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The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by
Pope Gregory XIII Pope Gregory XIII ( la, Gregorius XIII; it, Gregorio XIII; 7 January 1502 – 10 April 1585), born Ugo Boncompagni, was head of the Catholic Church and ruler of the Papal States from 13 May 1572 to his death in April 1585. He is best known for ...
as a modification of, and replacement for, the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...
. The principal change was to space leap years differently so as to make the average calendar year 365.2425 days long, more closely approximating the 365.2422-day 'tropical' or 'solar' year that is determined by the Earth's revolution around the Sun. The rule for leap years is: There were two reasons to establish the Gregorian calendar. First, the Julian calendar assumed incorrectly that the average solar year is exactly 365.25 days long, an overestimate of a little under one day per century, and thus has a leap year every four years without exception. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
es.See Wikisource English translation of the (Latin) 1582 papal bull '' Inter gravissimas''. Second, in the years since the
First Council of Nicaea The First Council of Nicaea (; grc, Νίκαια ) was a council of Christian bishops convened in the Bithynian city of Nicaea (now İznik, Turkey) by the Roman Emperor Constantine I in AD 325. This ecumenical council was the first effort ...
in AD 325, the excess leap days introduced by the Julian algorithm had caused the calendar to drift such that the (Northern) spring equinox was occurring well before its nominal 21 March date. This date was important to the Christian churches because it is fundamental to the calculation of the date of Easter. To reinstate the association, the reform advanced the date by 10 days: Thursday 4 October 1582 was followed by Friday 15 October 1582. In addition, the reform also altered the lunar cycle used by the Church to calculate the date for Easter, because astronomical new moons were occurring four days before the calculated dates. It is notable that whilst the reform introduced minor changes, the calendar continued to be fundamentally based on the same geocentric theory as its predecessor. The reform was adopted initially by the Catholic countries of Europe and their overseas possessions. Over the next three centuries, the
Protestant Protestantism is a branch of Christianity that follows the theological tenets of the Protestant Reformation, a movement that began seeking to reform the Catholic Church from within in the 16th century against what its followers perceived to b ...
and
Eastern Orthodox Eastern Orthodoxy, also known as Eastern Orthodox Christianity, is one of the three main branches of Chalcedonian Christianity, alongside Catholicism and Protestantism. Like the Pentarchy of the first millennium, the mainstream (or " canonical ...
countries also moved to what they called the ''Improved calendar'', with Greece being the last European country to adopt the calendar (for civil use only) in 1923. To unambiguously specify a date during the transition period (in contemporary documents or in history texts), both notations were given, tagged as 'Old Style' or 'New Style' as appropriate. During the 20th century, most non-
Western Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that id ...
countries also adopted the calendar, at least for civil purposes.


Description

The Gregorian calendar, like the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...
, is a solar calendar with 12 months of 28–31 days each. The year in both calendars consists of 365 days, with a leap day being added to February in the leap years. The months and length of months in the Gregorian calendar are the same as for the Julian calendar. The only difference is that the Gregorian reform omitted a leap day in ''three'' centurial years every 400 years and left the leap day unchanged. A leap year normally occurs every four years: the leap day, historically, was inserted by doubling 24 February there were indeed two days dated 24 February. However, for many years it has been customary to put the extra day at the end of the month of February, adding a
29 February February 29, also known as leap day or leap year day, is a date added to leap years. A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in mo ...
for the leap day. Before the 1969 revision of its General Roman Calendar, the Catholic Church delayed February feasts after the 23rd by one day in leap years; Masses celebrated according to the previous calendar still reflect this delay. Gregorian years are identified by consecutive year numbers. A calendar date is fully specified by the year (numbered according to a calendar era, in this case '' Anno Domini'' or Common Era), the month (identified by name or number), and the day of the month (numbered sequentially starting from 1). Although the calendar year currently runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar (see the "beginning of the year" section below). Calendar cycles repeat completely every 400 years, which equals 146,097 days. Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. During intervals that do not contain any century common years (such as 1900), the calendar repeats every 28 years, during which
February 29 February 29, also known as leap day or leap year day, is a date added to leap years. A leap day is added in various solar calendars (calendars based on the Earth's revolution around the Sun), including the Gregorian calendar standard in mos ...
will fall on each of the seven days of the week once and only once. All other dates of the year fall on each day exactly four times, each day of the week having gaps of 6 years, 5 years, 6 years, and 11 years, in that order.


