How it began
The Seljuk sultan Malik-Shah and his vizier Nizam al-Mulk invited Omar Khayyam to Isfahan to set up an observatory, where he led a group of astronomers for 18 years. The sultan commissioned a reform of the calendar in 1073. Khayyam was one of eight learned men employed on it, and the new calendar was adopted on 15 March 1079. It was called Jalali after Jalal al-Din, one of the sultan's names. Observations were made in Isfahan, Rey and Nishapur.
Before it existed
The reform answered the drift of the Islamic lunar calendar, whose year of 354 days moves through the seasons. That was unworkable for settled farming communities and for collecting taxes at harvest. Older Persian solar calendars were still known, but they had fallen into disorder for lack of official supervision.
How it works
The year began at the March equinox, the festival of Nowruz. Each month began when the Sun entered the next sign of the zodiac, so months could run from 29 to 32 days and varied slightly from year to year. Because the months followed the real position of the Sun, seasonal drift never exceeded one day and no leap-year rule was needed. Khayyam measured the length of the year as 365.24219858156 days. The cost was difficulty: the calendar required observation and full astronomical calculation.
Who pushed back
Nizam al-Mulk was murdered in 1092 and Malik-Shah died a month later. Funding for the observatory ceased, the observatory was abandoned, and Khayyam's further reform work was put on hold. The period of peace in which the astronomers had worked was over, and Khayyam himself came under attack from orthodox Muslims.
What lasted
The historian Edward Gibbon wrote that the Jalali computation of time "surpasses the Julian, and approaches the accuracy of the Gregorian style". The calendar remained in use for eight centuries. In 1925 Iran simplified its month lengths and calculation, producing the Solar Hijri calendar. Variants of the Jalali calendar are still in use in Iran and Afghanistan.
