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Gregorian calendar
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Gregorian calendar

The Gregorian calendar is the calendar currently used in the Western world. A modification of the Julian calendar, it was first proposed by Neapolitan doctor Aloysius Lilius, and promulgated by the eponymous Pope Gregory XIII on February 24, 1582 (Note: The document was dated 1581 as the year was at that time taken to commence in March).

The Gregorian calendar was devised because the mean year in the Julian Calendar was a little too long, causing the Vernal equinox to slowly drift earlier in the calendar year.

Table of contents
1 History
2 Proleptic Gregorian calendar
3 Months of the year
4 Accuracy
5 Calendar seasonal error
6 Numerical facts
7 External links



The motivation of the Catholic Church in adjusting the calendar was to have Easter celebrated at the time that had been agreed at the First Council of Nicaea in 325, ie. on the Sunday after the 14th day of the Moon that falls on or after the vernal equinox—which fell approximately on March 21 at that time. By the time of this council, the drift of the equinox since the introduction of the Julian calendar had already been noticed. Instead of modifying the calendar, the equinox was standardized at March 21 instead of the original March 24 or March 25. However by the 16th century, the equinox had drifted noticeably further.

Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by a 19-year cycle. However, that is an approximation that built up an error of 1 day every 310 years. So by the 16th century the lunar calendar was way out of phase with the real Moon too.

The fix for the equinox was to define that years divisible by 100 will be leap years only if they are divisible by 400 as well. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not.

When the new calendar was put in use, to correct the error already accumulated in the thirteen centuries since the council of Nicaea, a deletion of ten days was made. The last day of the Julian calendar was October 4, 1582 and this was followed by the first day of the Gregorian calendar October 15, 1582. The dates "5 October 1582" to "14 October 1582" (inclusive) exist only in the Proleptic Gregorian calendar, which is confined to special scientific contexts and has no relevance for dating ordinary historical events. This created some consternation, and the church was accused of stealing ten days of people's lives.

From 1 January 1622, the first day of the year was standardised as January 1. This was already the system used in Italy, Germany, and other places, but not universally (England, for example, began the year on March 25).

The terms Old Style or O.S. and New Style or N.S. are sometimes added to dates to identify which system is used. It is also sometimes necessary to indicate that the year itself had two different designations because of the change to the beginning of the year, for example, February 10, 1751/2.

The 19-year cycle used for the lunar calendar was also to be corrected by 1 day every 300 or 400 years (8 times in 2500 years) along with corrections for the years (1700, 1800, 1900, 2100 etc.) that are no longer leap years. In fact, a new method for computing the date of Easter was introduced.

Adoption outside of Roman Catholic nations

Very few countries accepted the new calendar immediately. Non-Catholic countries objected to adopting a Catholic invention. England, Scotland and thereby the rest of the British Empire (including part of what is now the United States) did not adopt it until 1752, by which time it was necessary to correct by eleven days (September 2, 1752 being followed by September 14, 1752). Again, people objected to the change—not because they literally thought days were being stolen from their lives, but because they were paid only for days actually worked, but were required to pay a full month's rent for the shortened September, which caused hardship.

Denmark-Norway and the Protestant parts of Germany adopted the new calendar in 1700, due to the influence of Ole RÝmer.

Sweden started to introduce the Gregorian Calendar in 1700, but it was decided to make the (by now) 11-day adjustment gradually over a period of 44 years, one day each leap year, rather than simultaneously. In the meantime, between 1700 and 1744, Sweden's calendar would correspond with neither the Julian calendar nor the Gregorian calendar. This concept in itself had great potential for endless confusion with Swedish dates compared with the rest of Europe. But that inherent compexity was compounded by the system being poorly administered, and some leap-year changes being missed, making it necessary to introduce the unique date 30th February in the year 1712. After a while it was decided to abandon the gradual change altogether. But rather than proceeding directly to the Gregorian calendar, Sweden decided to first revert to the Julian calendar for a time, before finally adopting the Gregorian calendar in one fell swoop in 1753, where February 17 was followed by March 1.

Russia did not accept the new calendar until 1918, with January 31 being followed by February 14. In consequence the anniversary of the so-called 'October Revolution' now falls in November.