Gregorian reform

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, 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 calculation of the date of Easter. Although a recommendation of the
First Council of Nicaea The First Council of Nicaea (; grc, Νίκαια ) was a council of Christian bishops convened in the Bithynian city of Nicaea (now İznik, Turkey) by the Roman Emperor Constantine I in AD 325. This ecumenical council was the first effort ...
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 for the issues which arose).


Background

Because the date of Easter is a function the '' computus'' of the date of the (northern hemisphere) spring
equinox A solar equinox is a moment in time when the Sun crosses the Earth's equator, which is to say, appears directly above the equator, rather than north or south of the equator. On the day of the equinox, the Sun appears to rise "due east" and se ...
, the
Catholic Church The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptized Catholics worldwide . It is among the world's oldest and largest international institutions, and has played a ...
considered unacceptable the increasing divergence between the canonical date of the equinox and observed reality. Easter is celebrated on the Sunday after the
ecclesiastical full moon An ecclesiastical full moon is formally the 14th day of the ecclesiastical lunar month (an ecclesiastical moon) in an ecclesiastical lunar calendar. The ecclesiastical lunar calendar spans the year with lunar months of 30 and 29 days which are int ...
on or after 21 March, which was adopted as an approximation to the March equinox. 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 c. 1200 estimated the error at seven or eight days.
Dante Dante Alighieri (; – 14 September 1321), probably baptized Durante di Alighiero degli Alighieri and often referred to as Dante (, ), was an Italian people, Italian Italian poetry, poet, writer and philosopher. His ''Divine Comedy'', origin ...
, writing c. 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 Johannes Müller von Königsberg (6 June 1436 – 6 July 1476), better known as Regiomontanus (), was a mathematician, astrologer and astronomer of the German Renaissance, active in Vienna, Buda and Nuremberg. His contributions were instrument ...
to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome. 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 The University of Salamanca ( es, Universidad de Salamanca) is a Spanish higher education institution, located in the city of Salamanca, in the autonomous community of Castile and León. It was founded in 1218 by King Alfonso IX. It is t ...
in 1515 and 1578, but the project was not taken up again until the 1540s, and implemented only under
Pope Gregory XIII Pope Gregory XIII ( la, Gregorius XIII; it, Gregorio XIII; 7 January 1502 – 10 April 1585), born Ugo Boncompagni, was head of the Catholic Church and ruler of the Papal States from 13 May 1572 to his death in April 1585. He is best known for ...
(r. 1572–1585).


Preparation

In 1545, the Council of Trent authorised Pope Paul III to reform the calendar, requiring that the date of the vernal equinox be restored to that which it held at the time of the
First Council of Nicaea The First Council of Nicaea (; grc, Νίκαια ) was a council of Christian bishops convened in the Bithynian city of Nicaea (now İznik, Turkey) by the Roman Emperor Constantine I in AD 325. This ecumenical council was the first effort ...
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 was sent to expert mathematicians outside the reform commission for comments. Some of these experts, including Giambattista Benedetti and 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. The reform adopted was a modification of a proposal made by the Calabrian doctor
Aloysius Lilius Aloysius Lilius (c. 1510 – 1576), also variously referred to as Luigi Lilio or Luigi Giglio, was an Italian doctor, astronomer, philosopher and chronologist, and also the "primary author" who provided the proposal that (after modifications) be ...
(or Lilio). 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 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. The German mathematician
Christopher Clavius Christopher Clavius, SJ (25 March 1538 – 6 February 1612) was a Jesuit German mathematician, head of mathematicians at the Collegio Romano, and astronomer who was a member of the Vatican commission that accepted the proposed calendar inve ...
, 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 Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was ...
also noticed discrepancies. 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 ''De revolutionibus orbium coelestium'' (English translation: ''On the Revolutions of the Heavenly Spheres'') is the seminal work on the heliocentric theory of the astronomer Nicolaus Copernicus (1473–1543) of the Polish Renaissance. The book ...
'') and
Erasmus Reinhold Erasmus Reinhold (22 October 1511 – 19 February 1553) was a German astronomer and mathematician, considered to be the most influential astronomical pedagogue of his generation. He was born and died in Saalfeld, Saxony. He was educated, und ...
('' Prutenic tables''). The three mean tropical years in Babylonian sexagesimals 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). 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' solution would have commended itself to the astronomers. 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. A commonly used value in Lilius's time, from the Alfonsine tables, is 365.2425463 days. 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). 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. 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. 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).


