"A.H." is an abbreviation used in many Western languages for "after hegira," or its Latin form, "Anno Hegirae." The hegira refers to a journey that Muslims believe the Prophet Mohammed took from Mecca to Medina in 622 A.D. Just as the Gregorian calendar begins with the birth of Christ, the Islamic calendar begins with this pivotal event. The Islamic calendar, which follows a lunar cycle, begins at year 1 A.H.
The phrase "A.H." is most commonly used when indicating a specific date in the Islamic calendar. For example, an event that occurred 100 years after the hegira would be rendered in the Islamic calendar as "101 A.H." -- the first year, listed as 1 A.H., plus one hundred years. (See Reference 1) This is similar to the Gregorian's calendar use of the abbreviation "A.D." meaning "Anno Domino," or "in year of our lord."
Before The Hegira
The phrase B.C. -- short for "before Christ" -- is used in the Gregorian calendar to denote the years that preceded the birth of Christ. Similarly, the Islamic calendar, the phrase "B.H.", an abbreviation of "before hegira," denotes the years preceding the hegira. However, when referring to the period after the birth of Christ and before the hegira, some Islamic texts also use the abbreviation "C.E.," which is an abbreviation for "Christian era."
Unlike the Gregorian calendar, which is based on the movement of the sun, the Islamic calendar, also known as the Hegira calendar, is based on the movements of the moon. This means that a year in the Islamic calendar is usually about 354 days or 355 days -- ten or eleven days shorter than the Gregorian calendar. This can lead to some confusion when converting a year in the Islamic calendar to a year in the Gregorian calendar, and vice versa.
Note About Conversion
It can be difficult to accurately convert dates from A.H. to A.D., as, under the Islamic calendar system, a new month begins not with the actual start of a new moon, but when a human actually sees a crescent moon at a specific location. Because this may occur up to several days after the moon actually appears, the calendar's timing can be thrown off, furthing shifting the conversion for certain dates away from the Gregorian calendar system.
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