The Universe of Discourse
Fri, 29 Feb 2008

Happy Leap Day! Persian edition
Roland Young has brought to my attention that the Persian calendar uses a hybrid 7/29 and 8/33 system. I was going to post this as an addendum to today's Leap Day article, but it got too long.

If I understand the rules correctly, to determine if a Persian year is a leap year, one applies the following algorithm to the Persian year number y. (Note that the current Persian year is not 2008, but 1386. Persian year 1387 will begin on the vernal equinox.) I will write a % b to denote the remainder when a is divided by b. Then:

  1. Let a = (y + 2345) % 2820.
  2. If a is 2819, y is a leap year. Otherwise,
  3. Let b = a % 128.
  4. If b < 29, let c = b. Otherwise, let c = (b - 29) % 33.
  5. If c = 0, y is not a leap year. Otherwise,
  6. If c is a multiple of 4, y is a leap year. Otherwise,
  7. y is not a leap year.
(Perl source code is available.)

This produces 683 leap years out of every 2820, which means that the average calendar year is 365.24219858 days.

How does this compare with the Dominus calendar? It is indeed more accurate, but I consider 683/2820 to be an unnecessarily precise representation of the vernal equinox year, especially inasmuch as the length of the year is changing. And the rule, as you see, is horrendous, requiring either a 2,820-entry lookup table or complicated logic.

Moreover, the Persian and Gregorian calendar are out of sync at present. Persian year 1387, which begins next month on the vernal equinox, is a leap year. But the intercalation will not take place until the last day of the year, around 21 March 2009. The two calendars will not sync up until the year 2092/1470, and then will be confounded only eight years later by the Gregorian 100-year exception. After that they will agree until 2124/1502. Clearly, even if it were advisable to switch to the Persian calendar, the time is not yet right.

I found this Frequently Asked Questions About Calendars page extremely helpful in preparing this article. The Wikipedia article was also useful. Thanks again to Roland Young for bringing this matter to my attention.

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