The Iranian calendar (also known as Persian calendar) is a solar calendar currently used in Iran and Afghanistan.
Persians have always been keen on the idea and importance of calendar system for as long as their documented history has been recorded. They were among the first cultures to use solar calendar systems and have always favoured the solar calendar. In general, the sun has always, even to this day, had a special meaning and great symbolic significance in the Iranian culture.
Today Iran uses a solar calendar with a system of leap years. Like the Gregorian calendar it is designed to approximate the vernal equinox tropical year of about 365.2424 days. The Iranian calendar is a more accurate approximation than the Gregorian calendar, but has a more complicated leap year rule. The Iranian calendar was revised in the 11th century by a panel of scientists including Omar Khayyam, one of the foremost mathematicians and astronomers of his time (but today he is better known in the West for his poetry). It was based on an older Persian calendar system which had existed for quite a long time.
The Iranian calendar was reintroduced in Persia in the year 1922. Afghanistan adopted the calendar in 1957, but in Afghanistan the Arabic names of the zodiacal signs for the corresponding months of the Persian calendar are used.
The Iranian solar calendar year begins with the midnight closest to the instant of the Northern spring equinox, when the sun enters the northern hemisphere; in other words, the start of Spring in the northern hemisphere. The calendar consists of 12 months with Persian names. The first six months are 31 days each, the next five 30 days, and the last month has 29 days but 30 days in leap years. The reason the first 6 months have 31 days and the rest 30, is not a random decision -- it has to do with the fact that the sun moves slightly more slowly along ecliptic in the northern spring and summer than in the northern autumn and winter.
The Persian new year is determined by noon-time observation of the Northern spring equinox. 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 (Norouz) of the next calendar year.
Typically leap years are devised and used by various solar calendar systems, usually every four years. Four-year leap years add 0.25 day to each year in the period. But that is a slight overcompensation compared to the actual behaviour of the sun. Remedying this overcompensation, after about every seven four-year leap years, the Persian solar calendar produces a five-year leap year, thus following a thirty-three year cycle for many centuries before interruptions by single twenty-nine year subcycles.
This general picture of the Persian calendar's leap-year behaviour contrasts with other ill-informed predictive algorithms which are based on confusion between the astronomers average tropical year (365.2422 days, approximated with mistaken near 128-year cycles) and the mean interval between spring equinoxes (365.2424 days, approximated here with a near 33-year cycle).
The Persian names are included in the parentheses.
- Farvardin (فروردین), 31 days
- Ordibehesht (اردیبهشت), 31 days
- Khordad (خرداد), 31 days
- Tir (تیر), 31 days
- Mordad (مرداد), 31 days
- Shahrivar (شهریور), 31 days
- Mehr (مهر), 30 days
- Aban (آبان), 30 days
- Azar (آذر), 30 days
- Dey (دی), 30 days
- Bahman (بهمن), 30 days
- Esfand (اسفند), 29 days (30 days in leap years)
The first day of this calendar year is also the day of the greatest festival of the year in Iran called Norouz (a single word made up of two parts, no and rouz, meaning "new day").
Calendar seasonal error
This image shows the difference between the Iranian Solar calendar 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. You can notice a gradual shift upwards over the 500 years shown.
By comparison, the Gregorian calendar, introduced in 1582, is almost as accurate in the long term, but has larger swings of seasonal errors over the centuries.
- THE JALAALI CALENDAR (http://www.kelsung.com/calendar/jalaali.htm)
- Khayam: A Persian Calendar Program (http://payvand.com/calendar)
- The Persian calendar for 3000 years (http://www.astro.uni.torun.pl/~kb/Papers/EMP/PersianC-EMP.htm)
- The Persian Calendar (http://www.tondering.dk/claus/cal/node6.html)
- The Iranian calendar (http://wwwusr.obspm.fr/~heydari/divers/calendar.html) in English and French.