfect" is described in detail in the body. We know that the orbit of planet earth is not a perfect metronome, so approximations will always be needed. For almost all of the people for almost all of the time, the niceties are entirely background radiation. --𝕁𝕄𝔽 (talk) 10:14, 14 October 2022 (UTC)

Reading on mobile, I see I didn't quite appreciate your question, my apologies. Would it meet your objection if the current more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun. were rephrased as more closely approximating the true duration of the time between vernal equinoxes, which is a little less than than the 365.25 days in the Julian calendar. I think we can say that without breaking WP:OR (and the present lead doesn't even say what the Julian figure is, so needs adding).

BTW, we shouldn't really say determined by the Earth's revolution around the Sun. since at the time the sun went around the earth. --𝕁𝕄𝔽 (talk) 17:10, 14 October 2022 (UTC)

Where angels fear to tread

I decided to be bold and give effect to this discussion so that the lead would read

The Gregorian calendar is the calendar used in most parts of the world.^ref It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, which is a little less than than the 365.25 days in the Julian calendar but which more closely approximates the true duration of the time between vernal equinoxes.

but that introduces duplication in close succession, because the next para (after stating the rule) reads

There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes.

Would it be too terse to delete There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception. ? --𝕁𝕄𝔽 (talk) 13:26, 18 October 2022 (UTC)

No, not delete. How about There were two reasons to establish the Gregorian calendar. First, the Julian calendar overestimated the length of the year by a little under one day per century, and thus has a leap year every four years without exception. Better? --𝕁𝕄𝔽 (talk) 15:06, 18 October 2022 (UTC)

The following discussion is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

---

If a tropical year is exactly 365.2422 days (=365 + 1211/5000 days), the Gregorian calendar or Revised Julian calendar should be 365.2422 days. To make it into 365.2422 days, the calendar needs a leap year rule that a leap day should be added every year which is a multiple of 4 and not a century year + every year which is a quarter-century year (i.e. a multiple of 250) + every year which is a quarter-decamillennium year (i.e. a multiple of 2500): thus the years 250, 750, 1250, 1750, 2250, 2500, 2750, 3250, 3750, 4250 and 4750 would be leap years.

Besides this I wonder why a calendar year starts in January, rather than in March. In the Roman calendar, a year started in March and ended in the next February. If a calendar year would be March-February rather than January-December, a leap day could be added in late-August rather than late-February, as late-August would be the mid of the year (if the year would be March-February), and I think a year should rather than in its beginning or its end have a leap day in its middle. 212.100.101.104 (talk) 23:24, 24 August 2023 (UTC)

On your point one, that may well be true and perhaps in a thousand years' time (if homo questionably sapiens still exists) the calendar may be revised. When it is, a future Wikipedia will document it as reality and until then, it cannot go in the article (see policies WP:CRYSTAL and WP:No original research). On your point two, see New Year's Day#New Year's Day in the older Julian calendar. --𝕁𝕄𝔽 (talk) 22:51, 26 August 2023 (UTC)

You must have meant "quarter-millennium year", not "quarter-century year", as years which are multiples of 250 are a quarter-millennium. 217.21.226.230 (talk) 11:09, 30 August 2023 (UTC)

I wonder how the difference from the Gregorian to the Julian calendar may be -13 days now (since 13 March 1900 Gregorian) and -10 days between March 1500 and March 1700. Between 1500 and 1700, the difference is supposed to have been twelve days, if applying the rule "no leap day in century years except those that are multiples of four hundred", as the century years 400, 800 and 1200 would be leap years; i.e. three century years of fifteen and 15 minus 3 = 12. If the difference is 10 days rather than 12 days after year 1500, a different leap year rule should have applied before the 16th century; i.e. a rule that a century year is a leap year only if being a multiple of 300 (which would make 300, 600, 900, 1200 and 1500 leap years), or a rule that a leap day shall be added every year which is a multiple of 4 and not a century year + every half-millennium year and quarter-millennium year (which would make the years 250, 500, 750, 1250 and 1500 leap years). 212.100.101.104 (talk) 23:41, 3 September 2023 (UTC)

Eleven days in 1700 (see Calendar (New Style) Act 1750#"Give us our eleven days!" – the calendar riot myth). But it really doesn't matter in the real world, as I have explained already. --𝕁𝕄𝔽 (talk) 16:02, 4 September 2023 (UTC)

The goal of the committee that devised the new calendar was to return the date of the northern hemisphere spring equinox to 21 March. This had been the traditional date since the Council of Nicea. They made the judgement this required skipping 10 days. Since they didn't state which city the correction was meant to be most valid for, it's hard to judge how accurate the correction was.

