Were the dates of saints days moved as part of the Gregorian calendar reforms?

Were the dates of saints days moved as part of the Gregorian calendar reforms?

I understand that if one were to go back in time and ask a participant at the battle of Agincourt what the date was, he would (if he knew) say that it was the 16th of October, he would also (more likely) say that it was - famously - St Crispin's Day:

This day is call'd the feast of Crispian.
He that outlives this day, and comes safe home,
Will stand a tip-toe when this day is nam'd,
And rouse him at the name of Crispian.

Currently, of course, we celebrate the anniversary of the event today, on the 25th of October - but we also call this day St Crispin's Day.

Did the dates of Saints Days move with the Gregorian calendar reforms? If so was this move done consistently, and at the time of the reforms or later?


The answer to your question is no. All fixed festivals remained on the same date, for example Christmas was celebrated on 25 December Julian, and then on 25 December Gregorian. However, the Gregorian reform did affect Easter and the movable feasts dependent on Easter.


Gregorian Calendar

In the year 1572, Ugo Boncompagni became Pope Gregory XIII and there was a crisis of the calendar - one of Christianity's most important dates was falling behind with respect to the seasons. Easter, which is based on the date of the vernal equinox (the first day of Spring), was being celebrated too early in the month of March. The cause of this calendrical confusion was the over 1,600 year-old Julian calendar, established by Julius Caesar in the year 46 BCE.

Julius Caesar took control over the chaotic Roman calendar, which was being exploited by politicians and others with the haphazard addition of days or months. It was a calendar horribly out-of-synch with the seasons of the earth, which are the result of the rotation of the earth around the sun. Caesar developed a new calendar of 364 1/4 days, closely approximating the length of the tropical year (the time it takes the earth to go around the sun from the beginning of spring to the beginning of spring). Caesar's calendar was normally 365 days long but included an extra day (a leap day) every four years to account for the extra one-quarter of a day. The intercalary (inserted into the calendar) day was added prior to February 25 each year.

Unfortunately, while Caesar's calendar was almost accurate, it wasn't quite accurate enough because the tropical year is not 365 days and 6 hours (365.25 days), but is approximately 365 days 5 hours 48 minutes, and 46 seconds (365.242199 days). Therefore, the calendar of Julius Caesar was 11 minutes and 14 seconds too slow. This added up to be a full day off every 128 years.

While it took from 46 BCE to 8 CE to get Caesar's calendar functioning properly (initially leap years were being celebrated every three years instead of every four), by the time of Pope Gregory XIII the one day every 128 years added up to a full ten days of error in the calendar. (Purely by luck did the Julian calendar happen to celebrate leap years on years divisible by four - during Caesar's time, the numbered years of today didn't exist).

A serious change needed to take place and Pope Gregory XIII decided to repair the calendar. Gregory was aided by astronomers in developing a calendar that would be more accurate than the Julian calendar. The solution they developed was almost perfect.

The new Gregorian calendar would continue to be comprised of 365 days with an intercalary added every four years (moved to after February 28 to make things easier) but there would be no leap year in years ending in "00" unless those years were divisible by 400. Therefore, the years 1700, 1800, 1900, and 2100 would not be a leap year but the years 1600 and 2000 would. This change was so accurate that today, scientists need only add leap seconds every few years to the clock in order to keep the calendar matching the tropical year.

Pope Gregory XIII issued a papal bull, "Inter Gravissimus" on February 24, 1582 that established the Gregorian calendar as the new and official calendar of the Catholic world. Since the Julian calendar had fallen ten days behind over the centuries, Pope Gregory XIII designated that October 4, 1582 would be officially followed by October 15, 1582. The news of the calendar change was disseminated across Europe. Not only would the new calendar be utilized but ten days would be "lost" forever, the new year would now begin on January 1 instead of March 25, and there would be a new method of determining the date of Easter.

Only a few countries were ready or willing to change to the new calendar in 1582. It was adopted that year in Italy, Luxembourg, Portugal, Spain, and France. The Pope was forced to issue a reminder on November 7 to nations that they should change their calendars and many did not heed the call. Had the calendar change been promulgated a century earlier, more countries would have been under Catholic rule and would have heeded the Pope's command. By 1582, Protestantism had spread across the continent and politics and religion were in disarray additionally, the Eastern Orthodox Christian countries would not change for many years.

Other countries later joined the fray over the following centuries. Roman Catholic Germany, Belgium, and the Netherlands switched by 1584 Hungary changed in 1587 Denmark and Protestant Germany switched by 1704 Great Britain and its colonies changed in 1752 Sweden changed in 1753 Japan changed in 1873 as part of Meiji's Westernization Egypt changed in 1875 Albania, Bulgaria, Estonia, Latvia, Lithuania, Romania, and Turkey all changed between 1912 and 1917 the Soviet Union changed in 1919 Greece switched to the Gregorian calendar in 1928 and finally, China changed to the Gregorian calendar after their revolution of 1949!

Change wasn't always easy, however. In Frankfurt as well as London, people rioted over the loss of days in their lives. With each change to the calendar around the world, laws established that people could not be taxed, paid, nor would interest accrue over the "missing" days. It was decreed that deadlines still had to take place in the correct number of "natural days" following the transition.

