The Encyclopedia Americana (1920)/Hegira
HEGIRA, hĕj'ĭ-ra, HEJRA, or HIJRA, an Arabic word meaning “going away,” commonly used to indicate Mohammed's flight from Mecca, in 622 A.D. In 639 Caliph Omar instituted a new Moslem calendar to begin with the first day of the first month of the year in which the flight took place. The Mohammedan year, as a lunar year, is shorter than ours by ten days, 21 hours and 14-2/5 seconds. A rough and ready method for finding the year in our calendar corresponding to a given year in the Mohammedan is to subtract from the latter 1/33 of itself and add 622 to the remainder. To find the precise year and day multiply the year of the Hegira by 970,224, strike off from the product six decimal figures and add 621.5774; this will give the year of the Christian era; and the day of the year is got by multiplying the decimal figures by 365.