Gregorian calendar

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Pope Gregory XIII
The Gregorian calendar is the most widely used calendar in the world today. It is a reform of the Julian calendar, first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 by papal bull Inter gravissimas.

Years in the reformed calendar continue the numbering system of the Julian calendar, which are numbered from the traditional Incarnation year of Jesus, which has been labeled the "anno Domini" (AD) era, and is sometimes labeled the "common era" (CE), otherwise known as "Christian Era".

The changes made by Gregory also corrected the drift in the civil calendar which arose because the mean Julian calendar year was slightly too long, causing the vernal equinox, and consequently the date on which Easter was being celebrated, to slowly drift forward in relation to the civil calendar and the seasons.

The Gregorian calendar system dealt with these problems by dropping 10 days to bring the calendar back into synchronization with the seasons, and adopting the following leap year rule:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1900 is not a leap year; the year 2000 is a leap year.

In the Julian calendar, all years exactly divisible by 4 are leap years.

Contents

Description

The Gregorian solar calendar is an arithmetical calendar. It counts days as the basic unit of time, grouping them into years of 365 or 366 days. The solar calendar repeats completely every 146,097 days, which fill 400 years, and which also happens to be 20,871 seven-day weeks. Of these 400 years, 303 (the "common years") have 365 days, and 97 - the leap years - have 366 days. This gives an average year length of exactly 365.2425 days - or 365 days, 5 hours, 49 minutes and 12 seconds.

A Gregorian year is divided into twelve months of irregular length (but note that there is a period of 153 days divided over 5 months in an alternating pattern from March to July that repeats from August to December):

No. Name Days
1 January 31
2 February 28 or 29
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

A calendar date is fully specified by the year (numbered by some scheme beyond the scope of the calendar itself), the month (identified by name or number), and the day of the month (numbered sequentially starting at 1).

Leap years are all years divisible by 4, with the exception of those divisible by 100, but not by 400. These 366-day years add a 29th day to February, which normally has 28 days. Thus, the essential ongoing differential feature of the Gregorian calendar, as opposed to the Julian calendar, is that the Gregorian omits 3 leap days every 400 years. This difference would have been more noticeable in modern memory were it not for the fact that the year 2000 was a leap year in both the Julian and Gregorian calendar systems.

The intercalary day in a leap year is known as a leap day. Since Roman times 24 February (bissextile) was counted as the leap day, but nowadays 29 February is regarded as the leap day in most countries.

Although the calendar year runs from 1 January to 31 December, sometimes year numbers were based on a different starting point within the calendar. Confusingly, the term "Anno Domini" is not specific on this point, and actually refers to a family of year numbering systems with different starting points for the years. (See the section below for more on this issue.)

Adoption

William Hogarth (c. 1755) painting which is the main source for "Give us our Eleven Days"
William Hogarth (c. 1755) painting which is the main source for "Give us our Eleven Days"

Though Gregory's reform was "enacted" in the most solemn of forms available to the Church, in fact the bull had no authority beyond the Catholic Church and the Papal States. The changes which he was "proposing" were changes to the civil calendar over which he had no authority. The changes required "adoption" by the civil authorities in each country to have legal effect.

The Nicene Council of 325 sought to devise rules whereby all Christians would celebrate Easter on the same day. In fact it took a very long time before Christians achieved that objective for the issues which arose). However, the bull Inter gravissimas became the law of the Catholic Church. It was not recognised, however, by Protestant Churches nor by Orthodox Churches and others. Consequently, the days on which Easter and related holidays were celebrated by different Christian Churches again diverged.

Adoption in Europe

Only four (Catholic) countries adopted the new calendar on the date specified by the bull. Other Catholic countries experienced some delay before adopting the reform; and non-Catholic countries, not being subject to the decrees of the Pope, initially rejected or simply ignored the reform altogether, although they all eventually adopted it. Hence, the dates 5 October 1582 to 14 October 1582 (inclusive) are still valid dates in many countries.

Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy implemented the new calendar on Friday, 15 October 1582, following Julian Thursday, 4 October 1582. The Spanish and Portuguese colonies adopted the calendar later due to the slowness of communication in those days. France adopted the new calendar on Monday, 20 December 1582, following Sunday, 9 December 1582. The Protestant Dutch provinces of Holland and Zeeland also adopted it in December of that year.

Most non-Catholic countries initially objected to adopting a Catholic invention, especially during the Counter-Reformation (of which Gregory was a leading proponent); some Protestants feared the new calendar was part of a plot to return them to the Catholic fold. In the Czech lands, Protestants resisted the calendar imposed by the Hapsburg Monarchy. In parts of Ireland, Catholic rebels until their defeat in the Nine Years' War kept the "new" Easter in defiance of the English-loyal authorities; later, Catholics practising in secret petitioned the Propaganda Fide for dispensation from observing the new calendar, as it signalled their disloyalty.

Denmark, [Norway and the Protestant states of Germany adopted the solar portion of the new calendar on Monday, 1 March 1700, following Sunday, 18 February 1700, due to the influence of Ole Rømer, but did not adopt the lunar portion. Instead, they decided to calculate the date of Easter astronomically using the instant of the vernal equinox and the full moon according to Kepler's Rudolphine Tables of 1627. They finally adopted the lunar portion of the Gregorian calendar in 1776. The remaining provinces of the Dutch Republic] also adopted the Gregorian calendar in 1700.

