Gregorian calendar

==Gregorian reform==
{{gallery | height=140 | width=200 
|File:Christopher Clavius.jpg| alt1= a portrait of [[Christopher Clavius]] (1538–1612), one of the main authors of the reform  |[[Christopher Clavius]] (1538–1612), one of the main authors of the reform
|File:Gregory XIII.jpg|alt2 = a portrait of Pope Gregory XIII by [[Lavinia Fontana]], sixteenth century |Pope Gregory XIII, portrait by [[Lavinia Fontana]], 16C
|File:Inter-grav.jpg|alt3= an image of the first page of the papal bull {{lang|la|[[Inter gravissimas]]}} | First page of the papal bull {{lang|la|[[Inter gravissimas]]}}
|File:Gregorianscher Kalender Petersdom.jpg| alt4 = a picture of Gregory's tomb showing Antonio Lilio presenting his printed calendar |Detail of the pope's tomb by [[Camillo Rusconi]] (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar.
}}

The Gregorian calendar was a reform of the Julian calendar. It was instituted by papal bull ''[[Inter gravissimas]]'' dated 24 February 1582 by Pope Gregory XIII,{{sfnp|Gregory XIII|1582}} after whom the calendar is named. The motivation for the adjustment was to bring the date for the celebration of Easter to the time of year in which it was celebrated when it was introduced by the early Church. The error in the Julian calendar (its assumption that there are exactly 365.25 days in a year) had led to the date of the equinox according to the calendar drifting from the observed reality, and thus an error had been introduced into the [[Computus|calculation of the date of Easter]]. Although a recommendation of the [[First Council of Nicaea]] in 325 specified that all Christians should celebrate Easter on the same day, it took almost five centuries before virtually all Christians achieved that objective by adopting the rules of the Church of Alexandria (see [[Easter#Controversies over the date|Easter]] for the issues which arose).{{efn|The last major Christian region to accept the Alexandrian rules was the [[Carolingian Empire]] (most of Western Europe) during 780–800. The last monastery in England to accept the Alexandrian rules did so in 931, and a few churches in southwest Asia beyond the eastern border of the [[Byzantine Empire]] continued to use rules that differed slightly, causing four dates for Easter to differ every 532 years.}}

===Background===
Because the [[date of Easter]] is a function{{snd}}the ''[[computus]]''{{snd}}of the date of the [[march equinox|spring equinox]] in the northern hemisphere, the [[Catholic Church]] considered unacceptable the increasing divergence between the [[Canon law of the Catholic Church|canonical date]] of the equinox and observed reality. Easter is celebrated on the Sunday after the [[ecclesiastical full moon]] on or after 21 March, which was adopted as an approximation to the March equinox.{{sfnp|Richards|2013|page=599}} European scholars had been well aware of the calendar drift since the early medieval period.

[[Bede]], writing in the 8th century, showed that the accumulated error in his time was more than three days. [[Roger Bacon]] in {{Circa|1200}} estimated the error at seven or eight days. [[Dante]], writing {{Circa|1300}}, was aware of the need for calendar reform. An attempt to go forward with such a reform was undertaken by [[Pope Sixtus IV]], who in 1475 invited [[Regiomontanus]] to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome.<ref>{{cite book | first1=Ari |last1=Ben-Menahem |title= Historical Encyclopedia of Natural and Mathematical Sciences | volume=1 |year=2009| url= https://books.google.com/books?id=9tUrarQYhKMC&pg=PA863 | page= 863|publisher=Springer |isbn=9783540688310 }}</ref> The increase of astronomical knowledge and the precision of observations towards the end of the 15th century made the question more pressing. Numerous publications over the following decades called for a calendar reform, among them two papers sent to the Vatican by the [[University of Salamanca]] in 1515 and 1578,{{sfnp|Carabias Torres|2012|page=241}} but the project was not taken up again until the 1540s, and implemented only under Pope Gregory XIII (r. 1572–1585).