First printed Gregorian 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 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.


Adoption

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 became the law of the Catholic Church in 1582, but it was not recognised by
Protestant Churches Protestantism is a branch of Christianity that follows the theological tenets of the Protestant Reformation, a movement that began seeking to reform the Catholic Church from within in the 16th century against what its followers perceived to b ...
, 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. This affected much of Roman Catholic Europe, as Philip was at the time ruler over Spain and Portugal as well as much of Italy. In these territories, as well as in the Polish–Lithuanian Commonwealth (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 Spanish might refer to: * Items from or related to Spain: **Spaniards are a nation and ethnic group indigenous to Spain **Spanish language, spoken in Spain and many Latin American countries **Spanish cuisine Other places * Spanish, Ontario, Can ...
and
Portuguese colonies The Portuguese Empire ( pt, Império Português), also known as the Portuguese Overseas (''Ultramar Português'') or the Portuguese Colonial Empire (''Império Colonial Português''), was composed of the overseas colonies, factories, and the l ...
followed somewhat later because of delay in communication. The other major Catholic power of Western Europe, France, adopted the change a few months later: 9 December was followed by 20 December. Many
Protestant Protestantism is a branch of Christianity that follows the theological tenets of the Protestant Reformation, a movement that began seeking to reform the Catholic Church from within in the 16th century against what its followers perceived to b ...
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 The Calendar (New Style) Act 1750 (24 Geo. II c.23), also known as Chesterfield's Act or (in American usage) the British Calendar Act of 1751, is an Act of the Parliament of Great Britain. Its purpose was for Great Britain and t ...
established a computation for the date of Easter that achieved the same result as Gregory's rules, without actually referring to him.
Britain Britain most often refers to: * The United Kingdom, a sovereign state in Europe comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands * Great Britain, the largest island in the United King ...
and the British Empire (including the eastern part of what is now the United States) adopted the Gregorian calendar in 1752. Sweden followed in 1753. Prior to 1917, Turkey used the lunar Islamic calendar with the 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 The ''Rumi'' calendar ( ota, رومي تقویم, , lit. "Roman calendar"), a specific calendar based on the Julian calendar, was officially used by the Ottoman Empire after Tanzimat (1839) and by its successor, the Republic of Turkey until 192 ...
). 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


Difference between Gregorian and Julian calendar dates

This section always places the intercalary day on even though it was always obtained by doubling (the (twice sixth) or bissextile day) until the late
Middle Ages In the history of Europe, the Middle Ages or medieval period lasted approximately from the late 5th to the late 15th centuries, similar to the post-classical period of global history. It began with the fall of the Western Roman Empire ...
. The Gregorian calendar is proleptic before 1582 (calculated backwards on the same basis, for years before 1582), and the difference between Gregorian and Julian calendar dates increases by three days every four centuries (all date ranges are inclusive). The following equation gives the number of days (actually, dates) that the Gregorian calendar is ahead of the Julian calendar, called the "secular difference" between the two calendars. A negative difference means the Julian calendar is ahead of the Gregorian calendar. :D = \left\lfloor\right\rfloor - \left\lfloor\right\rfloor - 2 where D is the secular difference and Y is the year using
astronomical year numbering Astronomical year numbering is based on AD/ CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0; the years before that are designated with negative numbers and the years after that are designated ...
, that is, use for BC years. \left\lfloor\right\rfloor means that if the result of the division is not an integer it is rounded down to the nearest integer. Thus during the 1900s, 1900/400 = 4, while during the −500s, −500/400 = −2. The general rule, in years which are leap years in the Julian calendar but not the Gregorian, is: Up to 28 February in the calendar being converted , add one day less or subtract one day more than the calculated value. Give February the appropriate number of days for the calendar being converted . When subtracting days to calculate the Gregorian equivalent of 29 February (Julian), 29 February is discounted. Thus if the calculated value is −4 the Gregorian equivalent of this date is 24 February.


