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In XI century C.E. the commission of astronomers under the leadership of Persian mathematician, astronomer and poet Omar Khayyam under instructions of Seljuq sultan Malik-Shah the solar calendar was elaborated, which was based on the 33 years cycle, consisting of 25 ordinary years (365 days each) and 8 leap years (366 days each).
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Omar Khayyam
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1st, 5th, 9th, 13th, 17th, 22nd, 26th and 30th were the leap years. The year started from the day of Vernal equinox and consisted of 12 months. Omar Khayyam calendar commission fixed original Vernal equinox for the epoch of March 15, 1079 C.E. according to the Julian calendar.
Medium length of the Omar Khayyam calendar year was:
(365 * 25 + 366 * 8) / 33 = 365.24242 days.
This quantity was only the 0.00022 part of the day more than a tropical year (365.24220 days). Thus, Khayyam's calendar is more accurate than the Gregorian one and makes the mistake of one day for 4500 years!
For unknown reasons Omar Khayyam's calendar was not brought into life by Malik-Shah, but Omar Khayyam's project is so good that it is impossible not to mention it.