===Preparation===
In 1545, the [[Council of Trent]] authorised [[Pope Paul III]] to reform the calendar, requiring that the date of the [[March equinox|vernal equinox]] be restored to that which it held at the time of the [[First Council of Nicaea]] in 325 and that an alteration to the calendar be designed to prevent future drift. This would allow for more consistent and accurate scheduling of the feast of Easter.

In 1577, a {{lang|la|Compendium}} was sent to expert mathematicians outside the reform commission for comments. Some of these experts, including [[Giambattista Benedetti]] and [[Giuseppe Moletti|Giuseppe Moleto]], believed [[Easter]] should be computed from the true motions of the Sun and Moon, rather than using a tabular method, but these recommendations were not adopted.{{sfnp|Ziggelaar|1983|pages=211, 214}} The reform adopted was a modification of a proposal made by the [[Calabria]]n doctor [[Aloysius Lilius]] (or Lilio).{{sfnp|Moyer|1983}}

Lilius's proposal included reducing the number of leap years in four centuries from 100 to 97, by making three out of four centurial years common instead of leap years. He also produced an original and practical scheme for adjusting the [[Epact|epacts of the Moon]] when calculating the annual date of Easter, solving a long-standing obstacle to calendar reform.

Ancient tables provided the Sun's mean longitude.{{efn|See, for example, {{lang|la|Tabule illustrissimi principis regis alfonsii}} (Prague 1401−4). A full set of Alphonsine Tables (including tables for mean motions, conjunctions of Sun and Moon, equation of time, spherical astronomy, longitudes and latitudes of cities, star tables, eclipse tables).<ref>{{cite book |title=Tabule illustrissimi principis regis alfonsii |trans-title=The tablet of the most illustrious prince King Alphonsus |last=John of Saxony |url=https://archive.org/details/ljs174/page/n3/mode/2up |date=1401  |language=la}}</ref> For an example of the information provided see Jacques Cassini, {{lang|fr|Tables astronomiques du soleil, de la lune, des planètes, des étoiles fixes, et des satellites de Jupiter et de Saturne}}, Table III.<ref>{{cite book |title=Tables astronomiques du soleil, de la lune, des planètes, des étoiles fixes, et des satellites de Jupiter et de Saturne |language=fr |trans-title=Astronomical tables of the sun, the moon, the planets, the fixed stars, and the satellites of Jupiter and Saturn |first=Jacques |last=Cassini |date=1740 |location=Paris |publisher=Imprimerie Royale |page=T10 |url=https://archive.org/details/s2id11854200/page/10/mode/1up}}</ref>}} The German mathematician [[Christopher Clavius]], the architect of the Gregorian calendar, noted that the tables agreed neither on the time when the Sun passed through the vernal equinox nor on the length of the mean tropical year. [[Tycho Brahe]] also noticed discrepancies.<ref>{{cite book |last=Dreyer |first=J L E |url=https://books.google.com/books?id=CdzSAgAAQBAJ&pg=PA52 |title=Tycho Brahe |date=2014 |publisher=Cambridge University Press |isbn=978-1-108-06871-0 |location=Cambridge |page=52 |quote=He remarks that both the Alphonsine and the Prutenic Tables are several hours wrong with regard to the time of the equinoxes and solstices.}}</ref><ref>{{cite book |url=https://books.google.com/books?id=q2xxatst4OQC&pg=PA29 |last=North |first=J |title=The Universal frame: historical essays in astronomy, natural philosophy and scientific method |location=London |date=1989 |page=29 |isbn=978-0-907628-95-8 |quote=He noted on one occasion that the ''Alphonsine tables'' differed from the ''Prutenic'' by nineteen hours as to the time of the vernal equinox of 1588.}}</ref> The Gregorian leap year rule (97 leap years in 400 years) was put forward by [[Petrus Pitatus]] of Verona in 1560. He noted that it is consistent with the tropical year of the [[Alfonsine tables]] and with the mean tropical year of Copernicus (''[[De revolutionibus]]'') and [[Erasmus Reinhold]] (''[[Prutenic tables]]''). The three mean tropical years in Babylonian [[sexagesimal]]s as the excess over 365 days (the way they would have been extracted from the tables of mean longitude) were 0;14,33,9,57 (Alfonsine), 0;14,33,11,12 (Copernicus) and 0;14,33,9,24 (Reinhold).{{efn|For an explanation of this notation, see [[Sexagesimal#Notations]].}} In decimal notation, these are equal to 0.24254606, 0.24255185, and 0.24254352, respectively. All values are the same to two sexagesimal places (0;14,33, equal to decimal 0.2425) and this is also the mean length of the Gregorian year. Thus Pitatus's solution would have commended itself to the astronomers.{{sfnp|Swerdlow|1986}}