Beginning of the year

The year used in dates during the Roman Republic and the Roman Empire was the consular year, which began on the day when consuls first entered office—probably 1 May before 222 BC, 15 March from 222 BC and 1 January from 153 BC. The Julian calendar, which began in 45, BC, continued to use 1 January as the first day of the new year. Even though the year used for dates changed, the civil year always displayed its months in the order January to December from the Roman Republican period until the present. During the Middle Ages, under the influence of the Catholic Church, many Western European countries moved the start of the year to one of several important Christian festivals—25 December ( Christmas), 25 March ( Annunciation), or Easter (France), while the Byzantine Empire began its year on 1 September and Russia did so on 1 March until 1492 when the new year was moved to 1 September. In common usage, 1 January was regarded as New Year's Day and celebrated as such, but from the 12th century until 1751 the legal year in England began on 25 March (
Lady Day In the Western liturgical year, Lady Day is the traditional name in some English-speaking countries of the Feast of the Annunciation, which is celebrated on 25 March, and commemorates the visit of the archangel Gabriel to the Virgin Mary, durin ...
). So, for example, the Parliamentary record lists the execution of Charles I on 30 January as occurring in 1648 (as the year did not end until 24 March), although later histories adjust the start of the year to 1 January and record the execution as occurring in 1649. Most Western European countries changed the start of the year to 1 January before they adopted the Gregorian calendar. For example, Scotland changed the start of the Scottish New Year to 1 January in 1600 (this means that 1599 was a short year). England, Ireland and the British colonies changed the start of the year to 1 January in 1752 (so 1751 was a short year with only 282 days). Later in 1752 in September the Gregorian calendar was introduced throughout Britain and the British colonies (see the section Adoption). These two reforms were implemented by the
Calendar (New Style) Act 1750 The Calendar (New Style) Act 1750 (24 Geo. II c.23), also known as Chesterfield's Act or (in American usage) the British Calendar Act of 1751, is an Act of the Parliament of Great Britain. Its purpose was for Great Britain and t ...
.Nørby, Toke
The Perpetual Calendar
In some countries, an official decree or law specified that the start of the year should be 1 January. For such countries, a specific year when a 1 January-year became the norm can be identified. In other countries, the customs varied, and the start of the year moved back and forth as fashion and influence from other countries dictated various customs. Neither the papal bull nor its attached canons explicitly fix such a date, though it is implied by two tables of saint's days, one labelled 1582 which ends on 31 December, and another for any full year that begins on 1 January. It also specifies its epact relative to 1 January, in contrast with the Julian calendar, which specified it relative to 22 March. The old date was derived from the Greek system: the earlier specified it relative to 1 January.


Dual dating

During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their numbered year on 1 January while others were still using some other date. Even before 1582, the year sometimes had to be double-dated because of the different beginnings of the year in various countries. Woolley, writing in his biography of John Dee (1527–1608/9), notes that immediately after 1582 English letter writers "customarily" used "two dates" on their letters, one OS and one NS.


