Persian Calendar

The Iranian calendar also known as Persian calendar or the Jalāli Calendar is a solar calendar currently used in Iran and Afghanistan. Beginning each year on the vernal equinox as precisely determined by astronomical observations from Tehran (or the 52.5°E meridian, which also defines IRST) and Kabul, this makes it more accurate than Gregorian Calendar, but harder to work out when a particular date would occur.

The Jalali calendar was introduced on 15 March, 1079 by the Seljuk Sultan Jalal al-Din Malik Shah I, based on the recommendations of a committee of astronomers, including Omar Khayyam, at the imperial observatory in his capital city of Isfahan (Iran). The calendar included the most accurate computation of the solar year at the time, and month computations were based on solar transits through the zodiac, a system integrating ideas from the Surya Siddhanta (India, 4th c. CE).

On February 21, 1911, the second Persian parliament tried to mandate 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”. 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.

The first six months have 31 days, the next five 30 days, and the last month has 29 days or 30 days in leap years. The reason the first six months have 31 days and the rest 30 has 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.

Solar calendar systems typically use leap years, usually every four years by adding 0.25 day to each year, but this is a slight overcompensation. Instead, 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, less accurate predictive algorithms are based on confusion between the astronomers 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).

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 (Norouz) of the next year. The calendar has 12 months with Persian names.

The first day of the calendar year is also the day of the greatest festival of the year in Iran, and surrounding regions, called Norouz (a single word made up of two parts, no (new) and rouz (day), meaning “new day”).

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