Lilius's proposals had two components. First, he proposed a correction to the length of the year. The mean [[tropical year]] is 365.24219 days long.{{sfnp|Meeus|Savoie|1992}} A commonly used value in Lilius's time, from the Alfonsine tables, is 365.2425463 days.{{sfnp|Moyer|1983}} As the average length of a Julian year is 365.25 days, the Julian year is almost 11 minutes longer than the mean tropical year. The discrepancy results in a drift of about three days every 400 years. Lilius's proposal resulted in an average year of 365.2425 days (see [[#Accuracy|Accuracy]]). At the time of Gregory's reform there had already been a drift of 10 days since the Council of Nicaea, resulting in the vernal equinox falling on 10 or 11 March instead of the ecclesiastically fixed date of 21 March, and if unreformed it would have drifted further. Lilius proposed that the 10-day drift should be corrected by deleting the Julian leap day on each of its ten occurrences over a period of forty years, thereby providing for a gradual return of the equinox to 21 March.

Lilius's work was expanded upon by Christopher Clavius in a closely argued, 800-page volume. He would later defend his and Lilius's work against detractors. Clavius's opinion was that the correction should take place in one move, and it was this advice that prevailed with Gregory.

The second component consisted of an approximation that would provide an accurate yet simple, rule-based calendar. Lilius's formula was a 10-day correction to revert the drift since the Council of Nicaea, and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. The proposed rule was that "years divisible by 100 would be leap years only if they were divisible by 400 as well".

The 19-year cycle used for the lunar calendar required revision because the astronomical new moon was, at the time of the reform, four days before the calculated new moon.{{sfnp|Richards|2013|page=599}} It was to be corrected by one day every 300 or 400 years (8 times in 2500 years) along with corrections for the years that are no longer leap years (i.e. 1700, 1800, 1900, 2100, etc.) In fact, a new method for computing the date of Easter was introduced. The method proposed by Lilius was revised somewhat in the final reform.{{sfnp|Ziggelaar|1983|page = 220}}

When the new calendar was put in use, the error accumulated in the 13 centuries since the Council of Nicaea was corrected by a deletion of 10 days. The Julian calendar day Thursday, 4 October 1582 was followed by the first day of the Gregorian calendar, Friday, 15 October 1582 (the cycle of weekdays was not affected).
{{clear}}

====First printed Gregorian calendar====
[[File:Reforma Gregoriana del Calendario Juliano.jpg|thumb|
''Lunario Novo, Secondo la Nuova Riforma della Correttione del l'Anno Riformato da N.S. Gregorio XIII'',{{efn|name=Lunario|"New Almanac according to the new reform for the correction of the year, [as] reformed by His Holiness Gregory XIII".}} printed in [[Rome]] by Vincenzo Accolti in 1582, one of the first printed editions of the new calendar]]
A month after having decreed the reform, the pope (with a brief of 3 April 1582) granted to one Antoni Lilio the exclusive right to publish the calendar for a period of ten years. The {{lang|it|Lunario Novo secondo la nuova riforma}}{{efn|name=Lunario}} was printed by Vincenzo Accolti, one of the first calendars printed in Rome after the reform, notes at the bottom that it was signed with papal authorization and by Lilio (''Con licentia delli Superiori... et permissu Ant(onii) Lilij''). The papal brief was revoked on 20 September 1582, because Antonio Lilio proved unable to keep up with the demand for copies.<ref>{{cite book| last1=Mezzi |first1=E. |last2= Vizza | first2=F. | title= Luigi Lilio Medico Astronomo e Matematico di Cirò | publisher= Laruffa Editore |location = Reggio Calabria | year=2010 |pages=14, 52 |isbn=9788872214817 }} citing as primary references: {{lang|it|Biblioteca Nazionale Centrale di Firenze}}, Magl. 5.10.5/a, [[Vatican Apostolic Archive]] A.A., Arm. I‑XVIII, 5506, f. 362r.</ref>

