Re: cal-persia.el disagrees with Iranian calendar in A.D. 2025
Ed Reingold <reingold@emr.cs.iit.edu> writes:
For the TZ data base, what is the point of making one assumption over the other? Neither will be known right or wrong for another 20 years.
It's mostly a matter of documenting the projected daylight-saving transitions. Currently we project through 2038 (when 32-bit time_t values roll over). We are adding support for 64-bit time_t values, though, and at some point we'll probably start projecting a bit further (50 years, say? we haven't decided). The cutoff is somewhat arbitrary, but going a few decades into the future helps give novices a better feel for the complexity of the projections. By the way, in researching this I found the following reference useful: M. Heydari-Malayeri (Paris Observatory), A concise review of the Iranian calendar (2005-02-15), <http://wwwusr.obspm.fr/~heydari/divers/ir-cal-eng.html> It mentions the March 20, 2025 discrepancy, and it has some interesting and not-altogether-positive things to say about the method used in GNU Emacs. I hadn't realized how controversial this area is.
For Emacs, it's irrelevant because it does not contain the Persian astronomical calendar
Thanks for clarifying this. Would it be appropriate to make the following change to the GNU Emacs user documentation, if only to help forestall future bug reports in this area? 2005-03-31 Paul Eggert <eggert@cs.ucla.edu> * calendar.texi (Calendar Systems): Mention that the Persian calendar implemented is the arithmetical calendar of Birashk. --- calendar.texi.~1.33.~ 2005-03-28 16:30:06 -0500 +++ calendar.texi 2005-03-31 01:46:45 -0500 @@ -691,6 +691,12 @@ Their calendar consists of twelve months days, the next five have 30 days, and the last has 29 in ordinary years and 30 in leap years. Leap years occur in a complicated pattern every four or five years. +The calendar implemented here is the arithmetical Persian calendar +championed by Birashk, based on a 2,820-year cycle. It differs from +the astronomical Persian calendar, which is based on astronomical +events. As of this writing the first future discrepancy is projected +to occur on March 20, 2025. It is currently not clear what the +official calendar of Iran will be that far into the future. @cindex Chinese calendar The Chinese calendar is a complicated system of lunar months arranged
On Thu, 2005-03-31 at 02:18 -0500, Paul Eggert wrote:
Ed Reingold <reingold@emr.cs.iit.edu> writes:
For the TZ data base, what is the point of making one assumption over the other? Neither will be known right or wrong for another 20 years.
By the way, in researching this I found the following reference useful:
M. Heydari-Malayeri (Paris Observatory), A concise review of the Iranian calendar (2005-02-15), <http://wwwusr.obspm.fr/~heydari/divers/ir-cal-eng.html>
It mentions the March 20, 2025 discrepancy, and it has some interesting and not-altogether-positive things to say about the method used in GNU Emacs. I hadn't realized how controversial this area is.
The issue is only controversial in academic circles. It's not controversial in legal circles at all. The text of the Iranian law, in effect since 1925, clearly mentions that the true solar year is the measure, and there is no arithmetic leap year calculation involved. There has never been any serious plan to change that law, and since the original law is passed by the Majlis (parliament), there are only two ways to change it: 1) The Guardian Council calling it void because of it being against Islam or the Islamic Republic Constitution, which would be a really weird thing to do, since it's Islamic (uses the year of Hegira of Muhammed as its first year), and the calendar being "hejri-e shamsi" (Solar Hegira-based) is mentioned in the Islamic Constitution. 2) The Majlis replacing it with a new law, which is again very improbable since: a) almost every calendar authority believes that one should stick to such an "accurate" calendar. This is based on my discussions with both the current calendar authority (the High Council of Calendar, appointed by Tehran University and empowered by the Board of Ministers), and the previous one (Dr Iraj Malekpour, same appointment and empowerment). b) It's very improbable that a majority of the Iranian Majlis will even understand any proposal (or importance of such a proposal) to adopt an arithmetic Persian calendar, specially since the real difference is very rare. And c) Iranians traditionally celebrate the moment of vernal equinox as the beginning of the new Persian year. (Please understand that I am personally among the supporters of an arithmetic Persian calendar.)
Thanks for clarifying this. Would it be appropriate to make the following change to the GNU Emacs user documentation, if only to help forestall future bug reports in this area?
I really believe that the suggested change in GNU Emacs is fine, but one should correct that single instance in tzdata and mention that an astronomical calendar is practically used. I'm not sure about Birashk's suggested leap year rule (which is based on the wrong and overly simplistic assumptions that the average length of the spring equinoctial year is the same as the average tropical year, that the average will remain the same forever, and that the length of an astronomical day is constant), but a very simpler one I checked (using 33-year cycles) will not differ from the astronomical one until sometime in the 2070s. roozbeh roozbeh
Roozbeh Pournader <roozbeh@farsiweb.info> writes:
The text of the Iranian law, in effect since 1925, clearly mentions that the true solar year is the measure, and there is no arithmetic leap year calculation involved. There has never been any serious plan to change that law....
Thanks for making this clear. I'll update the tz database accordingly; the only year this affects in the current database is 2025 Gregorian (both spring and fall transitions). I'll add Oscar's comments for years after 2037, as a warning to future maintainers.
I'm not sure about Birashk's suggested leap year rule ... but a very simpler one I checked (using 33-year cycles) will not differ from the astronomical one until sometime in the 2070s.
This suggests that cal-persia.el could be changed as well, as its practical use is (I think) for predicting dates in Iran. I'm hesitant to propose a change myself, though, as I'm not an expert in either the calendar or the code.
participants (2)
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Paul Eggert -
Roozbeh Pournader