Old Style and New Style dates

"Old Style" (O.S.) and "New Style" (N.S.) indicate dating systems before and after a calendar change, respectively. Usually, this is the change from the
Julian calendar The Julian calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the Roman calendar. It took effect on , by edict. It was designed with the aid of Greek mathematicians and astronomers such as Sosigenes of Alexandr ...
to the Gregorian calendar as enacted in various European countries between 1582 and the early 20th century. In England, Wales,
Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
and Britain's American colonies, there were two calendar changes, both in 1752. The first adjusted the start of a new year from
Lady Day In the Western liturgical year, Lady Day is the traditional name in some English-speaking countries of the Feast of the Annunciation, which is celebrated on 25 March, and commemorates the visit of the archangel Gabriel to the Virgin Mary, durin ...
(25 March) to 1 January (which
Scotland Scotland (, ) is a Countries of the United Kingdom, country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a Anglo-Scottish border, border with England to the southeast ...
had done from 1600), while the second discarded the Julian calendar in favour of the Gregorian calendar, removing 11 days from the September 1752 calendar to do so.Spathaky, Mik
Old Style and New Style Dates and the change to the Gregorian Calendar
. "Before 1752, parish registers, in addition to a new year heading after 24th March showing, for example '1733', had another heading at the end of the following December indicating '1733/4'. This showed where the Historical Year 1734 started even though the Civil Year 1733 continued until 24th March. ... We as historians have no excuse for creating ambiguity and must keep to the notation described above in one of its forms. It is no good writing simply 20th January 1745, for a reader is left wondering whether we have used the Civil or the Historical Year. The date should either be written 20th January 1745 OS (if indeed it was Old Style) or as 20th January 1745/6. The hyphen (1745-6) is best avoided as it can be interpreted as indicating a period of time."
To accommodate the two calendar changes, writers used
dual dating Dual dating is the practice, in historical materials, to indicate some dates with what appears to be duplicate, or excessive digits, sometimes separated by a hyphen, a slash or are placed one above the other. The need for dual dating arose from t ...
to identify a given day by giving its date according to both styles of dating. For countries such as Russia where no start of year adjustment took place, O.S. and N.S. simply indicate the Julian and Gregorian dating systems. Many
Eastern Orthodox Eastern Orthodoxy, also known as Eastern Orthodox Christianity, is one of the three main branches of Chalcedonian Christianity, alongside Catholicism and Protestantism. Like the Pentarchy of the first millennium, the mainstream (or " canonical ...
countries continue to use the older Julian calendar for religious purposes.


Proleptic Gregorian calendar

Extending the Gregorian calendar backwards to dates preceding its official introduction produces a proleptic calendar, which should be used with some caution. For ordinary purposes, the dates of events occurring prior to 15 October 1582 are generally shown as they appeared in the Julian calendar, with the year starting on 1 January, and no conversion to their Gregorian equivalents. For example, the Battle of Agincourt is universally considered to have been fought on 25 October 1415 which is Saint Crispin's Day. Usually, the mapping of new dates onto old dates with a start of year adjustment works well with little confusion for events that happened before the introduction of the Gregorian calendar. But for the period between the first introduction of the Gregorian calendar on 15 October 1582 and its introduction in Britain on 14 September 1752, there can be considerable confusion between events in continental western Europe and in British domains in English language histories. Events in continental western Europe are usually reported in English language histories as happening under the Gregorian calendar. For example, the Battle of Blenheim is always given as 13 August 1704. Confusion occurs when an event affects both. For example, William III of England set sail from the Netherlands on 11 November 1688 (Gregorian calendar) and arrived at Brixham in England on 5 November 1688 (Julian calendar). Shakespeare and
Cervantes Miguel de Cervantes Saavedra (; 29 September 1547 (assumed) – 22 April 1616 Old Style and New Style dates, NS) was an Early Modern Spanish writer widely regarded as the greatest writer in the Spanish language and one of the world's pre-emin ...
seemingly died on exactly the same date (23 April 1616), but Cervantes predeceased Shakespeare by ten days in real time (as Spain used the Gregorian calendar, but Britain used the Julian calendar). This coincidence encouraged
UNESCO The United Nations Educational, Scientific and Cultural Organization is a specialized agency of the United Nations (UN) aimed at promoting world peace and security through international cooperation in education, arts, sciences and culture. It ...
to make 23 April the World Book and Copyright Day. Astronomers avoid this ambiguity by the use of the Julian day number. For dates before the year 1, unlike the proleptic Gregorian calendar used in the international standard
ISO 8601 ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, w ...
, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a
year 0 A year zero does not exist in the Anno Domini (AD) calendar year system commonly used to number years in the Gregorian calendar (nor in its predecessor, the Julian calendar); in this system, the year is followed directly by year . However, the ...
and instead uses the ordinal numbers 1, 2, ... both for years AD and BC. Thus the traditional time line is 2 BC, 1 BC, AD 1, and AD 2. ISO 8601 uses astronomical year numbering which includes a year 0 and negative numbers before it. Thus the ISO 8601 time line is , 0000, 0001, and 0002.


