Hegira , (Arabic, hejrah, emigration, usually but incorrectly translated "flight;" the full expression is hejrat al-nabi, the migration of the prophet), the migration of Mohammed from Mecca to Medina. The most probable date assigned to this event is Sept. 13, 622. Mohammed died in 632, and seven or eight years afterward the caliph Omar, with the aid of Harmozan, a Persian, instituted a new calendar founded upon the hegira. But instead of commencing the new era with the actual date of the hegira, he began it with the first day of Moharrem, the first month of the Arabic year, corresponding to July 16, 622. The Arabian astronomers maintain that it was one day earlier, but their opinion has never been adopted in either official or popular practice. This date, July 16, is in most books of chronology given erroneously as the date of the hegira itself. The Mohammedan calendar is regulated entirely by the moon, without regard to the sun or the seasons. The year consists of 12 lunar months. The odd months contain each 30 days, the even months each 29 days except in intercalary years, when the 12th month also contains 30 days. The year thus consists of 354 or 355 days.

Whether a given year consists of one or the other number of days is decided as follows : the time from the commencement of the era is divided into cycles of 30 years. The year 1873, being the 1290th of the hegira, was the last year of the 43d cycle. In each of these cycles the 2d, 5th, 7th, 10th, 13th, 16th, 18th, 21st, 24th, 26th, and 29th years consist of 355 days, one day being added to the 12th month. The other years of the cycle consist each of 354 days. The Mohammedan year is thus on an average 10 days, 21 hours, and 15 seconds shorter than the tropical year, and consequently the first day of each Mohammedan year comes 10 or 11 days earlier than it did the preceding year, and thus retrogrades through all the different seasons. Hence the exact transfer of a Mohammedan date to our calendar is a very difficult and complicated operation. But the year can be fixed with sufficient accuracy in a simple manner. There are 11,694 days in 33 Mohammedan years, and 11,688 days in 32 Christian years; hence wo may assume that 33 Mohammedan are equal to 32 Christian years, and we have the following rule: subtract from the year of the hegira its 33d part, and add 622 ; the result is the year of the Christian era.

Thus to find the year corresponding to 1290 of the hegira: 1290-39 (i. e., 1290/33) + 622=1873 A. D. To turn a year of the Christian era into a year of the hegira, subtract 622 and add to the remainder the 32d part of itself. Thus Constantinople was taken by the Moslems in 1453 ; in what year of the hegira was it? 1453 - 622 = 831; adding 26 (i. e., 831/32), we have 857.