Leap year
A leap year (also known as an intercalary year or bissextile year) is a
An astronomical year lasts slightly less than 3651/4 days. The historic Julian calendar has three common years of 365 days followed by a leap year of 366 days, by extending February to 29 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 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.
In the lunisolar
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 months following the leap day (from 1 March through 28 February of the following year) will advance two days due to the extra day, thus leaping over one day in the week.
The length of a day is also occasionally corrected by inserting a
Leap years can present a problem in computing, known as the
Julian calendar
On 1 January 45 BC, by edict,
Prior to Caesar's creation of what would be the Julian calendar, February was already the shortest month of the year for Romans. In the Roman calendar (after the reform of Numa Pompilius that added January and February), all months except February had an odd number of days – 29 or 31. This was because of a Roman superstition that even numbers were unlucky.[5] When Caesar changed the calendar to follow the solar year closely, he made all months have 30 or 31 days, leaving February unchanged except in leap years.
Gregorian calendar
In the Gregorian calendar, the standard calendar in most of the world,[6] almost every fourth year is a leap year. Each leap year, the month of February has 29 days instead of 28. Adding one extra day in the calendar every four years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours.[7] However, this correction is excessive and the Gregorian reform modified the Julian calendar's scheme of leap years as follows:
Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the years 1600 and 2000 are.[8]
Whereas the Julian calendar year incorrectly summarised Earth's tropical year as 365.25 days, the Gregorian calendar makes these exceptions to follow a calendar year of 365.2425 days. This more closely resembles a mean tropical year of 365.2422 days. Over a period of four centuries, the accumulated error of adding a leap day every four years amounts to about three extra days. The Gregorian calendar therefore omits three leap days every 400 years, which is the length of its leap cycle. This is done by omitting 29 February in the three century years (multiples of 100) that are not multiples of 400.[9][10] The years 2000 and 2400 are leap years, but not 1700, 1800, 1900, 2100, 2200 and 2300. By this rule, an entire leap cycle is 400 years which total 146,097 days, and the average number of days per year is 365 + 1⁄4 − 1⁄100 + 1⁄400 = 365 + 97⁄400 = 365.2425.[11] (This rule could be applied to years before the Gregorian reform to create a proleptic Gregorian calendar,[12] though the result would not match any historical records.)
This graph shows the variations in date and time of the June Solstice. |
The Gregorian calendar was designed to keep the
Leap day in the Julian and Gregorian calendars
The intercalary day that usually occurs every four years is called leap day and is created by adding an extra day to February. This day is added to the calendar in leap years as a corrective measure because the Earth does not orbit the Sun in precisely 365 days. Since about the 15th century, this extra day has been 29 February, but when the Julian calendar was introduced, the leap day was handled differently in two respects. First, leap day fell within February and not at the end: 24 February was doubled to create, strangely to modern eyes, two days both dated 24 February.[14] Second, the leap day was simply not counted so that a leap year still had 365 days.[15]
Early Roman practice
The early
Julian reform
In Caesar's revised calendar, there was just one intercalary day – nowadays called the leap day – to be inserted every fourth year, and this too was done after 23 February. To create the intercalary day, the existing ante diem sextum Kalendas Martias (sixth day (inclusive: i.e. what we would call the fifth day before) before the
For legal purposes, the two days of the bis sextum were considered to be a single day, with the second sixth being intercalated; but in common practice by the year 238, when
In England, the Church and civil society continued the Roman practice whereby the leap day was simply not counted, so that a leap year was only reckoned as 365 days.
