what event marks year one of the islamic calendar
Islamic calendar The first day of the first month of the Islamic calendar (1 Muharram 1 AH) was set to the first new moon after the day the Prophet moved from Quba' to Medina (originally 26 Rabi' I on the pre-Islamic calendar[23]) i.e., Friday, 16 July 622 AD/CE, the equivalent civil tabular date (same daylight period) in the Julian calendar.[24][25] The Islamic day began at the preceding sunset on the evening of 15 July. This Julian date (16 July) was determined by medieval Muslim astronomers by projecting back in time their own tabular Islamic calendar, which had alternating 30- and 29-day months in each lunar year plus eleven leap days every 30 years. For example, al-Biruni mentioned this Julian date in the year 1000 AD/CE.[26] Although not used by either medieval Muslim astronomers or modern scholars to determine the Islamic epoch, the thin crescent moon would have also first become visible (assuming clouds did not obscure it) shortly after the preceding sunset on the evening of 15 July, 1.5 days after the associated dark moon (astronomical new moon) on the morning of 14 July.[27]