In Great Britain, Parliament legislated the change to the Gregorian calendar (by this time simply called the New Style calendar) in 1751 after two unsuccessful attempts at change in 1645 and 1699. They decreed that September 2, 1752 would be followed by September 14, 1752. Britain needed to add eleven days instead of ten because by the time Britain changed, the Julian calendar was eleven days off the Gregorian calendar and tropic year. This 1752 change also applied to the American colonies of Britain so the change was made in the pre-United States and pre-Canada at that time. Alaska didn't change calendars until 1867, when it transferred from a Russian territory to a part of the United States.

In the era after the change, dates were written with O.S. (Old Style) or N.S. (New Style) following the day so people examining records could understand whether they were looking at a Julian date or a Gregorian date. While George Washington was born on February 11, 1731 (O.S.), his birthday became February 22, 1732 (N.S.) under the Gregorian calendar. The change in the year of his birth was due to the change of when the change of the new year was acknowledged. Recall that prior to the Gregorian calendar, March 25 was the new year but once the new calendar was implemented, it became January 1. Therefore, since Washington was born between January 1 and March 25, the year of his birth became one year later upon the switch to the Gregorian calendar. (Prior to the 14th century, the new year change took place on December 25.)

Today, we rely on the Gregorian calendar to keep us almost perfectly in line with the rotation of the earth around the sun. Imagine the disruption to our daily lives if a new calendar change were required in this most modern era!


The date of Easter

Easter was the most important feast of the Christian church, and its place in the calendar determined the position of the rest of the church’s movable feasts (see church year). Because its timing depended on both the Moon’s phases and the vernal equinox, ecclesiastical authorities had to seek some way of reconciling lunar and solar calendars. Some simple form of computation, usable by nonastronomers in remote places, was desirable. There was no easy or obvious solution, and to make things more difficult there was no unanimous agreement on the way in which Easter should be calculated, even in a lunar calendar.

Easter, being the festival of the Resurrection, had to depend on the dating of the Crucifixion, which occurred three days earlier and just before the Jewish Passover. The Passover was celebrated on the 14th day of Nisan, the first month in the Jewish religious year—that is, the lunar month the 14th day of which falls on or next after the vernal equinox. The Christian churches in the eastern Mediterranean area celebrated Easter on the 14th of Nisan on whatever day of the week it might fall, but the rest of Christendom adopted a more elaborate reckoning to ensure that it was celebrated on a Sunday in the Passover week.

To determine precisely how the Resurrection and Easter Day should be dated, reference was made to the Gospels but, even as early as the 2nd century ce , difficulties had arisen, because the synoptic Gospels (Matthew, Mark, and Luke) appeared to give a different date from the Gospel According to John for the Crucifixion. This difference led to controversy that was later exacerbated by another difficulty caused by the Jewish reckoning of a day from sunset to sunset. The question arose of how the evening of the 14th day should be calculated, and some—the Quintodecimans—claimed that it meant one particular evening, but others—the Quartodecimans—claimed that it meant the evening before, since sunset heralded a new day. Both sides had their protagonists, the Eastern churches supporting the Quartodecimans, the Western churches the Quintodecimans. The question was finally decided by the Western church in favour of the Quintodecimans, though there is debate whether this was at the Council of Nicaea in 325 or later. The Eastern churches decided to retain the Quartodeciman position, and the church in Britain, which had few links with European churches at this time, retained the Quartodeciman position until Roman missionaries arrived in the 6th century, when a change was made. The dating of Easter in the Gregorian calendar was based on the decision of the Western church, which decreed that Easter should be celebrated on the Sunday immediately following the (Paschal) Full Moon that fell on or after the vernal equinox, which they took as March 21. The church also ordered that if this Full Moon fell on a Sunday, the festival should be held seven days later.

With these provisions in mind, the problem could be broken down into two parts: first, devising a simple but effective way of calculating the days of the week for any date in the year and, second, determining the date of the Full Moons in any year. The first part was solved by the use of a letter code derived from a similar Roman system adopted for determining market days. For ecclesiastical use, the code gave what was known as the Sunday, or dominical, letter.

The seven letters A through G are each assigned to a day, consecutively from January 1 so that January 1 appears as A, January 2 as B, to January 7 which appears as G, the cycle then continuing with January 8 as A, January 9 as B, and so on. Then in any year the first Sunday is bound to be assigned to one of the letters A–G in the first cycle, and all Sundays in the year possess that dominical letter. For example, if the first Sunday falls on January 3, C will be the dominical letter for the whole year. No dominical letter is placed against the intercalary day, February 29, but, since it is still counted as a weekday and given a name, the series of letters moves back one day every leap year after intercalation. Thus, a leap year beginning with the dominical letter C will change to a year with the dominical letter B on March 1 and in lists of dominical letters, all leap years are given a double letter notation, in the example just quoted, CB. It is not difficult to see what dominical letter or letters apply to any particular year, and it is also a comparatively simple matter to draw up a table of dominical letters for use in determining Easter Sunday. The possible dates on which Easter Sunday can fall are written down—they run from March 22 through April 25—and against them the dominical letters for a cycle of seven years. Once the dominical letter for a year is known, the possible Sundays for celebrating Easter can be read directly from the table. This system does not, of course, completely determine Easter to do so, additional information is required.

