Editing Year (section)

== Intercalation ==
Astronomical years do not have an [[integer]] number of days or lunar months. Any calendar that follows an astronomical year must have a system of [[Intercalation (timekeeping)|intercalation]] such as leap years.

=== Julian calendar ===
In the [[Julian calendar]], the average (mean) length of a year is 365.25 days. In a non-leap year, there are 365 days, in a leap year there are 366 days. A leap year occurs every fourth year during which a leap day is [[intercalation (timekeeping)#Solar calendars|intercalated]] into the month of February. The name "Leap Day" is applied to the added day.

In astronomy, the [[Julian year (astronomy)|Julian year]] is a unit of time defined as 365.25 days, each of exactly {{val|86400}} [[second]]s ([[SI base unit]]), totaling exactly 31,557,600 seconds in the Julian astronomical year.<ref>{{cite web |publisher= International Astronomical Union IAU |url=http://www.iau.org/science/publications/proceedings_rules/units/ |title=SI units |access-date=February 18, 2010}}</ref><ref>{{cite journal |first=George A. |last=Wilkins |url= http://www.iau.org/static/publications/stylemanual1989.pdf |title=The IAU Style Manual |year= 1989 |journal= IAU Transactions |volume= XXB}}</ref>

==== Revised Julian calendar ====
The [[Revised Julian calendar]], proposed in 1923 and used in some [[Eastern Orthodox Church]]es, has 218 leap years every 900 years, for the average (mean) year length of {{val|365.2422222}} days, close to the length of the mean tropical year, {{val|365.24219}} days (relative error of 9·10). In the year 2800 CE, the Gregorian and Revised Julian calendars will begin to differ by one calendar day.<ref>{{Cite journal |bibcode = 1924PA.....32..407S|title = The new calendar of the eastern churches|last1 = Shields |first1 = Miriam Nancy|journal = Popular Astronomy|year = 1924|volume = 32|pages = 407}}</ref>

=== Gregorian calendar ===
The [[Gregorian calendar]] aims to ensure that the [[northward equinox]] falls on or shortly before March 21 and hence it follows the [[tropical year|northward equinox year]], or [[tropical year]].<ref>{{Cite book |last=Ziggelaar |first=A. |year=1983 | chapter-url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?journal=grc..&year=1983&volume=book&letter=.&db_key=GEN&page_ind=230&plate_select=NO&data_type=GIF&type=SCREEN_GIF&classic=YES |chapter=The Papal Bull of 1582 Promulgating a Reform of the Calendar |editor1=G. V. Coyne|editor2=M. A. Hoskin |editor3=O. Pedersen  |title=Gregorian Reform of the Calendar: Proceedings of the Vatican Conference to Commemorate its 400th Anniversary |location=Vatican City |publisher=Pontifical Academy of Sciences|page=223}}</ref> Because 97 out of 400 years are leap years, the mean length of the Gregorian calendar year is {{val|365.2425}} days; with a relative error below one [[Parts per million|ppm]] (8·10) relative to the current length of the mean [[tropical year]] ({{gaps|365.242|189}} days) and even closer to the current ''March equinox year'' of {{gaps|365.242|374}} days that it aims to match. <!-- It is estimated that, by the year 4000 CE, the northward equinox will fall back by one day in the Gregorian calendar.{{cn|date=June 2022}} This not because of this difference, but due to [[Tidal acceleration|the slowing of the Earth's rotation]] and the associated lengthening of the day.{{cn}} [Statement suspended because uncited in over a year and unattributed. OR violation?] -->

=== Other calendars ===
{{see|Lunisolar calendar}}
Historically, lunisolar calendars intercalated entire [[leap month]]s on an observational basis. Lunisolar calendars have mostly fallen out of use except for liturgical reasons ([[Hebrew calendar]], various [[Hindu calendar]]s).

A modern adaptation of the historical [[Jalali calendar]], known as the [[Solar Hijri calendar]] (1925), is a purely [[solar calendar]] with an irregular pattern of leap days based on observation (or astronomical computation), aiming to place new year ([[Nowruz]]) on the day of [[March equinox|vernal equinox]] (for the time zone of [[Tehran]]), as opposed to using an algorithmic system of leap years.