Jalali calendar

The Jalali calendar is a solar calendar that was used in Iran (Persia), variants of which today are still in use in Iran as well as Afghanistan. It gains approximately 1 day on the Julian calendar every 128 years. The tropical Jalali calendar (Persian: گاهشماری جلالی or تقویم جلالی‎‎), which inherited some aspects from the Yazdgerdi calendar, was adopted on 15 March 1079 by the Seljuk Sultan Jalal al-Din Malik Shah I (for whom it was named), based on the recommendations of a committee of astronomers, including Omar Khayyam, at the imperial observatory in his capital city of Isfahan. Month computations were based on solar transits through the zodiac, a system integrating ideas taken from Hindu calendars. Later, some ideas from the Chinese-Uighur calendar (1258) were also incorporated. It remained in use for eight centuries. It arose out of dissatisfaction with the seasonal drift in the Islamic calendar which is due to that calendar being lunar instead of solar; a lunar year of 354 days, while acceptable to a desert nomad people, proved to be unworkable for settled, agricultural peoples, and the Iranian calendar is one of several non-lunar calendars adopted by settled Muslims for agricultural purposes (others include the Coptic calendar, the Julian calendar, and the Semitic calendars of the Near East). Sultan Jalal commissioned the task in 1073. Its work was completed well before the Sultan's death in 1092, after which the observatory would be abandoned.

The year was computed from the vernal equinox (Nowruz), 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 of the Surya Siddhanta (Surya=solar, Siddhanta=analysis, 4th century), 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 the two last years of the Jalali calendar had:

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