Iranian calendar

The Iranian calendar or Solar Hejri (
19 March || Demise of Muhammad and Martyrdom of Imam Hassan || |
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20 March || Nationalization of the oil industries || |
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4 June || Anniversary of the passing of Imam Khomeini|| || 1989
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5 June || Anniversary of the uprising against the Shah || |
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3 October || Martyrdom of Imam Ali || |
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15 October || End of Ramadan || Eid-e-Fitr |
- ] .

The year was computed from the vernal equinox, and each month was determined by the transit of the sun into the corresponding zodiac region, a system that incorporated improvements on the ancient Indian system ofthe Surya Siddhanta("Surya"=solar, "Siddhanta"=analysis, 4th c. CE), also the basis of most Hindu calendars. Since the solar transit times can have 24-hour variations, the length of the months vary slightly in different years (each month can be between 29 and 32 days). For example, the months in two last years of the Jalali calendar had:
* 1303

* 1302

Because months were computed based on precise times of solar transit between zodiacal regions, seasonal drift never exceeded one day, and also there was no need for a leap year in the Jalali calendar. However, this calendar was very difficult to compute; it required the full ephemeris computations / actual observations to determine solar motion trajectories. Some claim that simplifications introduced in the intervening years may have introduced a system with 8 leap days in every cycle of 33 years. (Different rules, such as the 2820-year cycle, have also been accredited to Khayyam). However, the original Jalali calendar based on observations (or predictions) of solar transit would not have needed either leap years or seasonal adjustments.

The team also came up with the computation of the length of a solar year as 365.24219858156 days. The number of decimal digits reflects their high confidence in this computation. Though it may not have been known at the time, the length of theyear is changing in the sixth decimal place over a person'slifetime. Nonetheless, the result is astoundingly accurate: the length ofthe year at various points are:

* 365.2421986 days: Isfahan team, 1079
* 365.242196 days: end of the 19th century
* 365.242190 days: end of 20th c. (today)

However, owing to the variations in month lengths, and also the difficulty in computing the calendar itself, the Iranian calendar was modified to simplify these aspects in 1925 (1304 AP).

Iranian Calendar Reform: 1925

On February 21, 1911, the second Persian parliament mandated government use of a simplified calendric computation system based on the solar calendar. The present Iranian calendar was legally adopted on March 31, 1925, under the early Pahlavi dynasty. The law said that the first day of the year should be the first day of spring in "the true solar year", "as it has been" (کماکان). It also fixed the number of days in each month, which previously varied by year with the tropical zodiac. It revived the ancient Persian names, which are still used. It specified the origin of the calendar (Hegira of Muhammad from Mecca to Medina in 622 CE). It also deprecated the 12-year cycles of the Chinese-Uighur calendar which were not officially sanctioned but were commonly used.

The first six months (Farvardin–Shahrivar) have 31 days, the next five (Mehr–Bahman) have 30 days, and the last month (Esfand) has 29 days or 30 days in leap years. The reason the first six months have 31 days and the rest 30 may have to do with the fact that the sun moves slightly more slowly along the ecliptic in the northern spring and summer than in the northern autumn and winter.Fact|date=November 2007

Afghanistan legally adopted this calendar in 1957,Fact|date=November 2007 but with different month names. The Afghan Pashto language in Afghanistan uses the Pashto names of the zodiac signs. The Afghan Dari language in Afghanistan, uses the Dari names of the zodiac signs.

The Persian calendar produces a five-year leap year interval after about every seven four-year leap year intervals. It usually follows a 33-year cycle with occasional interruptions by single 29-year or 37-year subcycles. By contrast, some less accurate predictive algorithms are suggestion based on confusion between average tropical year (365.2422 days, approximated with near 128-year cycles or 2820-year great cycles) and the mean interval between spring equinoxes (365.2424 days, approximated with a near 33-year cycle).

