Editing Islamic calendar (section)

==Length of year==
The mean duration of a [[tropical year]] is 365.24219 days, while the long-term average duration of a [[lunar month#Synodic month|synodic month]] is 29.530587981 days. Thus the average lunar year (twelve new moons) is 10.87513 days shorter than the average solar year (365.24219 &minus; (12 × 29.530587981)), causing months of the Hijri calendar to advance about eleven days earlier each year, relative to the equinoxes.{{efn|The precise number varies, depending on the length of the [[synodic month]] (the time it takes the Moon to reach the same visual [[lunar phase|phase]]. This varies notably throughout the year.<ref name="AA">Jean Meeus, ''Astronomical Algorithms'' (Richmond, VA: Willmann-Bell, 1998) p 354. From 1900–2100, the shortest time from one new moon to the next is 29&nbsp;days, 6&nbsp;hours, and 35&nbsp;min, and the longest 29&nbsp;days, 19&nbsp;hours, and 55&nbsp;min.</ref>)}}  "As a result," says the ''[[Astronomical Almanac]]'', "the cycle of twelve lunar months regresses through the seasons over a period of about 33 &#91;[[tropical year|solar]]&#93; years".<ref name="Navy Almanac">{{Cite book|title=Explanatory Supplement to the Astronomical Almanac |last=Richards |first=E. G. |publisher=University Science Books |year=2012 |isbn=978-1-891389-85-6 |editor-last=Urban |editor-first=Sean E. |location=Mill Valley, CA |pages=606 |chapter=Calendars |editor-last2=Seidelmann |editor-first2=P. Kenneth |chapter-url=http://aa.usno.navy.mil/publications/docs/c15_usb_online.pdf |url-status=dead |archive-url=https://web.archive.org/web/20190430134555/http://aa.usno.navy.mil/publications/docs/c15_usb_online.pdf |archive-date=30 April 2019}}</ref>{{efn|This means that there are 34 Hijri years to 33 Gregorian years.}}