Greece followed suit in 1924. The majority of Eastern Orthodoxy did not accept the change to the new calendar for liturgical purposes, regardless of the new civil date. This includes the Orthodox Church of Russia, which maintains the Julian calendar for religious purposes while accepting the use of the Gregorian for purely secular purposes. Some Orthodox Christians may go so far as to identify themselves as Old Calendarist and assert that under the Julian Calendar the eternal liturgy in Heaven was reflected on earth by the liturgical calendar and that the change meant that Heaven and Earth would be out of tune. However, most recognize that an ecclesiastic calendar need not be identical to the civil calendar.

Technically the Orthodox church does not use the Gregorian Calendar, but a Revised Julian calendar, but these will only start to differ in 2800.

Japan replaced the traditional lunisolar calendar with a solar calendar compatible to the Gregorian Calendar in 1873, but the Common Era has not been officially adopted. Official representation of years is based on imperial eras (Meiji; M1=AD1867, Taisho; T1=AD1912, Showa; S1=AD1926, Heisei; H1=AD1989, and so on future). The Common Era (seireki) is nonetheless widely accepted by civilians and to a less extent by government agencies.

Proleptic Gregorian calendar

The Gregorian calendar can for certain purposes be extended to dates preceding its official introduction, producing the Proleptic Gregorian Calendar. However this proleptic calendar should be used with great caution. For ordinary purposes, the dates of events occurring prior to 15 October 1582 should be shown as they appeared in the Julian calendar, and not converted into their Gregorian equivalents.

However, events occurring in countries where the Gregorian calendar was introduced later than 15 October 1582 are a little more contentious. For example, in Great Britain and its overseas possessions (then including the American colonies), the new calendar was not introduced until 14 September 1752. How, then, should we date events occurring in Britain in the 170 years between 1582 and 1752? The answer depends very much on the context, but in all cases the writer should make it absolutely clear which calendar is being used. It would be absurd to go back and change all historical records in Britain deriving from this period, however it is often highly desirable to translate particular OS dates into their NS equivalents, such as where the context includes reference to other countries that had already converted to NS before Britain did. Unless we do this, we have absurdities such as William and Mary of Orange seeming to arrive in London to accept the English crown, a week or so before they left the Netherlands; and Shakespeare and Cervantes apparently dying on exactly the same date, when in fact Cervantes predeceased Shakespeare by 10 days in real time. In order to make these kinds of historical comparisons meaningful, the proleptic Gregorian calendar must be used.

Months of the year

The Gregorian calendar's year is divided into 12 months:

No.NameLength (days)
2February28 or 29


The Gregorian calendar improves the approximation made by the Julian calendar by skipping 3 Julian leap days in every 400 years, giving an average year of 365.2425 mean solar dayss long, which has an error of about 1 day per 3000 years with respect to the mean tropical year but less than half this error with respect to the vernal equinox tropical year of 365.2424 days.

This is substantially more accurate than the 1 day in about 130 years error of the Julian calendar.

Also on any timescale over 3000 years it is expected that changes in the Earth's orbit and unpredictable rotation make it improbable that long term accuracy can be gained by any rule change requiring further regular skipping of Julian leap days.

Calendar seasonal error

This image shows the difference between the Gregorian calendar and the seasons.
The Y axis is "days error" and the X axis is Gregorian calendar years.
Each point represents a single date on a given year. The error shifts by about 1/4 day per year. Years that are multiples of 100 but not 400 are NOT leap years. This causes a correction on years 1700, 1800, 1900, 2100, 2300.
For instance, these corrects cause December 23, 1903 to be the latest December solstice, and December 20, 2096 to be the earliest solstice, nearly 2.5 days variations with the seasonal event.
Compare this error to the Iranian calendar. It also uses leap years mostly every 4th year, but instead of waiting 100 years for exceptions, it corrects the error with a 5 year period after 28 - 4 year cycles, for a total of 8 leap years every 33 years. This calendar keeps the seasonal variations within 30 hours.

Numerical facts

An average year is 365.2425 days = 52.1775 weeks, 8,765.82 hours = 525,949.2 minutes = 31,556,952 seconds.

A common year is 365 days = 8,760 hours = 525,600 minutes = 31,536,000 seconds.

A leap year is 366 days = 8,784 hours = 527,040 minutes = 31,622,400 seconds.

(Some years may also contain a leap second.)

See also common year starting on Sunday and dominical letter.

The 400-year cycle of the Gregorian calendar has 146097 days and hence exactly 20871 weeks. So for example the days of week in Gregorian 1603 were exactly the same as for 2003. Also this causes more months to begin on a Sunday (and hence have Friday 13) than any other day of the week.

See also: 0 (year)

External links