Islamic calendar

The Islamic calendar or Muslim calendar (also called 'Hijri calendar') is the calendar used to date events in predominantly Muslim countries, and used by Muslims everywhere to determine the proper day on which to celebrate Muslim holy days. It is a purely lunar calendar having 12 lunar months in a year of about 354 days. Because this lunar year is about 11 days shorter than the solar year, Muslim holy days, although celebrated on fixed dates in their own calendar, usually shift 11 days earlier each successive solar year, such as a year of the Gregorian calendar. Islamic years are also called Hijra years because the first year was the year during which the Hijra occurred—Muhammad's emigration from Mecca to Medina. Thus each numbered year is designated either H or AH, the latter being the initials of the Latin anno Hegirae (in the year of the Hijra).

Months

Each month has either 29 or 30 days, but usually in no discernible order. Traditionally, the first day of each month was the day (beginning at sunset) of the first sighting of the lunar crescent (the hilal) shortly after sunset. If the hilal was not observed immediately after the 29th day of a month, either because clouds blocked its view or because the western sky was still too bright when the moon set, then the day that began at that sunset was the 30th. Such a sighting had to be made by one or more trustworthy men testifying before a committee of Muslim leaders. Determining the most likely day that the hilal could be observed was a motivation for Muslim interest in astronomy, which put Islam in the forefront of that science for many centuries. This traditional practice is still followed in a few parts of the world, like Pakistan and Jordan. However, in most Muslim countries astronomical rules are followed which allow the calendar to be determined in advance, which is not the case using the traditional method. Malaysia, Indonesia, and a few others begin each month at sunset on the first day that the moon sets after the sun (moonset after sunset). In Egypt, the month begins at sunset on the first day that the moon sets at least five minutes after the sun.

Related Topics:
Month - Astronomy - Pakistan - Jordan - Malaysia - Indonesia - Egypt

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The official Umm al-Qura calendar of Saudi Arabia used a substantially different astronomical method until recent years http://www.jas.org.jo/sau.html. Before AH 1420 (before April 18, 1999), if the moon's age at sunset in Riyad was at least 12 hours, then the day ending at that sunset was the first day of the month. This often caused the Saudis to celebrate holy days one or even two days before other Muslim countries, including the dates for the Hajj, which can only be dated using Saudi dates because it is performed in Mecca. During one memorable year during the AH 1380s (the 1970s), different Muslim countries ended the fast of Ramadan on each of four successive days! The celebrations became more uniform beginning in AH 1420. For AH 1420-22, if moonset occurred after sunset at Mecca, then the day beginning at that sunset was the first day of a Saudi month, essentially the same rule used by Malaysia, Indonesia, and others (except for the location from which the hilal was observed). Since the beginning of AH 1423 (March 16, 2002), the rule has been clarified a little by requiring the geocentric conjunction of the sun and moon to occur before sunset, in addition to requiring moonset to occur after sunset at Mecca. This ensures that the moon has moved past the sun by sunset, even though the sky may still be too bright immediately before moonset to actually see the crescent.

Related Topics:
Umm al-Qura - Saudi Arabia - April 18 - 1999 - Riyad - Hajj - 1970s - March 16 - 2002

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The moon sets progressively later than the sun for locations further west, thus western Muslim countries are more likely to celebrate some holy day one day earlier than an eastern Muslim country.

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Microsoft uses the "Kuwaiti algorithm" to convert Gregorian dates to the Islamic ones. It is based on statistical analysis of historical data from Kuwait.

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There exists a variation of the Islamic calendar known as the tabular Islamic calendar in which months are worked out by arithmetic rules rather than by observation or astronomical calculation. It has a 30-year cycle in with 11 years are leap years with 355 days instead of 354 days. In the long term, it is accurate to one day in about 2500 years. It also deviates up to about 1 or 2 days in the short term.

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