This must provide dates for Full Moons throughout the year, and for this a lunar cycle like the Metonic cycle was originally used. Tables were prepared, again using the range of dates on which Easter Sunday could appear, and against each date a number from one through 19 was placed. This number indicated which of the 19 years of the lunar cycle would give a Full Moon on that day. From medieval times these were known as golden numbers, possibly from a name used by the Greeks for the numbers on the Metonic cycle or because gold is the colour used for them in manuscript calendars.

The system of golden numbers was introduced in 530, but the numbers were arranged as they should have been if adopted at the Council of Nicaea two centuries earlier and the cycle was taken to begin in a year when the New Moon fell on January 1. Working backward, chronologers found that this date had occurred in the year preceding 1 ce , and therefore the golden number for any year is found by adding one to the year and dividing that sum by 19. The golden number is the remainder or, if there is no remainder, 19.

To compute the date of Easter, the medieval chronologer computed the golden number for the year and then consulted his table to see by which date this number lay. Having found this date, that of the first Full Moon after March 20, he consulted his table of dominical letters and saw the next date against which the dominical letter for that year appeared this was the Sunday to be designated Easter. The method, modified for dropping centennial leap years as practiced in the Gregorian calendar, is still given in the English prayer book, although it was officially discarded when the Gregorian calendar was introduced.

The system of golden numbers was eventually rejected because the astronomical Full Moon could differ by as much as two days from the date they indicated. It was Lilius who had proposed a more accurate system based on one that had already been in use unofficially while the Julian calendar was still in force. Called the epact—the word is derived from the Greek epagein, meaning “to intercalate”—this was again a system of numbers concerned with the Moon’s phases, but now indicating the age of the Moon on the first day of the year, from which the age of the Moon on any day of the year may be found, at least approximately, by counting, using alternately months of 29 and 30 days.

The epact as previously used was not, however, completely accurate because, like the golden number, it had been based on the Metonic cycle. This 19-year cycle was in error, the discrepancy amounting to eight days every 2,500 years. A one-day change on certain centennial years was then instituted by making the computed age of the Moon one day later seven times, at 300-year intervals, and an eighth time after a subsequent 400 years. This operation was known as the lunar correction, but it was not the only correction required there was another.

Because the Gregorian calendar used a more accurate value for the tropical year than the Julian calendar and achieved this by omitting most centennial leap years, Clavius decided that, when the cycle of epacts reached an ordinary centennial year, the number of the epact should be reduced by one this reduction became known as the solar correction.

One advantage of the epact number was that it showed the age of the Moon on January 1 and so permitted a simple calculation of the dates of New Moon and Full Moon for the ensuing year. Another was that it lent itself to the construction of cycles of 30 epact numbers, each diminishing by one from the previous cycle, so that, when it became necessary at certain centennial years to shift from one cycle to another, there would still be a cycle ready that retained a correct relationship between dates and New Moons.

For determining Easter, a table was prepared of the golden numbers, one through 19, and below them the cycles of epacts for about 7,000 years after this time, all the epact cycles are repeated. A second table was then drawn up, giving the dates of Easter Full Moons for different epact numbers. Once the epact for the year was known, the date of the Easter Full Moon could be immediately obtained, while consultation of a table of dominical letters showed which was the next Sunday. Thus, the Gregorian system of epacts, while more accurate than the old golden numbers, still forced the chronologer to consult complex astronomical tables.


Gregorian calendar

The Gregorian calendar is the calendar used in most of the world. [1] It was introduced in October 1582 by Pope Gregory XIII as a minor modification of the Julian calendar, reducing the average year from 365.25 days to 365.2425 days, and adjusting for the drift in the 'tropical' or 'solar' year that the inaccuracy had caused during the intervening centuries.

The calendar spaces leap years to make its average year 365.2425 days long, approximating the 365.2422-day tropical year that is determined by the Earth's revolution around the Sun. The rule for leap years is:

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. [2]

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. The Gregorian reform shortened the average (calendar) year by 0.0075 days to stop the drift of the calendar with respect to the equinoxes. [3] Second, in the years since the First Council of Nicaea in AD 325, [Note 1] 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. [3] 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.

The reform was adopted initially by the Catholic countries of Europe and their overseas possessions. Over the next three centuries, the Protestant and Eastern Orthodox 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. [4] 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 countries also adopted the calendar, at least for civil purposes.

Description

The Gregorian calendar, like the Julian calendar, 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 occurred every 4 years, and the leap day was historically inserted by doubling 24 February. However, it is now customary to number the days of February sequentially with no gaps, and 29 February is typically considered 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. [5]

A year is divided into twelve months
No. Name Length in days
1 January 31
2 February 28 (29 in leap years)
3 March 31
4 April 30
5 May 31
6 June 30
7 July 31
8 August 31
9 September 30
10 October 31
11 November 30
12 December 31

Gregorian years are identified by consecutive year numbers. [6] 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. [Note 2] [Note 3] Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is 365 + 97 / 400 days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. [Note 4]

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, [3] 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 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). [Note 5]

Background

Because the date of Easter is a function – the computus – of the date of the (northern hemisphere) spring equinox, the Catholic Church 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 on or after 21 March, which was adopted as an approximation to the March equinox. [8] 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, 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 to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome. [9] 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, [10] 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 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 Compendium 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. [11] The reform adopted was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio). [12]

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. [13] 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. [14] 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 sexagesimals as the excess over 365 days (the way they would have been extracted from the tables of mean longitude) were 014,33,9,57 (Alfonsine), 014,33,11,12 (Copernicus) and 014,33,9,24 (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 (014,33, 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. [15]

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. [16] A commonly used value in Lilius's time, from the Alfonsine tables, is 365.2425463 days. [12] 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. [8] 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. [17]

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).