Sweden's relationship with the Gregorian Calendar was a difficult one. Sweden started to make the change from the Julian calendar and towards the Gregorian calendar in 1700, but it was decided to make the (then 11 day) adjustment gradually, by excluding the leap days (29 February) from each of 11 successive leap years, 1700 to 1740. In the meantime, not only would the Swedish calendar be out of step with both the Julian calendar and the Gregorian calendar for 40 years, but the difference would not be constant but would change every 4 years. This strange system clearly had great potential for endless confusion when working out the dates of Swedish events in this 40 year period. To make matters worse, the system was poorly administered and the leap days that should have been excluded from 1704 and 1708 were not excluded. The Swedish calendar (according to the transition plan) should have been 8 days behind the Gregorian, but was still in fact 10 days behind. King Charles XII wisely recognised that the gradual change to the new system was not working and abandoned it.

However, rather than proceeding directly to the Gregorian calendar, it was decided to revert to the Julian calendar. This was achieved by introducing the unique date 30 February in the year 1712, adjusting the discrepancy in the calendars from 10 back to 11 days. Sweden finally adopted the Gregorian calendar in 1753, when Wednesday, 17 February was followed by Thursday, 1 March. Since Finland was under Swedish rule at that time, it did the same.

Britain and the British Empire (including the eastern part of what is now the United States) 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 to account for 29 February 1700 (Julian). After 1753, the British tax year in Britain continued to operate on the Julian calendar and began on 5 April, which was the "Old Style" new tax year of 25 March. A 12th skipped Julian leap day in 1800 changed its start to 6 April. It was not changed when a 13th Julian leap day was skipped in 1900, so the tax year in the United Kingdom still begins on 6 April.

In Alaska, the change took place when Friday, 6 October, 1867 was followed again by Friday, 18 October after the US purchase of Alaska from Russia, which was still on the Julian calendar. Instead of 12 days, only 11 were skipped, and the day of the week was repeated on successive days, because the International Date Line was shifted from Alaska's eastern to western boundary along with the change to the Gregorian calendar.

In Russia the Gregorian calendar was accepted after the October Revolution (so named because it took place in October 1917 in the Julian calendar). On 24 January 1918 the Council of People's Commissars issued a Decree that Wednesday, 31 January]] 1918 was to be followed by Thursday, 14 February 1918.

The last country of Eastern Orthodox Europe to adopt the Gregorian calendar was Greece on Thursday, 1 March 1923, following Wednesday, 15 February 1923.

Adoption in Eastern Asia

The Republic of China (ROC) formally adopted the Gregorian calendar at its founding on 1 January 1912, but China soon descended into a period of warlordism with different warlords using different calendars. With the unification of China under the Kuomintang in October 1928, the Nationalist Government decreed that effective 1 January 1929 the Gregorian calendar would be used. However, China retained the Chinese traditions of numbering the months and a modified Era System, backdating the first year of the ROC to 1912; this system is still in use in Taiwan where this ROC government retains control. Upon its foundation in 1949, the People's Republic of China continued to use the Gregorian calendar with numbered months, but abolished the ROC Era System and adopted Western numbered years.

Japan replaced the traditional lunisolar calendar with the Gregorian calendar on 1 January 1873, but, like China, continued to number the months, and used reign names instead of the Common Era: Meiji 1=1868, Taisho 1=1912, Showa 1=1926, Heisei 1=1989, and so on. The "Western calendar" (西暦, seireki) using western year numbers, is also widely accepted by civilians and to a lesser extent by government agencies.

Korea started using the Gregorian calendar on 1 January 1896 due to Japanese influence. The lunisolar Korean calendar used immediately before that day was based on the lunisolar Chinese calendar.

Adoption by Orthodox Churches

Despite all the civil adoptions, none of the national Orthodox Churches recognized it. Instead, a Revised Julian calendar was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule. There will be no difference between the two calendars until 2800. The Orthodox churches of Constantinople, Alexandria, Antioch, Greece, Cyprus, Romania, and Bulgaria adopted the Revised Julian calendar, so these New calendarists would celebrate the Nativity along with the Western churches on 25 December in the Gregorian calendar until 2800.

The Orthodox churches of Jerusalem, Russia, Serbia, Georgia, Poland and the Greek Old Calendarists did not accept the Revised Julian calendar. All these Old Calendarists continue to celebrate the Nativity on 25 December in the Julian calendar, which is 7 January in the Gregorian calendar until 2100.

All of the other Eastern churches, the Oriental Orthodox churches (Coptic, Ethiopian, Eritrean, Syrian, Armenian) and the Assyrian Church, continue to use their own calendars, which usually result in fixed dates being celebrated in accordance with the Julian calendar.

All Eastern churches continue to use the Julian Easter with the sole exception of the Finnish Orthodox Church, which has adopted the Gregorian Easter.

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" (OS) and "New Style" (NS) are sometimes added to dates to identify which system is used in the British Empire and other countries that did not immediately change. Because the Calendar Act of 1750 altered the start of the year, 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).

Use of dates from historical documents in present-day documents

There was a great deal of confusion when the calendar changed, and the confusion continues today. Other months were 'affected' by the change, for instance, although the calendar 'moved' March to January, and every person automatically became 11 days older by fiat, the change actually often occurred at another time of year, for instance, in September of 1752 in the British Empire, including the American colonies. Consequently, the actual date of the change over created confusion regarding the months other than January through March, and can be seen in quite a few months, but not in the American Colonies in December and August, for reasons that are not yet known. In some cases, historians did not differentiate between the years - leading some researchers to guess between two years when interpreting the information.

When 'translating' dates from historical documents to current documents for dates that have been incorrectly double dated by historians, both years should be entered into present-day documents until a copy of the original primary source verifying which year was written in the 'official record'. Often errors have been perpetuated from the early 19th century and still exist today. When 'translating' dates from historical documents to current documents for dates that have been correctly double dated by historians, the standard practice is to enter the earlier year first, and the later year second.

See also

External Links


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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Gregorian calendar".

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