Editing Intercalation (timekeeping) (section)

== Lunar calendars ==
{{more|Nasi'|Islamic calendar#Astronomical considerations}}
In principle, lunar calendars do not employ intercalation because they do not seek to synchronise with the seasons, and the motion of the moon is astronomically predictable. But religious lunar calendars rely on actual observation.

The [[Islamic calendar|Lunar Hijri calendar]], the purely lunar calendar observed by most of Islam, depends on actual observation of the first crescent of the moon and thus has no intercalation. 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 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.<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 }}</ref> 

The [[tabular Islamic calendar]], used in Iran, has 12 lunar months that usually alternate between 30 and 29 days every year, but an intercalary day is added to the last month of the year 12 times in a 33-year cycle. Some historians also linked the pre-Islamic practice of [[Nasi']] to intercalation.