... and by (b) the statute de anno bissextili, it is provided, quod computentur dies ille excrescens et dies proxime præcedens pro unico dii, so as in computation that day excrescent is not accounted.[21]
29 February
Replacement (by 29 February) of the awkward practice of having two days with the same date appears to have evolved by custom and practice; the etymological origin of the term "bissextile" seems to have been lost.[14] In England in the fifteenth century, "29 February" appears increasingly often in legal documents – although the records of the proceedings of the House of Commons of England continued to use the old system until the middle of the sixteenth century.[14] It was not until the passage of the Calendar (New Style) Act 1750 that 29 February was formally recognised in British law.[22][b]
Liturgical practices
In the
In the Eastern Orthodox Church, the feast of St. John Cassian is celebrated on 29 February, but he is instead commemorated at Compline on 28 February in non-leap years. The feast of St. Matthias is celebrated in August, so leap years do not affect his commemoration, and, while the feast of the First and Second Findings of the Head of John the Baptist is celebrated on 24 February, the Orthodox church calculates days from the beginning of the current month, rather than counting down days to the Kalends of the following month, this is not affected. Thus, only the feast of St. John Cassian and any movable feasts associated with the Lenten or Pre-Lenten cycles are affected.
Folk traditions
In Ireland and Britain, it is a
According to Felten: "A play from the turn of the 17th century, 'The Maydes Metamorphosis,' has it that 'this is leape year/women wear breeches.' A few hundred years later, breeches wouldn't do at all: Women looking to take advantage of their opportunity to pitch woo were expected to wear a scarlet petticoat – fair warning, if you will."[30]
In Finland, the tradition is that if a man refuses a woman's proposal on leap day, he should buy her the fabrics for a skirt.[31]
In France, since 1980, a satirical newspaper titled La Bougie du Sapeur is published only on leap year, on 29 February.[citation needed]
In Greece, marriage in a leap year is considered unlucky.[32] One in five engaged couples in Greece will plan to avoid getting married in a leap year.[33]
In February 1988 the town of Anthony, Texas, declared itself the "leap year capital of the world", and an international leapling birthday club was started.[34]
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Woman capturing man with butterfly-net
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Women anxiously awaiting January 1
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Histrionically preparing
Birthdays
A person born on February 29 may be called a "leapling" or a "leaper".[35] In common years, they usually celebrate their birthdays on 28 February. In some situations, 1 March is used as the birthday in a non-leap year, since it is the day following 28 February.
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 bound to serve the pirates until his 21st birthday (that is, when he turns 88 years old, since 1900 was not a leap year) rather than until his 21st year.
For legal purposes, legal birthdays depend on how local laws count time intervals.
Taiwan
The Civil Code of Taiwan since 10 October 1929,[36] implies that the legal birthday of a leapling is 28 February in common years:
If a period fixed by weeks, months, and years does not commence from the beginning of a week, month, or year, it ends with the ending of the day which precedes the day of the last week, month, or year which corresponds to that on which it began to commence. But if there is no corresponding day in the last month, the period ends with the ending of the last day of the last month.[37]
Hong Kong
Since 1990 non-retroactively, Hong Kong considers the legal birthday of a leapling 1 March in common years:[38]
- The time at which a person attains a particular age expressed in years shall be the commencement of the anniversary corresponding to the date of [their] birth.
- 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.
This rule gives an average year length of 365.242222 days. This is a very good approximation to the mean tropical year, but because the vernal equinox year is slightly longer, the Revised Julian calendar, for the time being, does not do as good a job as the Gregorian calendar at keeping the vernal equinox on or close to 21 March.
Baháʼí calendar
The Baháʼí calendar is a solar calendar composed of 19 months of 19 days each (361 days). Years begin at Naw-Rúz, on the
Bengali, Indian and Thai calendars
The Revised
The
Chinese calendar
The
Hebrew calendar
The Hebrew calendar is lunisolar with an
In addition, the Hebrew calendar has postponement rules that postpone the start of the year by one or two days. These postponement rules reduce the number of different combinations of year length and starting
One reason for this rule is that Yom Kippur, the holiest day in the Hebrew calendar and the tenth day of the Hebrew year, now must never be adjacent to the weekly Sabbath (which is Saturday), i.e., it must never fall on Friday or Sunday, in order not to have two adjacent Sabbath days. However, Yom Kippur can still be on Saturday. A second reason is that Hoshana Rabbah, the 21st day of the Hebrew year, will never be on Saturday. These rules for the Feasts do not apply to the years from the Creation to the deliverance of the Hebrews from Egypt under Moses. It was at that time (cf. Exodus 13) that the God of Abraham, Isaac and Jacob gave the Hebrews their "Law" including the days to be kept holy and the feast days and Sabbaths.
