Editing Full moon (section)

==Lunar and lunisolar calendars==
{{Main|Lunar calendar}}
[[File:Christmas full moons 1977-2091.gif|thumb|The December 2015 full moon coincided with Christmas.<ref>{{Cite web |url=https://www.nasa.gov/feature/goddard/rare-full-moon-on-christmas-day |title=Rare Full Moon on Christmas Day, NASA |date=16 December 2015 |access-date=2015-12-23 |archive-date=2021-05-06 |archive-url=https://web.archive.org/web/20210506160209/https://www.nasa.gov/feature/goddard/rare-full-moon-on-christmas-day/ |url-status=live }}</ref> This last occurred in 1977 (for the American timezones). A small horizontal [[libration]] is visible comparing their appearances. By the 19-year [[metonic cycle]] the full moon will repeat on Christmas Day in 2034, 2053, 2072, and 2091.<ref>{{Cite web |url=http://www.chicagotribune.com/news/weather/ct-wea-1221-asktom-20151220-column.html |title=Ask Tom: How unusual is a full moon on Christmas Day? |website=[[Chicago Tribune]] |date=20 December 2015 |access-date=2015-12-23 |archive-date=2017-06-29 |archive-url=https://web.archive.org/web/20170629141733/http://www.chicagotribune.com/news/weather/ct-wea-1221-asktom-20151220-column.html |url-status=live }}</ref>]]

Most pre-modern calendars the world over were [[lunisolar]], combining the solar year with the lunation by means of [[intercalary month]]s.<ref>see e.g., {{Cite book | first = Bonnie | last = Blackburn | display-authors = etal | date = 1999 | title = The Oxford Companion to the Year | isbn = 0-19-214231-3 | publisher = Oxford University Press | url-access = registration | url = https://archive.org/details/oxfordcompaniont00blac }}</ref><ref>{{Cite book| first = Edward M. | last = Reingold |display-authors=etal | date= 2001 | title = Calendrical Calculations: The Millennium Edition |title-link= Calendrical Calculations | isbn = 0-521-77752-6 | publisher = Cambridge University Press}}</ref> The Julian calendar abandoned this method in favour of a purely solar reckoning while conversely the 7th-century [[Islamic calendar]] opted for a purely lunar one.

A continuing lunisolar calendar is the [[Hebrew calendar]]. Evidence of this is noted in the dates of [[Passover]] and Easter in Judaism and Christianity, respectively. Passover falls on the full moon on 15 Nisan of the Hebrew calendar. The date of the Jewish [[Rosh Hashana]] and [[Sukkot]] festivals along with all other [[Jewish holidays]] are dependent on the dates of the new moons.<ref>[[Leviticus]] 23:4–7, 33–35.</ref> Another continuing lunisolar calendar is the [[Chinese calendar]], which places many of its most important festivals, such as [[Chinese New Year]], the [[Dragon Boat Festival]], and the [[Mid-Autumn Festival]], according to lunisolar dates.

===Intercalary months===
{{Main|Intercalary month}}

In lunisolar calendars, an intercalary month occurs seven times in the 19 years of the [[Metonic cycle]], or on average every 2.7 years (19/7). In the [[Hebrew calendar]] this is noted with a periodic extra month of [[Adar]] in the early spring. The [[Chinese calendar]], by contrast, can place its intercalary month anywhere in the year, and does so based on finding the first month within a thirteen-month winter-solstice-to-winter-solstice period to include no [[solar term|major solar term]].