Also, there was no intent to renumber years before 1583; historical dates were just left as they were. Jc3s5h (talk) 20:59, 4 September 2023 (UTC)

However, I think the Gregorian or revised Julian calendar should let a year run from March to February, like the Roman calendar, rather than January to December. It would be more realistic to add a leap day in the end of August rather than February, as late-August would be mid-year if the year were March-February, and adding an extra day in mid-year is more realistic than adding an extra day in the end or beginning of the year.

P.S.: The rule that a leap day should be added in February was kept at the switch from the Roman to Julian calendar, despite the change of the year from March-February to January-December. 212.100.101.104 (talk) 14:24, 6 September 2023 (UTC)

WP:TALK#TOPIC states "Talk pages are for discussing the article, not for general conversation about the article's subject...." Proposed changes to calendars made by Wikipedia editors do not belong on talk pages (or anywhere else in Wikipedia). Jc3s5h (talk) 15:16, 6 September 2023 (UTC)

The discussion above is closed. Please do not modify it. Subsequent comments should be made on the appropriate discussion page. No further edits should be made to this discussion.

Swapping description and lead sections?

Would it be better to move a bunch of the stuff in "Description" into the lead section, and move what's now in the lead section to a "History" section? The lead section right now really assumes you already are very familiar with the Gregorian calendar, which reads a bit odd to me. AapeliV (talk) 00:32, 1 January 2023 (UTC)

Semi-protected edit request on 4 February 2023

Please remove this sentence:

Because the date of Easter is a function – the computus – of the date of the (northern hemisphere) spring equinox

and add this one:

Because the date of Easter is a function – the computus – of the date of the spring equinox in the northern hemisphere

While it's slightly longer, it flows better without the parenthetical phrase. 192.180.91.15 (talk) 11:48, 4 February 2023 (UTC)

 Done – small jars tc 12:24, 4 February 2023 (UTC)

Realignment

It probably should be made more clear that the Gregorian calendar reform did not realign the dates with the beginning of the Julian calendar, but rather realigned the dates to the Julian dates that the Catholic Church were use to using for Easter calculations in the 3rd century (already off by a couple days). Proleptic Gregorian Jan 1, 10CE is not the same day as Julian Jan 1, 10CE (and definitely off for dates before 4CE as the Julian calendar's leap years were being incorrectly applied for its first few decades, only fully corrected by the end of the first decade CE). — al-Shimoni (talk) 16:52, 29 December 2023 (UTC)

I’m not Jewish! BUT why isn’t the Jewish Calendar listed in the list of calendars, which is the true calendar the world should be operating on⁉️🧐🧐🧐 78.149.113.1 (talk) 14:18, 1 January 2024 (UTC)

I am Jewish and have no idea what you're talking about. The Rabbinic Jewish calendar is listed in the infobox but is misidentified as the Hebrew calendar.

There are three Hebrew calendars in use today: The Rabbinic Jewish Calendar, which starts on 1 Tishri. The Karaite Jewish calendar, which starts on 1 Aviv (which starts on the first new moon after the ripening of the barley in Israel). And the Samaritan calendar, which starts on 1 Elul. The years are the same for the two Jewish calendars; they just start at different times (one in the spring, one in the autumn). The Samaritan Hebrew calendar uses a different system of numeration and calculation altogether.

2601:645:C57F:74A0:71FE:65DD:4352:40EF (talk) 01:56, 3 March 2024 (UTC)

Unfortunately per WP:COMMONNAME, the Rabinical calendar is the one most widely known outside Judaism. Life's a bitch. See also Islamic calendar, meaning the Sunni Lunar Hijri calendar getting priority over the Shia Solar Hijri calendar. --𝕁𝕄𝔽 (talk) 14:40, 3 March 2024 (UTC)

Roman Republic not Roman Emprie

One of the tables shows 153 BC as the date the Roman Empire adopted 1 January as New Year's Day. There was NO Roman Empire in 153 BC. The Roman Empire did not exist until 27 BC. when Augustus became the first Emperor.

Julius Caesar had not even been born yet in 153 BC, which was VERY much within the days of the Roman REPUBLIC not the Roman empire.

2601:645:C57F:74A0:71FE:65DD:4352:40EF (talk) 01:50, 3 March 2024 (UTC)