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.
The official Umm al-Qura calendar of Saudi Arabia used a substantially different astronomical method until recent years. 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 predominantly Muslim populated 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. Strictly speaking, the Umm al-Qura calendar is intended for civil purposes only. Their makers are well aware of the fact that the first visual sighting of the lunar crescent (hilal) can occur up to two days after the date calculated in the Umm al-Qura calendar. Since AH 1419 (1998/99) several official hilal sighting committees have been set up by the government of Saudi Arabia to determine the first visual sighting of the lunar crescent at the begin of each lunar month. Nevertheless, the religious authorities of Saudi Arabia also allow the testimony of less experienced observers and thus often announces the sighting of the lunar cresc ent on a date when none of the official committees could see the lunar crescent. In nearly all of these cases, a retrospective analysis indicates that these extremely early reports of the lunar crescent are impossible and are based on false sightings.
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.
Microsoft uses the "Kuwaiti algorithm" to convert Gregorian dates to the Islamic ones. It is based on statistical analysis of historical data from Kuwait.
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.
Forbidding intercalary months
In the ninth year after the Hijra, Muslims believe God forbade the intercalary month. This is expressed in the Qur'an (9:36-37):
The number of months with Allah has been twelve months by Allah's ordinance since the day He created the heavens and the earth. Of these four are known as sacred; That is the straight usage, so do not wrong yourselves therein, and fight the Pagans.
Verily the transposing (of a prohibited month) is an addition to Unbelief: The Unbelievers are led to wrong thereby: for they make it lawful one year, and forbidden another year, of months forbidden by Allah and make such forbidden ones lawful. The evil of their course seems pleasing to them. But Allah guideth not those who reject Faith.
This prohibition was repeated by Muhammad during his last sermon on Mount Arafat which was delivered during his farewell pilgrimage to Mecca on 9 Dhu al-Hijja AH 10 (this paragraph is often deleted from the sermon by its modern editors as now unimportant):
O People, the unbelievers indulge in tampering with the calendar in order to make permissible that which Allah forbade, and to forbid that which Allah has made permissible. With Allah the months are twelve in number. Four of them are holy, three of these are successive and one occurs singly between the months of Jumada and Shaban.
The three successive holy months are Dhu al-Qada, Dhu al-Hijja, and Muharram, thus excluding an intercalary month before Muharram. The single holy month is Rajab.
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