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The Persian calendar is a type of solar calendar, which is officially used in Iran and Afghanistan. It is one of the most accurate calendars in the world.

Calendar structure[]

A year is divided into 12 months, each with a pre-defined length. Similar to the Gregorian calendar, leap days are inserted periodically to keep the calendar in sync with the seasons. Each year begins on the date of the vernal equinox as determined by astronomical calculations, making it an observational calendar, although a few mathematical rules have been suggested for determining which years are leap years.

Month names and lengths in the Persian calendar
Month number Month name Length Approximate Gregorian date
1 Farvardin 31 March 21 - April 20
2 Ordibehesht 31 April 21 - May 21
3 Khordad 31 May 22 - June 21
4 Tir 31 June 22 - July 22
5 Mordad 31 July 23 - August 22
6 Shahrivar 31 August 23 - September 22
7 Mehr 30 September 23 - October 22
8 Aban 30 October 23 - November 21
9 Azar 30 November 22 - December 21
10 Dey 30 December 22 - January 20
11 Bahman 30 January 21 - February 19
12 Esfand 29 (30 in leap years) February 20 - March 20

Year counts[]

The year counts in the Persian calendar start from AD 622, i.e. the year of Mohammed's emigration to Medina. Years in the Persian calendar are designated "AP" (anno persico, or "Persian year").

Apart from the starting point of the year counts, the Persian calendar shares no similarities with the Islamic calendar.

Leap year rules[]

The Persian calendar is based on astronomical observations. Its years begin with the midnight closest to the vernal equinox, which is calculated for the Iranian time zone (GMT+3:30). This also determines the length of the month of Esfand.

A number of arithmetical rules have been suggested for determining leap years. The most common (and complex) one is so overwhelmingly accurate that it takes over 100,000 years for the calendar to deviate from the solar year by 1 day, barring the gradual slowing of Earth's rotation. These rules are as follows:

The calendar is divided into periods of 2820 years. These periods are then divided into 88 cycles whose lengths follow this pattern:

29, 33, 33, 33, 29, 33, 33, 33, 29, 33, 33, 33, ...

This gives 2816 years. The total of 2820 years is achieved by extending the last cycle by 4 years (for a total of 37 years).

If you number the years within each cycle starting with 0, then leap years are the years that are divisible by 4, except that the year 0 is not a leap year. For example, within a 29-year cycle, the years numbered 4, 8, 12, 16, 20, 24, and 28 are leap years with 30 days in Esfand, while the rest are common years with 29 days in the month of Esfand.

How accurate are these rules? As mentioned above, it takes over 100,000 years for the calendar to accumulate an error of 1 day. The approximation to the tropical year under this set of rules is 365 683/2820 days, as it gives 683 leap days in 2820 years. This means the average calendar year is 365.2421986 days long, which is a much better approximation than the 365.2425 days in the Gregorian calendar.

References[]

  • Bikos, Konstantin. “The Persian Solar Hijri Calendar.” Timeanddate.com, www.timeanddate.com/calendar/persian-calendar.html. Accessed 25 July 2021.
  • Tøndering, Claus. “The Persian Calendar.” The Calendar FAQ, www.tondering.dk/claus/cal/persian.php. Accessed 25 July 2021.
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