Hegira, or Hejra (Arabic), the "emigration" of Memammed (q.v.) from Mecca to Medina in 622 A.D., which. 17 years later, was made by the Kalif Omar the starting-point of the Mohammedan calendar. The new era began with the first new moon in the month Moharrem, which is generally supposed to have fallen on July 16th, but, according to Caussin de Perceval, the true date is April 19th. The Mohammedan era is reckoned by lunar years of 354 and 355 days.