Leap year: Difference between revisions

{{Short description|Calendar year containing an additional day}}

{{other uses}}

A '''leap year''' (also known as an '''intercalary year''' or '''bissextile year''') is a [[calendar year]] that contains an additional day (or, in the case of a [[lunisolar calendar]], a month) compared to a [[common year]]. The 366th day (or 13th month) is added to keep the calendar year synchronised with the [[astronomical year]] or [[seasonal year]].<ref name="Meeus">{{citation|last1 = Meeus | first1=Jean | date = 1998 | title = Astronomical Algorithms | publisher = Willmann-Bell | page=62}}</ref> Since astronomical events and seasons do not repeat in a [[Natural number|whole number]] of days, calendars having a constant number of days each year will unavoidably drift over time with respect to the event that the year is supposed to track, such as seasons. By inserting ("[[Intercalation (timekeeping)|intercalating]]") an additional day—a '''leap day'''—or month—a '''[[leap month]]'''—into some years, the drift between a civilization's dating system and the physical properties of the [[Solar System]] can be corrected.

 

An astronomical year lasts slightly less than 365{{sfrac|4}}&nbsp;days. The historic [[Julian calendar]] has three [[common year]]s of 365&nbsp;days followed by a leap year of 366&nbsp;days, by extending February to 29&nbsp;days rather than the common 28. The [[Gregorian calendar]], the world's most widely used civil calendar, makes a further adjustment for the small error in the Julian algorithm. Each leap year has 366&nbsp;days instead of 365. This extra leap day occurs in each year that is a multiple of 4, except for years evenly divisible by 100 but not by 400.

 

 

The term ''leap year'' probably comes from the fact that a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, but the day of the week in the 12&nbsp;months following the leap day (from 1 March through 28 February of the following year) will advance two&nbsp;days due to the extra day, thus leaping over one&nbsp;day in the week.<ref>{{citation | first = Douglas | last = Harper | url = http://www.etymonline.com/index.php?term=leap+year&allowed_in_frame=0 | title = leap year | work = Online Etymology Dictionary | date = 2012 | access-date = 15 August 2012 | archive-date = 21 August 2012 | archive-url = https://web.archive.org/web/20120821214540/http://www.etymonline.com/index.php?term=leap+year&allowed_in_frame=0 | url-status = live }}</ref><ref>{{citation |url=http://www.oxforddictionaries.com/us/definition/american_english/leap-year|archive-url=https://web.archive.org/web/20150913171136/http://www.oxforddictionaries.com/us/definition/american_english/leap-year|url-status=dead|archive-date=13 September 2015|title=leap year |website=Oxford US Dictionary | access-date = 6 January 2020}}</ref> For example, 1 March was on a Friday in 2024, then it will be on Saturday in 2025, Sunday in 2026, and Monday in 2027, but then will "leap" over Tuesday to fall on a Wednesday in 2028.<!-- Please do not change this list until after 2028 /at the very earliest/; leaving it March 1 would do no harm at all, as this is the day that immediately follows February 29 in leap years and February 28 in common years. -->

The Gregorian calendar was designed to keep the [[vernal equinox (Northern Hemisphere)|vernal equinox]] on or close to 21 March, so that the date of [[Easter]] (celebrated on the Sunday after the ecclesiastical [[full moon]] that falls on or after 21 March) remains close to the vernal equinox.<ref>{{citation | first = E. G. | last = Richards | date = 1998 | title = Mapping time: The Calendar and its History | publisher = Oxford University Press | pages = 250–1 | isbn = 0-19-286205-7}}</ref> The "[[Gregorian calendar#Accuracy|Accuracy]]" section of the "[[Gregorian calendar]]" article discusses how well the Gregorian calendar achieves this design goal, and how well it approximates the [[tropical year]].

 

{{Main|February 29|Bissextus}}

{{Redirect|Leap day|other uses|Leap Day (disambiguation){{!}}Leap Day}}

<gallery mode="packed" widths="350px" caption="1908 postcards">

File:PostcardLeapYearBeCarefulClara1908.jpg|Woman capturing man with butterfly-net

File:PostcardTheMaidensVowIn1908.jpg|Histrionically preparing

</gallery>

 

===Birthdays===

 

Technically, a leapling will have fewer ''birthday anniversaries'' than their age in years. This phenomenon may be exploited for dramatic effect when a person is declared to be only a quarter of their actual age, by counting their leap-year birthday anniversaries only. For example, in [[Gilbert and Sullivan]]'s 1879 [[comic opera]] ''[[The Pirates of Penzance]]'', Frederic (the pirate apprentice) discovers that he is [[Indentured servitude|bound to serve]] the pirates until his 21st ''birthday'' (that is, when he turns 88&nbsp;years old, since 1900 was not a leap year) rather than until his 21st ''year''.

# Where a person has been born on February 29 in a leap year, the relevant anniversary in any year other than a leap year shall be taken to be March 1.

# This section shall apply only where the relevant anniversary falls on a date after the date of commencement of this Ordinance.}}

===Revised Julian calendar===

The [[Revised Julian calendar]] adds an extra day to February in years that are multiples of four, except for years that are multiples of 100 that do not leave a remainder of 200 or 600 when divided by 900. This rule agrees with the rule for the Gregorian calendar until 2799. The first year that dates in the Revised Julian calendar will not agree with those in the Gregorian calendar will be 2800, because it will be a leap year in the Gregorian calendar but not in the Revised Julian calendar.

* [[Sansculottides]]

* [[Zeller's congruence]]

 

* [[Leap year starting on Monday]]