Details

The Iranian calendar year begins at the start of Spring in the northern hemisphere: on the midnight between the two consecutive solar noons which include the instant of the Northern spring equinox, when the sun enters the northern hemisphere. If between two consecutive noons the sun's altitude rises through its equinoctial altitude, then the first noon is on the last day of one calendar year and the second noon is on the first day (Nowruz) of the next year.The calendar has 12 months with Persian names.

Month names

The first day of the calendar year is also the day of the greatest festival of the year in Iran, Afghanistan and surrounding regions, called Nowruz (two morphemes: IPA|now (new) and IPA|ruz (day), meaning "new day").

Days of the week

In the Iranian calendar, every week begins on Saturday and ends on Friday. The days of the week are called: IPA|ʃanbeh (شنبه in native script), IPA|jekʃanbeh (یکشنبه), IPA|doʃanbeh (دوشنبه), IPA|seʃanbeh (سه شنبه), IPA|tʃahɒrʃanbeh (چهارشنبه), IPA|pandʒʃanbeh (پنجشنبه), and IPA|dʒomhe (جمعه originally in Arabic) or IPA|ɒdineh (آدینه) (in IPA|pɒrsi). In most Islamic countries, IPA|dʒumʕa is the holiday.

Calculating the day of the week is easy, using an anchor date. One good such date is Sunday, 1 Farvardin 1372, which equals 21 March 1993. Assuming the 33-year cycle approximation, move back by one weekday to jump ahead by one 33-year cycle. Similarly, to jump back by one 33-year cycle, move ahead by one weekday.

As in the Gregorian calendar, dates move forward exactly one day of the week with each passing year, except if there is an intervening leap day when they move two days. The anchor date 1 Farvardin 1372 is chosen so that its 4th, 8th, ..., 32nd anniversaries come immediately after leap days, yet the anchor date itself does not immediately follow a leap day.

Seasonal error

The image below shows the difference between the Iranian calendar (using the 33-year arithmetic approximation) and the seasons. The Y axis is "days error" and the X axis is Gregorian calendar years. Each point represents a single date on a given year. The error shifts by about 1/4 day per year, and is corrected by a leap year every 4th year regularly, and one 5 year leap period to complete a 33-year cycle. One can notice a gradual shift upwards over the 500 years shown. The Gregorian calendar, introduced in 1582, is almost as accurate in the long term, but has larger swings of seasonal errors over centuries.

References

External links

* [http://pmc.khone.ir/ Persian Multi Calendar] This is a free and open source calendar program running under Windows, containing all three calendars Jalali (Persian), Hijri (Islamic) and Gregorian with much more features.
* [http://www.alavi.us/toolbar/ An Iranian calendar toolbar for Internet Explorer and Firefox]
* [http://www.aaahoo.com/main/dispCalendar.asp?ULanID=E&PR=y Online Persian Calendar from aaahoo portal]
* [http://www.payvand.com/calendar Online Persian Calendar Generator and Convertor]
* [http://www.nongnu.org/afghancalendar Online Afghan Calendar with Gregorian, Hejrah-e shamsi and Hejrah-e qamari dates]
* [http://www.iranchamber.com/calendar/converter/iranian_calendar_converter.php An online Jalali(shamsi)/Gregorian/Islamic(hijri) Date Convertor]
* [http://www.tondering.dk/claus/cal/node7.html The Persian Calendar] : How the leap years are calculated
* [http://msdn2.microsoft.com/en-us/library/system.globalization.persiancalendar.aspx System.Globalization.PersianCalendar class documentation in MSDN Library] (The implementation of Persian Calendar in Microsoft .NET Framework 2.0)
* [http://www.alavi.us/jcal/ An Interactive Iranian Calendar]
* [http://www.calendar-ir.narod.ru/ An online Persian/Gregorian date convertor, Persian calendar for mobile (j2me)]
* [http://ahura.TheLalis.com The Zoroastrian Calendar]
* [http://www.iranian.com/History/April99/Months/index.html Meaning of the names of the months in the Persian Calendar]
* [http://farhadi.ir/works/jalalijscalendar Iranian Calendar implemented in JavaScript]
* [http://psoft.ir/ Sun Calendar] This is a Persian solar calendar plus some useful tools such as date converters.