Months

The Gregorian calendar continued to employ the Julian months, which have Latinate names and irregular numbers of days: * January (31 days), from Latin ', "Month of Janus",. the
Roman god Roman mythology is the body of myths of ancient Rome as represented in the literature and visual arts of the Romans. One of a wide variety of genres of Roman folklore, ''Roman mythology'' may also refer to the modern study of these representat ...
of gates, doorways, beginnings and endings * February (28 days in common and 29 in leap years), from Latin ', "Month of the
Februa Lupercalia was a pastoral festival of Ancient Rome observed annually on February 15 to purify the city, promoting health and fertility. Lupercalia was also known as ''dies Februatus'', after the purification instruments called ''februa'', the b ...
", the Roman festival of purgation and purification,.. cognate with fever, the Etruscan death god
Februus Februus is an ancient Italic god of purifications, who was worshipped by both the Romans and Etruscans. He was also worshipped as the god of the underworld by the Etruscans. For them, Februus was also the god of riches (money and gold) and death, ...
("Purifier"), and the Proto-Indo-European word for sulfur *
March March is the third month of the year in both the Julian and Gregorian calendars. It is the second of seven months to have a length of 31 days. In the Northern Hemisphere, the meteorological beginning of spring occurs on the first day of March ...
(31 days), from Latin ', "Month of
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
",. the Roman
war god War is an intense armed conflict between states, governments, societies, or paramilitary groups such as mercenaries, insurgents, and militias. It is generally characterized by extreme violence, destruction, and mortality, using regul ...
* April (30 days), from Latin ', of uncertain meaning. but usually derived from some form of the verb ' ("to open") or the name of the goddess Aphrodite * May (31 days), from Latin ', "Month of Maia",. a Roman vegetation goddess whose name is cognate with Latin ' ("great") and English ''major'' * June (30 days), from Latin ', "Month of Juno",. the Roman goddess of
marriage Marriage, also called matrimony or wedlock, is a culturally and often legally recognized union between people called spouses. It establishes rights and obligations between them, as well as between them and their children, and between ...
,
childbirth Childbirth, also known as labour and delivery, is the ending of pregnancy where one or more babies exits the internal environment of the mother via vaginal delivery or caesarean section. In 2019, there were about 140.11 million births glob ...
, and rule * July (31 days), from Latin ', "Month of Julius Caesar", the month of Caesar's birth, instituted in 44BC. as part of his calendrical reforms * August (31 days), from Latin ', "Month of Augustus", instituted by Augustus in 8BC in agreement with July and from the occurrence during the month of several important events during his rise to power. *
September September is the ninth month of the year in both the Julian and Gregorian calendars, the third of four months to have a length of 30 days, and the fourth of five months to have a length of fewer than 31 days. September in the Northern H ...
(30 days), from Latin ', "seventh month", of the ten-month Roman year of Romulus BC. * October (31 days), from Latin ', "eighth month", of the ten-month Roman year of Romulus BC. * November (30 days), from Latin ', "ninth month", of the ten-month Roman year of Romulus BC. * December (31 days), from Latin ', "tenth month", of the ten-month Roman year of Romulus BC. Europeans sometimes attempt to remember the number of days in each month by memorizing some form of the traditional verse " Thirty Days Hath September". It appears in Latin, Italian,. French and Portuguese, and belongs to a broad oral tradition but the earliest currently attested form of the poem is the English marginalia inserted into a
calendar of saints The calendar of saints is the traditional Christian method of organizing a liturgical year by associating each day with one or more saints and referring to the day as the feast day or feast of said saint. The word "feast" in this context d ...
:. Variations appeared in ''
Mother Goose The figure of Mother Goose is the imaginary author of a collection of French fairy tales and later of English nursery rhymes. As a character, she appeared in a song, the first stanza of which often functions now as a nursery rhyme. This, howeve ...
'' and continue to be taught at schools. The unhelpfulness of such involved mnemonics has been parodied as "Thirty days hath September/ But all the rest I can't remember" but it has also been called "probably the only sixteenth-century poem most ordinary citizens know by heart".. A common nonverbal alternative is the
knuckle mnemonic The knuckle mnemonic is a mnemonic device for remembering the number of days in the months of the Julian and Gregorian calendars. Methods One-handed One form of the mnemonic is done by counting on the knuckles of one's hand to remember the numbe ...
, considering the knuckles of one's hands as months with 31 days and the lower spaces between them as the months with fewer days. Using two hands, one may start from either pinkie knuckle as January and count across, omitting the space between the index knuckles (July and August). The same procedure can be done using the knuckles of a single hand, returning from the last (July) to the first (August) and continuing through. A similar mnemonic is to move up a piano keyboard in
semitone A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music, and it is considered the most dissonant when sounded harmonically. It is defined as the interval between two adjacent no ...
s from an F key, taking the white keys as the longer months and the black keys as the shorter ones.