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 Lunario Novo secondo la nuova riforma [a] 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. [18]

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 and the Papal States. 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 Inter gravissimas became the law of the Catholic Church in 1582, but it was not recognised by 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. [19] 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 [ citation needed ] (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 1582. The Spanish and Portuguese colonies followed somewhat later de facto because of delay in communication. [20]

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. [21]

Britain 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 Hegira 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 Hegira 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.

Year Country/-ies/Areas
1582 Spain, Portugal, France, Poland, Italy, Catholic Low Countries, Luxemburg, and colonies
1584 Kingdom of Bohemia, some catholic Swiss cantons [Note 6]
1610 Prussia
1648 Alsace
1682 Strasbourg
1700 'Germany', [Note 7] Protestant Low Countries, Norway, Denmark, some protestant Swiss cantons [Note 6]
1752 Great Britain, Ireland, and colonies
1753 Sweden and Finland
1873 Japan
1875 Egypt
1896 Korea
1912 China, Albania
1915 Latvia, Lithuania
1916 Bulgaria
1917 Ottoman Empire
1918 Russia, Estonia
1919 Romania, Yugoslavia [Note 8]
1923 Greece
1926 Turkey (common era years Gregorian dates in use since 1917 Ottoman adoption)
2016 Saudi Arabia

Difference between Gregorian and Julian calendar dates

Conversion from Julian to Gregorian dates. [22]
Gregorian range Julian range Difference
From 15 October 1582
to 28 February 1700
From 5 October 1582
to 18 February 1700
10 days
From 1 March 1700
to 28 February 1800
From 19 February 1700
to 17 February 1800
11 days
From 1 March 1800
to 28 February 1900
From 18 February 1800
to 16 February 1900
12 days
From 1 March 1900
to 28 February 2100
From 17 February 1900
to 15 February 2100
13 days
From 1 March 2100
to 28 February 2200
From 16 February 2100
to 14 February 2200
14 days

This section always places the intercalary day on 29 February even though it was always obtained by doubling 24 February (the bissextum (twice sixth) or bissextile day) until the late Middle Ages. 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. [23]

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 from, 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 into. 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. [24]

Beginning of the year

Country Start numbered year
on 1 January
Adoption of
Gregorian calendar
Roman Empire 153 BC
Denmark Gradual change from
13th to 16th centuries [25]
1700
Papal States 1583 1582
Holy Roman Empire (Catholic states) 1544 1583
Spain, Poland, Portugal 1556 1582
Holy Roman Empire (Protestant states) 1559 1700 [Note 7]
Sweden 1559 1753
France 1564 [27] 1582 [n 1]
Southern Netherlands 1576 [28] 1582
Lorraine 1579 1582 [Note 9]
Dutch Republic 1583 1582
Scotland 1600 [29] [30] 1752
Russia 1700 [31] 1918
Tuscany 1750 [32] 1582 [33]
Great Britain and the British Empire
except Scotland
1752 [29] 1752
Republic of Venice 1522 1582

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 AUC 532 (222 BC), 15 March from AUC 532 (222 BC) and 1 January from AUC 601 (153 BC). [34] The Julian calendar, which began in AUC 709 (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 (supposed Nativity of Jesus), 25 March (Annunciation), or Easter (France), [35] 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. [36]

In common usage, 1 January was regarded as New Year's Day and celebrated as such, [37] but from the 12th century until 1751 the legal year in England began on 25 March (Lady Day). [38] 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), [39] although later histories adjust the start of the year to 1 January and record the execution as occurring in 1649. [40]

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. [41]

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, [ citation needed ] and another for any full year that begins on 1 January. [ citation needed ] 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 Supputatio Romana specified it relative to 1 January.

  1. ^ In 1793 France abandoned the Gregorian calendar in favour of the French Republican Calendar. This change was reverted in 1805.

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. [42]

Old Style and New Style dates

"Old Style" (OS) and "New Style" (NS) are sometimes added to dates to identify which calendar reference system is used for the date given. In Britain and its colonies, where the Calendar Act of 1750 altered the start of the year, [Note 10] and also aligned the British calendar with the Gregorian calendar, there is some confusion as to what these terms mean. They can indicate that the start of the Julian year has been adjusted to start on 1 January (NS) even though contemporary documents use a different start of year (OS) or to indicate that a date conforms to the Julian calendar (OS), formerly in use in many countries, rather than the Gregorian calendar (NS). [40] [43]

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 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 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, the traditional proleptic Gregorian calendar (like the Julian calendar) does not have a year 0 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 −0001 , 0000, 0001, and 0002.

