An Etymological Dictionary of Astronomy and Astrophysics

A religious and strictly → lunar calendar which follows the visibility of the lunar crescent after → conjunction and ignores the seasons (see also → synodic month). The year, which consists of 12 months of 29 or 30 days, is approximately 354 days long (→ lunar year of 354.3672 days). Because the calendar follows a purely lunar cycle, each month begins 10 or 11 days earlier each year in relation to the 365-day → solar year. As a result, the cycle of 12 lunar months regresses through the seasons over a period of 33 years. For religious purposes, Muslims begin the months with the first visibility of the lunar crescent. The month length may be 30 or 29 days during four or three successive months respectively. However, astronomers consider a calendar with months of alternately 30 and 29 days. The 33-year period contains 11 → leap years of 355 days. The origin of the Islamic era is considered to be the migration (Hijra) of Prophet Muhammad from Mecca to Medina on 16 July, A.D. 622.
It was Caliph Umar (died 644) who, 17 years after the actual event, poised the migration as the beginning of the Muslim era.

See also: From Islam, literally “submission” (to God); → calendar.