Weeks

In conjunction with the system of months, there is a system of weeks. A physical or electronic calendar provides conversion from a given date to the weekday and shows multiple dates for a given weekday and month. Calculating the day of the week is not very simple, because of the irregularities in the Gregorian system. When the Gregorian calendar was adopted by each country, the weekly cycle continued uninterrupted. For example, in the case of the few countries that adopted the reformed calendar on the date proposed by Gregory XIII for the calendar's adoption, Friday, 15 October 1582, the preceding date was Thursday, 4 October 1582 (Julian calendar). Opinions vary about the numbering of the days of the week.
ISO 8601 ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, w ...
, in common use worldwide, starts with Monday=1; printed monthly calendar grids often list Mondays in the first (left) column of dates and Sundays in the last. In North America, the week typically begins on Sunday and ends on Saturday.


Accuracy

The Gregorian calendar improves the approximation made by the Julian calendar by skipping three Julian leap days in every 400 years, giving an average year of 365.2425 mean solar days long. This approximation has an error of about one day per 3,030 years with respect to the current value of the
mean tropical year A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time f ...
. However, because of the precession of the equinoxes, which is not constant, and the movement of the
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any elli ...
(which affects the Earth's orbital speed) the error with respect to the ''astronomical'' vernal equinox is variable; using the average interval between vernal equinoxes near 2000 of 365.24237 days implies an error closer to 1 day every 7,700 years. By any criterion, the Gregorian calendar is substantially more accurate than the 1 day in 128 years error of the Julian calendar (average year 365.25 days). In the 19th century, Sir John Herschel proposed a modification to the Gregorian calendar with 969 leap days every 4000 years, instead of 970 leap days that the Gregorian calendar would insert over the same period. This would reduce the average year to 365.24225 days. Herschel's proposal would make the year 4000, and multiples thereof, common instead of leap. While this modification has often been proposed since, it has never been officially adopted. On time scales of thousands of years, the Gregorian calendar falls behind the astronomical seasons. This is because the Earth's speed of rotation is gradually slowing down, which makes each day slightly longer over time (see tidal acceleration and leap second) while the year maintains a more uniform duration.


Calendar seasonal error

This image shows the difference between the Gregorian calendar and the astronomical seasons. The ''y''-axis is the date in June and the ''x''-axis is Gregorian calendar years. Each point is the date and time of the June solstice in that particular year. The error shifts by about a quarter of a day per year. Centurial years are ordinary years, unless they are divisible by 400, in which case they are leap years. This causes a correction in the years 1700, 1800, 1900, 2100, 2200, and 2300. For instance, these corrections cause 23 December 1903 to be the latest December solstice, and 20 December 2096 to be the earliest solstice—about 2.35 days of variation compared with the astronomical event.