Months

The Gregorian calendar continued to employ the Julian months, which have Latinate names and irregular numbers of days:

    (31 days), from Latin mēnsis Iānuārius, "Month of Janus", [44] the Roman god of gates, doorways, beginnings and endings (28 days in common and 29 in leap years), from Latin mēnsis Februārius, "Month of the Februa", the Roman festival of purgation and purification, [45][46]cognate with fever, [45] the Etruscandeath godFebruus ("Purifier"), [citation needed] and the Proto-Indo-European word for sulfur[45] (31 days), from Latin mēnsis Mārtius, "Month of Mars", [47] the Roman war god[46] (30 days), from Latin mēnsis Aprīlis, of uncertain meaning [48] but usually derived from some form of the verb aperire ("to open") [49] or the name of the goddess Aphrodite[46][52] (31 days), from Latin mēnsis Māius, "Month of Maia", [53] a Roman vegetation goddess[46] whose name is cognate with Latin magnus ("great") [53] and English major (30 days), from Latin mēnsis Iūnius, "Month of Juno", [54] the Roman goddess of marriage, childbirth, and rule [46] (31 days), from Latin mēnsis Iūlius, "Month of Julius Caesar", the month of Caesar's birth, instituted in 44 BC [55] as part of his calendrical reforms[46] (31 days), from Latin mēnsis Augustus, "Month of Augustus", instituted by Augustus in 8 BC in agreement with July and from the occurrence during the month of several important events during his rise to power [56] (30 days), from Latin mēnsis september, "seventh month", of the ten-month Roman year of Romulusc. 750 BC [57] (31 days), from Latin mēnsis octōber, "eighth month", of the ten-month Roman year of Romulus c. 750 BC [58] (30 days), from Latin mēnsis november, "ninth month", of the ten-month Roman year of Romulus c. 750 BC [59] (31 days), from Latin mēnsis december, "tenth month", of the ten-month Roman year of Romulus c. 750 BC [60]

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, [61] Italian, [62] and French, [63] 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 c. 1425 : [64] [65]

Thirti dayes hath novembir
April june and Septembir.
Of xxviij is but oon
And alle the remenaunt xxx and j. [64]

Thirty days have November,
April, June, and September.
Of 28 is but one
And all the remnant 30 and 1.

Variations appeared in Mother Goose 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" [66] but it has also been called "probably the only sixteenth-century poem most ordinary citizens know by heart". [67] A common nonverbal alternative is the knuckle mnemonic, 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 semitones 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, 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. [68] This approximation has an error of about one day per 3,030 years [69] with respect to the current value of the mean tropical year. However, because of the precession of the equinoxes, which is not constant, and the movement of the perihelion (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 [70] 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. [71] 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. [72]

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 seasonal event.


3. Pope Gregory didn’t design the Gregorian calendar.

Although Pope Gregory authorized this new calendar, and it’s named after him, he didn’t come up with it on his own. As pointed in a Vox article, he “appointed a commission, led by physician Aloysius Lilius and astronomer Christopher Clavius, to solve the problem.” After five years, fixed the problem.

“First, let’s eliminate those extra ten days and get back on schedule. Okay, those ten days are gone. Next, let’s tweak the system of leap years. We’ll have leap years every four years except on centennial years that aren’t divisible by 400.” That explains why there was a leap year in 2000, but not in 1900, 1800, or 1700. And, it also squashes that misconception that leap year take placed every four years.

Furthermore, “This changed the length of the average year to 365.2425 days. While not spot-on, it’s close enough and more accurate than the Julian calendar.


Gregorian Calendar Reform

Pope Gregory XIII's major changes were algorithms to calculate movable feasts and a new system of leap years that got rid of leap years in years that are divisible by 100 but not 400. Pope Gregory also deleted ten days from the 1592 calendar year in order to accommodate a shift in the equinox.

A variety of calendars culminate around the year 2000. Calendar Convergence shows the common end of calendar cycles from the Hopi, the Ancient Greeks, the Early Egyptian Christians, the Mayan, and the Indian Vedic tradition. Planets Alignments in 2000 shows an alignment of the seven planets on May 5, 2000.

From Calendar Convergence (6) and Planets Alignments (7)

U. Glessmer. "The Otot-Texts (4Q319) and the Problem of Intercalations in the Context of the 364-Day Calendar" in:
Qumranstudien: Vortraege und Beitraege der Teilnehmer des Qumranseminars auf dem internationalen Treffen der Society of Biblical Lit., Muenster, 25-26. Juli 1993 [Hans-Peter Mueller zum 60. Geburtstag]. Schriften des Institutum Judaicum Delitzschianum Bd. 4. Ed. H.J. Fabry et al. Goettingen 1996, 125-164.


Contents

In the Kingdom of Great Britain and its possessions, the Calendar (New Style) Act 1750 introduced two concurrent changes to the calendar. The first, which applied to England, Wales, Ireland and the British colonies, changed the start of the year from 25 March to 1 January with effect from 1 January 1752: [5] Scotland had already made this aspect of the changes, on 1 January 1600. [6] [7] The second (in effect [a] ) adopted the Gregorian calendar in place of the Julian calendar. Thus "New Style" can either refer to the start of year adjustment, or to the adoption of the Gregorian calendar.