===Adoption <span class="anchor" id="Timeline"></span>===
{{main|Adoption of the Gregorian calendar}}
Although Gregory's reform was enacted in the most solemn of forms available to the Church, the bull had no authority beyond the Catholic Church (of which he was the supreme religious authority) and the [[Papal States]] (which he personally ruled). The changes that he was proposing were changes to the civil calendar, over which he had no authority. They required adoption by the civil authorities in each country to have legal effect.

The bull {{lang|la|Inter gravissimas}} became the law of the Catholic Church in 1582, but it was not recognised by [[Protestantism|Protestant Churches]], [[Eastern Orthodox Churches]], [[Oriental Orthodox Churches]], and a few others. Consequently, the days on which Easter and related holidays were celebrated by different Christian Churches again diverged.

On 29 September 1582, [[Philip II of Spain]] decreed the change from the Julian to the Gregorian calendar.<ref>{{cite book |last=Kamen |first=Henry |date=1998 |title=Philip of Spain |url= https://books.google.com/books?id=wyWourPR5S8C&q=gregorian%20calendar|publisher=Yale University Press |page=248 |isbn=978-0300078008}}</ref> This affected much of Roman Catholic Europe, as Philip was at the time ruler over [[Iberian Union|Spain and Portugal]] as well as [[History of Italy (1559–1814)|much of Italy]]. In these territories, as well as in the [[Polish–Lithuanian Commonwealth]]<ref>{{Cite web|last=Cohen|first=Jennie|title=6 Things You May Not Know About the Gregorian Calendar|url=https://www.history.com/news/6-things-you-may-not-know-about-the-gregorian-calendar|access-date=23 July 2021|website=HISTORY|date=23 August 2018 |language=en}}</ref> (ruled by [[Anna Jagiellon]]) and in the Papal States, the new calendar was implemented on the date specified by the bull, with Julian Thursday, 4 October 1582, being followed by Gregorian Friday, 15 October. The [[Spanish Empire|Spanish]] and [[Portuguese Empire|Portuguese colonies]] followed somewhat later {{lang|la|de facto}} because of delay in communication.<ref>{{cite web| url=http://hdl.loc.gov/loc.wdl/wdl.2837 |title ="Pragmatica" on the Ten Days of the Year |year =1584 | publisher= [[World Digital Library]]}}: the first known South American imprint, produced in 1584 by Antonio Ricardo, of a four-page edict issued by King Philip II of Spain in 1582, decreeing the change from the Julian to the Gregorian calendar. {{Dead link|date=May 2022}}</ref> The other major Catholic power of Western Europe, France, adopted the change a few months later: 9 December was followed by 20 December.<ref>{{cite web|url=https://www.tondering.dk/claus/cal/gregorian.php|publisher=Tondering.dk|title=The Calendar FAQ: The Gregorian Calendar|accessdate=3 May 2022}}</ref>

Many [[Protestant]] countries initially objected to adopting a Catholic innovation; some Protestants feared the new calendar was part of a plot to return them to the Catholic fold. For example, the British could not bring themselves to adopt the Catholic system explicitly: the Annexe to their [[Calendar (New Style) Act 1750]] established a computation for the date of Easter that achieved the same result as Gregory's rules, without actually referring to him.<ref>{{cite web |url=https://www.legislation.gov.uk/apgb/Geo2/24/23/section/3 |title=Calendar (New Style) Act 1750, Section 3|publisher = Parliament of Great Britain | via=National Archives}}</ref>

[[Kingdom of Great Britain|Britain]] and the [[British Empire]] (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752. [[Swedish calendar|Sweden]] followed in 1753.