Years consisting of 12 months have between 353 and 355 days. In a k'sidra ("in order") 354-day year, months have alternating 30 and 29 day lengths. In a chaser ("lacking") year, the month of
Islamic calendars
The observed and calculated versions of the lunar
The
The Hijri-Shamsi calendar, also adopted by the
Coptic and Ethiopian calendars
The Coptic calendar has 13 months, 12 of 30 days each, and one at the end of the year of 5 days, or 6 days in leap years. The Coptic Leap Year follows the same rules as the Julian Calendar so that the extra month always has six days in the year before a Julian Leap Year.[45] The Ethiopian calendar has twelve months of thirty days plus five or six epagomenal days, which comprise a thirteenth month.[46]
See also
- Century leap year
- Calendar reform includes proposals that have not (yet) been adopted.
- Leap second
- Leap week calendar
- Leap year bug
- Sansculottides
- Zeller's congruence
- February 30
- Leap year starting on Monday
- Leap year starting on Tuesday
- Leap year starting on Wednesday
- Leap year starting on Thursday
- Leap year starting on Friday
- Leap year starting on Saturday
- Leap year starting on Sunday
- Orbital period
Notes
- ^ Statute concerning [the] leap year and leap day
The day of the leap year, and the day before, shall be holden for one day.[20] - ^ Though it appears in earlier Government proclamations, such as one of 1619.[23]
- ^ Virtually no laws of Margaret survive. Indeed, none concerning her subjects are recorded in the twelve-volume Acts of the Parliaments of Scotland (1814–75) covering the period 1124–1707 (two laws concerning young Margaret herself are recorded on pages 424 & 441–2 of volume I).
Sources
- Cheney, Christopher Robert, ed. (2000) [1945]. A Handbook of Dates for students of British History. Revised by Michael Jones. Cambridge: ISBN 9780521778459.
- Pollard, A F (1940). "New Year's Day and Leap Year in English History". The English Historical Review. 55 (218 (April 1940)): 177–193. JSTOR 553864.
References
- ^ Meeus, Jean (1998), Astronomical Algorithms, Willmann-Bell, p. 62
- ^ Harper, Douglas (2012), "leap year", Online Etymology Dictionary, archived from the original on 21 August 2012, retrieved 15 August 2012
- ^ "leap year", Oxford US Dictionary, archived from the original on 13 September 2015, retrieved 6 January 2020
- ^ Astronomical almanac online glossary, US Naval Observatory, 2020, archived from the original on 23 February 2022, retrieved 28 January 2022
- Encyclopedia Britannica, retrieved 31 May 2023
- ' calendar designed by a commission assembled by Pope Gregory XIII in the sixteenth century.
- ^ Lerner, Ed. K. Lee; Lerner, Brenda W. (2004), "Calendar", The Gale Encyclopedia of Science, Detroit, MI: Gale
- ^ Introduction to Calendars Archived 2019-06-13 at the Wayback Machine. (10 August 2017). United States Naval Observatory.
- ^ United States Naval Observatory (14 June 2011), Leap Years, archived from the original on 15 October 2007, retrieved 9 April 2014
- ^ Lerner & Lerner 2004, p. 681.
- ISBN 9781891389856
- ^ Doggett, L.E. (1992), "Calendars", in Seidelmann, P. K. (ed.), Explanatory Supplement to the Astronomical Almanac (2nd ed.), Sausalito, CA: University Science Books, pp. 580–1
- ISBN 0-19-286205-7
- ^ a b c Pollard (1940), p. 188.
- ^ Cheney (2000), p 145, footnote 1.
- ^ According to Christian Ludwig Ideler (1825)
- ^ Key, Thomas Hewitt (2013) [1875], Calendarium, University of Chicago,
the intermediate days are in all cases reckoned backward upon the Roman principle already explained of counting both extremes.
- ^ Pollard (1940), p. 186.
- ^ a b Cheney (2000), Page 145, Footnote 1.