Start of year adjustment Edit

When recording British history, it is usual to quote the date as originally recorded at the time of the event, but with the year number adjusted to start on 1 January. [8] The latter adjustment may be needed because the start of the civil calendar year was not always 1 January and was altered at different times in different countries. [b] From 1155 to 1752, the civil or legal year in England began on 25 March (Lady Day) [9] [10] so for example, the execution of Charles I was recorded at the time in Parliament as happening on 30 January 1648 (Old Style). [11] In newer English language texts this date is usually shown as "30 January 1649" (New Style). [12] The corresponding date in the Gregorian calendar is 9 February 1649, the date by which his contemporaries in some parts of continental Europe would have recorded his execution.

The O.S./N.S. designation is particularly relevant for dates which fall between the start of the "historical year" (1 January) and the legal start date, where different. This was 25 March in England, Wales, Ireland and the colonies until 1752 and until 1600 in Scotland.

During the years between the first introduction of the Gregorian calendar in continental Europe and its introduction in Britain, contemporary usage in England started to change. [13] In Britain, 1 January was celebrated as the New Year festival, [14] but the "year starting 25th March was called the Civil or Legal Year, although the phrase Old Style was more commonly used." [13] To reduce misunderstandings about the date, it was normal in parish registers to place a new year heading after 24 March (for example "1661") and another heading from the end of the following December, 1661/62, a form of dual dating to indicate that in the following twelve weeks or so, the year was 1661 Old Style but 1662 New Style. [15] Some more modern sources, often more academic ones (e.g. the History of Parliament) also use the 1661/62 style for the period between 1 January and 24 March for years before the introduction of the New Style calendar in England. [16]

Adoption of the Gregorian calendar Edit

Through the enactment of the Calendar (New Style) Act 1750, the Kingdom of Great Britain, the Kingdom of Ireland and the British Empire (including much of what is now the eastern part of the United States and Canada) adopted the Gregorian calendar in 1752, by which time it was necessary to correct by 11 days. Wednesday, 2 September 1752, was followed by Thursday, 14 September 1752. Claims that rioters demanded "Give us our eleven days" grew out of a misinterpretation of a painting by William Hogarth. [2]

Beginning in 1582, the Gregorian calendar replaced the Julian in Roman Catholic countries. This change was implemented subsequently in Protestant and Orthodox countries, usually at much later dates. When Old Style and New Style notation is encountered, the British adoption date is not necessarily intended. The 'start of year' change and the calendar system change were not always adopted concurrently. Similarly, civil and religious adoption may not have happened at the same time or even at all. In the case of Eastern Europe, for example, all of those assumptions would be incorrect.

Russia Edit

In Russia, new style dates came into use in early 1918, when 31 January 1918 was followed by 14 February 1918: there is a 13-day difference between Old Style and New Style dates since 1 March 1900. [17]

It is common in English-language publications to use the familiar Old Style and/or New Style terms to discuss events and personalities in other countries, especially with reference to the Russian Empire and the very beginning of Soviet Russia. For example, in the article "The October (November) Revolution," the Encyclopædia Britannica uses the format of "25 October (7 November, New Style)" to describe the date of the start of the revolution. [18]

Greece Edit

Other countries in Eastern Orthodoxy eventually adopted Gregorian (or new style) dating for their civil calendars but most of these continue to use the Julian calendar for religious purposes. Greece was the last to do so, in 1923. [19] Here too, there is a 13-day difference between Old Style and New Style dates in modern Greek history.

The Americas Edit

The European colonies of the Americas adopted the new style calendar when their mother countries did. In what is now the continental United States, the French and Spanish possessions did so about 130 years earlier than the British colonies. In practice, however, most surviving written records of what is now the United States are from Britain's Thirteen Colonies, where the British Calendar Act of 1751 was applied fourteen years before the United States declared independence. Canadian records may reflect both traditions but the language used in the record is likely to be a good indicator of which calendar was being used for the dates given. The same logic applies to the Caribbean islands.

In Alaska, the change took place after the United States purchased Alaska from Russia. Friday, 6 October 1867 was followed by Friday, 18 October. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because at the same time the International Date Line was moved, from following Alaska's eastern border with Canada to following its new western border, now with Russia. [20]

Usually, the mapping of New Style dates onto Old Style dates with a start of year adjustment works well with little confusion for events before the introduction of the Gregorian calendar. For example, the Battle of Agincourt is well known to have been fought on 25 October 1415, which is Saint Crispin's Day. However, 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. Events in Continental Western Europe are usually reported in English-language histories by using the Gregorian calendar. For example, the Battle of Blenheim is always given as 13 August 1704. However, confusion occurs when an event involves both. For example, William III of England arrived at Brixham in England on 5 November (Julian calendar), after he had set sail from the Netherlands on 11 November (Gregorian calendar) 1688. [21]

The Battle of the Boyne in Ireland took place a few months later on 1 July 1690 (Julian calendar). That maps to 11 July (Gregorian calendar), conveniently close to the Julian date of the subsequent (and more decisive) Battle of Aughrim on 12 July 1691 (Julian). The latter battle was commemorated annually throughout the 18th century on 12 July, [22] following the usual historical convention of commemorating events of that period within Great Britain and Ireland by mapping the Julian date directly onto the modern Gregorian calendar date (as happens, for example, with Guy Fawkes Night on 5 November). The Battle of the Boyne was commemorated with smaller parades on 1 July. However, both events were combined in the late 18th century, [22] and continue to be celebrated as "The Twelfth".