Prior to 1917, Turkey used the lunar [[Islamic calendar]] with the [[Hijri year|Hijri era]] for general purposes and the Julian calendar for fiscal purposes. The start of the fiscal year was eventually fixed at 1 March and the year number was roughly equivalent to the [[Hijri year]] (see [[Rumi calendar]]). As the solar year is longer than the lunar year this originally entailed the use of "escape years" every so often when the number of the fiscal year would jump. From 1 March 1917 the fiscal year became Gregorian, rather than Julian. On 1 January 1926, the use of the Gregorian calendar was extended to include use for general purposes and the number of the year became the same as in most other countries.

====Adoption by country====
{{hatnote|This is a brief summary. For a comprehensive table, see [[List of adoption dates of the Gregorian calendar by country]]}}
{| class="wikitable"
! Year
! Country/-ies/Areas 
|-
| 1582
| [[Spain]], [[Portugal]], [[France]], [[Polish-Lithuanian Commonwealth]], [[Italy]], Catholic [[Low Countries]], [[Luxembourg]], and colonies thereof
|-
| 1584
| [[Kingdom of Bohemia]], some Catholic [[Swiss cantons]]{{efn|name="CH"|In the [[Old Swiss Confederacy]], [[Helvetic Republic]], or [[Switzerland]], adoptions were made between 1584 and 1811. Some Catholic cantons switched in 1584, some Protestant in 1700/1701. For a complete list see [[List of adoption dates of the Gregorian calendar per country]].}}
|-
| 1610
| [[Prussia]]
|-
| 1648
| [[Alsace]]
|-
| 1682
| [[Strasbourg]]
|-
| 1700
| Protestant [[Low Countries]], [[Norway]], [[Denmark]], some Protestant [[Swiss cantons]]{{efn| name="CH"}}
|-
| 1752
| [[Kingdom of Great Britain|Great Britain]], [[Kingdom of Ireland|Ireland]], and the [[British Empire#"First" British Empire (1707–1783)|"First" British Empire (1707–1783)]]
|-
| 1753
| [[Sweden]] and [[Finland]]
|-
| 1873
| [[Japan]]
|-
| 1875
| [[Egypt]]
|-
| 1896
| [[Korea]]
|-
| 1912
| [[China]], [[Albania]]
|-
| 1915
| [[Latvia]], [[Lithuania]]
|-
| 1916
| [[Bulgaria]]
|-
| 1917
| [[Ottoman Empire]]
|-
| 1918
| [[Ukraine]], [[Russia]], [[Estonia]]
|-
| 1919
| [[Romania]], [[Yugoslavia]]{{efn|1919 in the regions comprising the former Kingdoms of Serbia and Montenegro (present-day Kosovo, Montenegro, Serbia and North Macedonia). The western and northern regions of what became Yugoslavia were already using the Gregorian calendar. For example, most of [[Slovenia]] adopted the Gregorian calendar at the same time as [[Austria]] in 1583. Coastal [[Croatia]], which was at the time ruled by [[Venice]], adopted the Gregorian calendar in 1582. Inland [[Croatia]], ruled by the [[House of Habsburg|Habsburgs]], adopted it in 1587 along with Hungary. The Gregorian calendar was used in [[Bosnia and Herzegovina]] since the 16th century by the [[Catholic Church|Catholic]] population and was formally adopted for government use in 1878 following occupation by [[Austria-Hungary]].}}
|-
| 1923
| [[Greece]]
|-
| 1926
| [[Turkey]] ([[common era]] years; Gregorian dates in use since 1917 Ottoman adoption)
|-
| 2016
| [[Saudi Arabia]]
|}