Editing Islamic calendar (section)

==Astronomical considerations==
{{more citations needed section|date=October 2016}}
Due to the Islamic calendar's reliance on certain variable methods of observation to determine its month-start-dates, these dates sometimes vary slightly from the month-start-dates of the astronomical lunar calendar,{{Which|date=September 2022}} which are based directly on astronomical calculations. Still, the Islamic calendar roughly approximates the astronomical-lunar-calendar system, seldom varying by more than three days from it. Both the Islamic calendar and the astronomical-lunar-calendar take no account of the [[solar year]] in their calculations, and thus both of these strictly lunar based calendar systems have no ability to reckon the timing of the four seasons of the year.

In the astronomical-lunar-calendar system, a year of 12 lunar months is 354.37 days long. In this calendar system, lunar months begin precisely at the time of the monthly "conjunction", when the Moon is located most directly between the Earth and the Sun. The month is defined as the average duration of a revolution of the Moon around the Earth (29.53 days). By convention, months of 30 days and 29 days succeed each other, adding up over two successive months to 59 full days. This leaves only a small monthly variation of 44 minutes to account for, which adds up to a total of 24 hours (i.e., the equivalent of one full day) in 2.73 years. To settle accounts, it is sufficient to add one day every three years to the lunar calendar, in the same way that one adds one day to the Gregorian calendar every four years.<ref>Emile Biémont, Rythmes du temps, Astronomie et calendriers, De Borck, 2000, 393p.</ref> The technical details of the adjustment are described in [[Tabular Islamic calendar]].

The Islamic calendar, however, is based on a different set of conventions being used for the determination of the month-start-dates.<ref>{{cite web |url=http://tabsir.net/?p=633#more-633 |website=tabsir.net |title=Issues in the Islamic Calendar |first=Khalid |last=Chraibi |date=16 August 2008 |access-date=18 August 2008 |archive-date=6 October 2008 |archive-url=https://web.archive.org/web/20081006083025/http://tabsir.net/?p=633#more-633 |url-status=live }}</ref> Each month still has either 29 or 30 days, but due to the variable method of observations employed, there is usually no discernible order in the sequencing of either 29 or 30-day month lengths. Traditionally, the first day of each month is the day (beginning at sunset) of the first sighting of the ''hilal'' (crescent moon) shortly after sunset. If the ''hilal'' is not observed immediately after the 29th day of a month (either because clouds block its view or because the western sky is still too bright when the moon sets), then the day that begins at that sunset is the 30th. Such a sighting has 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. Still, due to the fact that both lunar reckoning systems are ultimately based on the lunar cycle itself, both systems still do roughly correspond to one another, never being more than three days out of synchronisation with one another.
[[File:استهلال ماه رمضان در شهر قم، عکاس مصطفی معراجی، بلندی های بوستان علوی قم 12.jpg|alt=One man watches another, who looks through a modern telescope|thumb|Muslim clerics observe the moon.]]

This traditional practice for the determination of the start-date of the month is still followed in the overwhelming majority of Muslim countries. For instance, [[Saudi Arabia]] uses the sighting method to determine the beginning of each month of the Hijri calendar. Since AH 1419 (1998/99), several official ''hilal'' sighting committees have been set up by the government to determine the first visual sighting of the lunar crescent at the beginning of each lunar month. Nevertheless, the religious authorities also allow the testimony of less experienced observers and thus often announce the sighting of the lunar crescent on a date when none of the official committees could see it.

Each Islamic state proceeds with its own monthly observation of the new moon (or, failing that, awaits the completion of 30 days) before declaring the beginning of a new month on its territory. However, the lunar crescent becomes visible only some 17 hours after the conjunction, and only subject to the existence of a number of favourable conditions relative to weather, time, geographic location, as well as various astronomical parameters.<ref>Karim Meziane et [[Nidhal Guessoum]]: La visibilité du croissant lunaire et le ramadan, La Recherche n° 316, janvier 1999, pp. 66–71.</ref> Given the fact that the moon sets progressively later than the sun as one goes west, with a corresponding increase in its "age" since conjunction, Western Muslim countries may, under favorable conditions, observe the new moon one day earlier than eastern Muslim countries. Due to the interplay of all these factors, the beginning of each month differs from one Muslim country to another, during the 48-hour period following the conjunction. The information provided by the calendar in any country does not extend beyond the current month.

A number of Muslim countries try to overcome some of these difficulties by applying different astronomy-related rules to determine the beginning of months. Thus, [[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. A detailed analysis of the available data shows, however, that there are major discrepancies between what countries say they do on this subject, and what they actually do. In some instances, what a country says it does is impossible.<ref>{{cite web|url=http://www.moonsighting.com/calculation-or-sighting.html|title=Calculations or Sighting for starting an Islamic month|website=www.moonsighting.com|access-date=16 April 2017|archive-url=https://web.archive.org/web/20170508214424/http://www.moonsighting.com/calculation-or-sighting.html|archive-date=8 May 2017|url-status=dead}}</ref><ref>{{cite web|url=http://oumma.com/Le-mois-islamique-est-il-universel|title=Le mois islamique est-il universel ou national ?|author=Oumma|date=23 June 2010|work=Oumma|access-date=20 September 2010|archive-date=5 October 2016|archive-url=https://web.archive.org/web/20161005080558/http://oumma.com/Le-mois-islamique-est-il-universel|url-status=live}}</ref>

Due to the somewhat variable nature of the Islamic calendar, in most Muslim countries, the Islamic calendar is used primarily for religious purposes, while the Solar-based Gregorian calendar is still used primarily for matters of [[commerce]] and [[agriculture]].