Because of the differences, British writers and their correspondents often employed two dates, which is called dual dating, more or less automatically. Letters concerning diplomacy and international trade thus sometimes bore both Julian and Gregorian dates to prevent confusion. For example, Sir William Boswell wrote to Sir John Coke from The Hague a letter dated "12/22 Dec. 1635". [21] In his biography of John Dee, The Queen's Conjurer, Benjamin Woolley surmises that because Dee fought unsuccessfully for England to embrace the 1583/84 date set for the change, "England remained outside the Gregorian system for a further 170 years, communications during that period customarily carrying two dates". [23] In contrast, Thomas Jefferson, who lived while the British Isles and colonies eventually converted to the Gregorian calendar, instructed that his tombstone bear his date of birth by using the Julian calendar (notated O.S. for Old Style) and his date of death by using the Gregorian calendar. [24] At Jefferson's birth, the difference was eleven days between the Julian and Gregorian calendars and so his birthday of 2 April in the Julian calendar is 13 April in the Gregorian calendar. Similarly, George Washington is now officially reported as having been born on 22 February 1732, rather than on 11 February 1731/32 (Julian calendar). [25]

There is some evidence that the calendar change was not easily accepted. Many British people continued to celebrate their holidays "Old Style" well into the 19th century, [c] a practice that the author Karen Bellenir considered to reveal a deep emotional resistance to calendar reform. [26]

The change arose from the realisation that the correct figure for the number of days in a year is not 365.25 (365 days 6 hours) as assumed by the Julian calendar but slightly less (c. 365.242 days): the Julian calendar has too many leap years. The consequence was that the basis for the calculation of the date of Easter, as decided in the 4th century, had drifted from reality. The Gregorian calendar reform also dealt with the accumulated difference between these figures, between the years 325 and 1582 by skipping 10 days to set the ecclesiastical date of the equinox to be 21 March, the median date of its occurrence at the time of the First Council of Nicea in 325.

Countries that adopted the Gregorian calendar after 1699 needed to skip the additional day for each subsequent new century that the Julian calendar had added since then. When the British Empire did so in 1752, the gap had grown to eleven days [d] when Russia did so (as its civil calendar) in 1918, thirteen days needed to be skipped.


Contents

The Gregorian calendar, like the Julian calendar, 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 occurred every 4 years, and the leap day was historically inserted by doubling 24 February. However, it is now customary to number the days of February sequentially with no gaps, and 29 February is typically considered 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. [5]

A year is divided into twelve months
No. Name Length in days
1 January 31
2 February 28 (29 in leap years)
3 March 31
4 April 30
5 May 31
6 June 30
7 July 31
8 August 31
9 September 30
10 October 31
11 November 30
12 December 31

Gregorian years are identified by consecutive year numbers. [6] 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. [Note 2] [Note 3] Of these 400 years, 303 are regular years of 365 days and 97 are leap years of 366 days. A mean calendar year is 365 + 97 / 400 days = 365.2425 days, or 365 days, 5 hours, 49 minutes and 12 seconds. [Note 4]

Christopher Clavius (1538–1612), one of the main authors of the reform

Pope Gregory XIII, portrait by Lavinia Fontana, 16C.

First page of the papal bull Inter gravissimas

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, [3] 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 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). [Note 5]

Background

Because the date of Easter is a function – the computus – of the date of the (northern hemisphere) spring equinox, the Catholic Church 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 on or after 21 March, which was adopted as an approximation to the March equinox. [8] 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, 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 to the Vatican for this purpose. However, the project was interrupted by the death of Regiomontanus shortly after his arrival in Rome. [9] 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, [10] 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 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 Compendium 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. [11] The reform adopted was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio). [12]

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. [13] 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. [14] 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 sexagesimals as the excess over 365 days (the way they would have been extracted from the tables of mean longitude) were 014,33,9,57 (Alfonsine), 014,33,11,12 (Copernicus) and 014,33,9,24 (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 (014,33, 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. [15]

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. [16] A commonly used value in Lilius's time, from the Alfonsine tables, is 365.2425463 days. [12] 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. [8] 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. [17]

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 Lunario Novo secondo la nuova riforma [a] 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. [18]

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 and the Papal States. 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 Inter gravissimas became the law of the Catholic Church in 1582, but it was not recognised by 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. [19] 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 [ citation needed ] (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 1582. The Spanish and Portuguese colonies followed somewhat later de facto because of delay in communication. [20]

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. [21]

Britain 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 Hegira 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 Hegira 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

