By order of Seljuk Sultan Malik Shah, Omar Khayyam and a group of astronomers at the Isfahan observatory created the Jalali calendar. They calculated the solar year as 365.2421985 days—with an error of only 1 day in 5,000 years.
Fun Facts
The Jalali calendar predates the Gregorian calendar (1582 CE) by 500 years and is more accurate! The Gregorian has an error of 1 day in 3,330 years, while the Jalali has 1 day in 5,000 years. Khayyam, besides mathematics and astronomy, is famous for his Rubaiyat poetry. This calendar, with modifications, is used today as the "Solar Hijri" calendar in Iran and Afghanistan.
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