- ^ a b Ruffhead, Owen (1763), The Statutes at Large, from Magna Charta to the End of the Last Parliament, Mark Basket, p. 20, retrieved 21 October 2021 (21 Hen, III)
- ^ Coke, Edward (1628). "Cap. 2, Of Villenage". First Part of the Institutes of the Lawes of England. p. 136 left (Sect. 202, line 3).
- ^ Pickering, Danby, ed. (1765), The Statutes at Large: from the 23rd to the 26th Year of King George II, vol. 20, Cambridge: Charles Bathurst, p. 194, retrieved 28 January 2020 (calendar at the end of the Act)
- ^ Bond, John James (1875), "Preface", Handy Book of Rules and Tables for Verifying Dates With the Christian Era Giving an Account of the Chief Eras and Systems Used by Various Nations...' (4th ed.), London: George Bell & Sons, p. xix
- Archive.org
- Archive.org
- ^ Cheney (2000), p. 8.
- Liber Extra, 5. 40. 14. 1
- ^ Mikkelson, B.; Mikkelson, D.P. (2010), "The Privilege of Ladies", The Urban Legends Reference Pages, snopes.com
- ^ Ha-Redeye, Omar (28 February 2016). "The Leap Year Proposal Law". Slaw. Retrieved 8 April 2024.
- ^ Felten, E. (23 February 2008), "The Bissextile Beverage", Wall Street Journal, archived from the original on 12 August 2017, retrieved 12 August 2017
- ^ Hallett, S. (29 February 2012), "Leap Year Proposal: What's The Story Behind It?", Huffington Post, archived from the original on 21 October 2014, retrieved 21 December 2015
- ^ "A Greek Wedding", Anagnosis Books, archived from the original on 10 February 2012, retrieved 12 January 2012
- ^ "Teaching Tips 63", Developing Teachers, archived from the original on 2 March 2012, retrieved 12 January 2012
- ^ Anthony – Leap Year Capital of the World, Time and Date, 2008, archived from the original on 9 November 2011, retrieved 6 November 2011
- ^ "29 February: 29 things you need to know about leap years and their extra day", Mirror, 28 February 2012, archived from the original on 2 January 2016, retrieved 7 December 2015
- ^ Legislative History of the Civil Code of the Republic of China, archived from the original on 28 December 2016, retrieved 19 July 2011
- ^ "Article 121 Civil Code", Part I General Principles of the Republic of China, archived from the original on 4 March 2021, retrieved 19 July 2011
- ^ Age of Majority (Related Provisions) Ordinance (Ch. 410 Sec. 5), Hong Kong Department of Justice, 30 June 1997, archived from the original on 18 May 2013, retrieved 19 July 2011 (Enacted in 1990).
- ^ Exodus 23:15, Exodus 34:18, Deuteronomy 15:1, Deuteronomy 15:13
- ^ The Islamic leap year, Time and Date AS, n.d., archived from the original on 3 March 2020, retrieved 29 February 2012
- ^ Leap year trivia you might want to know, GMA News, n.d., archived from the original on 15 May 2013, retrieved 29 February 2012
- ^ Bromberg, Irv, Fixed Arithmetic Calendar Cycle Jitter, University of Toronto, archived from the original on 24 October 2019, retrieved 24 October 2019
- Bibcode:2004astro.ph..9620H, archivedfrom the original on 16 July 2011, retrieved 19 December 2010
- ^ Hijri-Shamsi Calendar, Al Islam, 2015, archived from the original on 26 January 2017, retrieved 18 April 2015,
The time frame in these months is the same as [...] the months of a Christian calendar.
- ^ Fr Tadros Y Malaty (1988), The Coptic Calendar and Church of Alexandria., The Monastery of St. Macarius Press, The Desert of Scete, archived from the original on 13 September 2022, retrieved 13 September 2022
- ^ "Ethiopia: The country where a year lasts 13 months", BBC News, 10 September 2021, archived from the original on 18 September 2022, retrieved 17 September 2022
External links
- Gray, Meghan. "29 Leap Year". Numberphile. Brady Haran. Archived from the original on 22 May 2017. Retrieved 6 April 2013.
- Famous Leapers
- Leap Day Campaign: Galileo Day
- History Behind Leap Year National Geographic Society