Year Country/-ies/Areas
1582 Spain, Portugal, France, Poland, Italy, Catholic Low Countries, Luxemburg, and colonies
1584 Kingdom of Bohemia, some catholic Swiss cantons [Note 6]
1610 Prussia
1648 Alsace
1682 Strasbourg
1700 'Germany', [Note 7] Protestant Low Countries, Norway, Denmark, some protestant Swiss cantons [Note 6]
1752 Great Britain, Ireland, and colonies
1753 Sweden and Finland
1873 Japan
1875 Egypt
1896 Korea
1912 China, Albania
1915 Latvia, Lithuania
1916 Bulgaria
1917 Ottoman Empire
1918 Russia, Estonia
1919 Romania, Yugoslavia [Note 8]
1923 Greece
1926 Turkey (common era years Gregorian dates in use since 1917 Ottoman adoption)
2016 Saudi Arabia
Conversion from Julian to Gregorian dates. [22]
Gregorian range Julian range Difference
From 15 October 1582
to 28 February 1700
From 5 October 1582
to 18 February 1700
10 days
From 1 March 1700
to 28 February 1800
From 19 February 1700
to 17 February 1800
11 days
From 1 March 1800
to 28 February 1900
From 18 February 1800
to 16 February 1900
12 days
From 1 March 1900
to 28 February 2100
From 17 February 1900
to 15 February 2100
13 days
From 1 March 2100
to 28 February 2200
From 16 February 2100
to 14 February 2200
14 days

This section always places the intercalary day on 29 February even though it was always obtained by doubling 24 February (the bissextum (twice sixth) or bissextile day) until the late Middle Ages. 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. [23]

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 from, 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 into. 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. [24]

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 AUC 532 (222 BC), 15 March from AUC 532 (222 BC) and 1 January from AUC 601 (153 BC). [34] The Julian calendar, which began in AUC 709 (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 (supposed Nativity of Jesus), 25 March (Annunciation), or Easter (France), [35] 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. [36]

In common usage, 1 January was regarded as New Year's Day and celebrated as such, [37] but from the 12th century until 1751 the legal year in England began on 25 March (Lady Day). [38] 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), [39] although later histories adjust the start of the year to 1 January and record the execution as occurring in 1649. [40]

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. [41]

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, [ citation needed ] and another for any full year that begins on 1 January. [ citation needed ] 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 Supputatio Romana specified it relative to 1 January.


Ancient calendar’s New Year’s Day lets us have a fresh start today | Commentary

Happy New Year! Wait, isn’t it a little late for that? Yes — under our current Gregorian calendar. For hundreds of years, however, March 25, not Jan. 1, was the first day of the year in Europe and its colonies. It’s a tradition worth reviving, if only informally, because it calls to mind what a new year represents.

In the West, Jan. 1 has been a common but by no means universal day to start the year. The ancient Romans used a 10-month calendar that started March 1 and ended with December, from the Latin “decem,” meaning 10th. Sometime in the period of the Republic (509 to 27 B.C.)­, January and February joined the calendar, and the first day of the year shifted to Jan. 1. Julius Caesar codified the new year’s date in his 45 B.C. reforms that produced the Julian calendar.

Still, local variations abounded as Rome became an empire. In the Eastern Mediterranean, for example, Sept. 23, the birthday of Emperor Augustus, began the year.

Christians, growing more numerous in late antiquity, wanted a calendar that reflected their own cosmology. Popular dates for Christian new years included Easter, though it moved from year to year, and Christmas, which at least stayed fixed.

The most widely used date, however, was March 25, the Feast of the Annunciation, the day celebrating when the Virgin Mary was visited by the Archangel Gabriel and accepted God’s plan for her to bear Jesus. The symbolism was clear: time should be reckoned by when the savior became man, not by the birth of a worldly emperor or by pagan tradition.

The practice of starting a year on March 25 still varied, though. Some places in Spain, Portugal, and southern France kept the Roman system and calculated their years from 38 B.C., starting each year in January. An attempt at a definitive reform came in 1582, when Pope Gregory XIII addressed the Julian calendar’s inability to set the correct date for Easter. As a secondary measure, the Gregorian calendar restored New Year’s Day to Jan. 1.

But Protestant countries, dismissing Jan. 1 as mere popery, resisted at first, though they gradually fell in line, some more gradually than others. Britain and its colonies gave in only when Dec. 31, 1751, was followed by Jan. 1, 1752.

Returning March 25 to the head of the calendar has obvious benefits for Christians, especially those Christians who honor Mary’s role in salvation history. It reminds them that Jesus became man at a specific moment in time because of Mary’s “yes.” Their lives, like the year itself, should begin with a similar “yes” to God.

But a March 25 new year has more than sectarian appeal. It reminds everyone that calendars are part science and part cultural constructions. Back in 2018, the physicist Neil deGrasse Tyson tweeted that “New Years Day on the Gregorian Calendar is a cosmically arbitrary event, carrying no Astronomical significance at all.” His trolling had a point: it’s culture that gives particular dates their meaning.

Different cultures figure their time differently, even today. According to the Jewish calendar, the year is currently 5781, figured from the date of creation. Under the Muslim calendar it is 1442, counting from when the Prophet Muhammad traveled to Medina. Many Asian countries, meanwhile, celebrate the lunar new year. In China, Feb. 12, 2021, ushered in the year 4718.


The more advanced leap year formula makes the Gregorian calendar far more accurate than the Julian. However, it is not perfect either. Compared to the tropical year, it is off by one day every 3236 years.

Although the Gregorian calendar is named after Pope Gregory XIII, it is an adaptation of a calendar designed by Luigi Lilio (also known as Aloysius Lilius), who was an Italian doctor, astronomer, and philosopher. He was born around 1510 and died in 1576, six years before his calendar was officially introduced.


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