Calendar · a Light

Omar Khayyam helped fix a calendar still used in Iran

The Seljuk sultan Malik Shah asked for a solar calendar that would keep the seasons in place, and a committee of astronomers, among them the poet and mathematician Omar Khayyam, worked at an observatory in Isfahan to produce one. The Jalali calendar they delivered begins on 15 March 1079, with the year starting at the exact moment the sun crosses the equator at the spring equinox rather than by a fixed rule. Its year tracks the sun more closely than the Julian calendar then in use in Europe, and more closely than the Gregorian reform that came five centuries later. A direct descendant of it is the official calendar of Iran and Afghanistan today.

Isfahan · epoch 15 March 1079