Proposed reforms

The following are proposed reforms of the Gregorian calendar: * Holocene calendar * International Fixed Calendar (also called the ''International Perpetual calendar'') *
World Calendar The World Calendar is a proposed reform of the Gregorian calendar created by Elisabeth Achelis of Brooklyn, New York in 1930. Features The World Calendar is a 12-month, perennial calendar with equal quarters. Each quarter begins on a Sunday ...
*
World Season Calendar yi, יצחק אזימאװ , birth_date = , birth_place = Petrovichi, Russian SFSR , spouse = , relatives = , children = 2 , death_date = , death_place = Manhattan, New York City, U.S. , nationality = Russian (1920–1922)Soviet (192 ...
* Leap week calendars ** Pax Calendar **
Symmetry454 The Symmetry454 calendar (Sym454) is a proposal for calendar reform created by Irv Bromberg of the University of Toronto, Canada. It is a perennial solar calendar that conserves the traditional month pattern and 7-day week, has symmetrical equal ...
** Hanke–Henry Permanent Calendar


See also

*
Calendar (New Style) Act 1750 The Calendar (New Style) Act 1750 (24 Geo. II c.23), also known as Chesterfield's Act or (in American usage) the British Calendar Act of 1751, is an Act of the Parliament of Great Britain. Its purpose was for Great Britain and t ...
*
Calendar reform Calendar reform or calendrical reform is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar design. Principles The prime objective of a calendar is to unambiguo ...
* Conversion between Julian and Gregorian calendars *
Doomsday rule The Doomsday rule, Doomsday algorithm or Doomsday method is an algorithm of determination of the day of the week for a given date. It provides a perpetual calendar because the Gregorian calendar moves in cycles of 400 years. The algorithm for ...
* French revolutionary calendar * Hebrew calendar * Dionysius Exiguus * Inter gravissimas in English – Wikisource * Julian day * History of calendars *
ISO 8601 ISO 8601 is an international standard covering the worldwide exchange and communication of date and time-related data. It is maintained by the Geneva-based International Organization for Standardization (ISO) and was first published in 1988, w ...
, an international standard for the representation of dates and times, which uses the Gregorian calendar (see Section 3.2.1). *
List of adoption dates of the Gregorian calendar per country This is a list of adoption dates of the Gregorian calendar by country. For explanation, see the article about the Gregorian calendar. Except where stated otherwise, the transition was a move by the civil authorities from the Julian to the Grego ...
* List of calendars *
Old Calendarists Old Calendarists (Greek: ''palaioimerologitai'' or ''palaioimerologites''), also known as Old Feasters (''palaioeortologitai''), Genuine Orthodox Christians or True Orthodox Christians (GOC; ), are traditionalist groups of Eastern Orthodox Chri ...
**
Greek Old Calendarists Old Calendarists (Greek: ''palaioimerologitai'' or ''palaioimerologites''), also known as Old Feasters (''palaioeortologitai''), Genuine Orthodox Christians or True Orthodox Christians (GOC; ), are traditionalist groups of Eastern Orthodox Chri ...
*
Revised Julian calendar The Revised Julian calendar, or less formally the new calendar, is a calendar proposed in 1923 by the Serbian scientist Milutin Milanković as a more accurate alternative to both Julian and Gregorian calendars. At the time, the Julian calendar ...
(Milanković) – used in Eastern Orthodoxy Precursors of the Gregorian reform *
Johannes de Sacrobosco Johannes de Sacrobosco, also written Ioannes de Sacro Bosco, later called John of Holywood or John of Holybush ( 1195 – 1256), was a scholar, monk, and astronomer who taught at the University of Paris. He wrote a short introduction to the Hi ...
, ''De Anni Ratione'' ("On reckoning the years"), c. 1235 * Roger Bacon, ''
Opus Majus The ''Opus Majus'' (Latin for "Greater Work") is the most important work of Roger Bacon. It was written in Medieval Latin, at the request of Pope Clement IV, to explain the work that Bacon had undertaken. The 878-page treatise ranges over all ...
'' ("Greater Work"), c. 1267


Notes


Citations


References

* *. * * * * * * * * * * * * * * * * * * * * *


External links

*
Calendar Converter





The Perpetual Calendar Gregorian Calendar adoption dates for many countries.


* ttps://web.archive.org/web/20051228123115/http://www.tondering.dk/claus/calendar.html The Calendar FAQ– Frequently Asked Questions about Calendars
Today's date (Gregorian) in over 800 more-or-less obscure foreign languages
{{DEFAULTSORT:Gregorian Calendar 1580s introductions 1582 establishments 1582 establishments in Europe Articles which contain graphical timelines