The Gregorian calendar, also called the Western calendar and the Christian calendar, is internationally the most widely accepted and used civil calendar.
The calendar was a reform in 1582 to the Julian calendar. The motivation for the reform was to bring the date for the celebration of Easter to the time of the year in which the First Council of Nicaea had agreed upon in 325.
Because the spring equinox was tied to the celebration of Easter, the Roman Catholic Church considered this steady drift in the date of Easter undesirable.
The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar, eventually adopting the Gregorian reform for the sake of convenience in international trade.
The last European country to adopt the Gregorian calendar was Greece in 1923.
The calendar was introduced by Pope Gregory XIII, after whom the calendar was named, by papal bull Inter gravissimas dated February 24, 1582.
The Council of Trent had approved a plan in 1563 for correcting the Julian 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 a 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.
The reform adopted as the Gregorian calendar was a modification of a proposal made by the Calabrian doctor Aloysius Lilius (or Lilio).
Lilius’s proposal included reducing the number of leap years in four centuries from 100 to 97, by making 3 out of 4 centurial years common instead of leap years: this part of the proposal had been suggested before by, among others, Pietro Pitati.
Lilius 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.
Lilius’s proposals had two components. Firstly, he proposed a correction to the length of the year. The mean tropical year is 365.24219 days long, while the mean vernal equinox year is 365.2424 days. As the average length of a Julian year is 365.25 days, the Julian year is almost 11 minutes longer than the mean 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.
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 March 11 instead of the ecclesiastically fixed date of March 21, and if unreformed it would drift 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 40 years, thereby providing for a gradual return of the equinox to March 21. 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 which prevailed with Gregory.
The second component consisted of an approximation which 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.
Because the spring equinox was tied to the date of Easter, the Catholic Church considered the seasonal drift in the date of Easter undesirable. The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the moon (computed using the Metonic cycle) that falls on or after the vernal equinox, which they placed on March 21. However, the Church of Rome still regarded March 25 as the equinox (until 342) and used a different cycle to compute the day of the moon. In the Alexandrian system, since the 14th day of the Easter moon could fall at earliest on March 21 its first day could fall no earlier than March 8 and no later than April 5. This meant that Easter varied between March 22 and April 25. In Rome, Easter was not allowed to fall later than April 21, that being the day of the Parilia or birthday of Rome and a pagan festival. The first day of the Easter moon could fall no earlier than March 5 and no later than April 2.
Easter was the Sunday after the 15th day of this moon, whose 14th day was allowed to precede the equinox. Where the two systems produced different dates there was generally a compromise so that both churches were able to celebrate on the same day. By the 10th century all churches (except some on the eastern border of the Byzantine Empire) had adopted the Alexandrian Easter, which still placed the vernal equinox on March 21, although Bede had already noted its drift in 725—it had drifted even further by the 16th century.
Worse, the reckoned Moon that was used to compute Easter was fixed to the Julian year by a 19-year cycle. However, that approximation built up an error of one day every 310 years, so by the 16th century the lunar calendar was out of phase with the real Moon by four days.
The 19-year cycle used for the lunar calendar was also 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.
Pope Gregory dropped 10 days to bring the calendar back into synchronization with the seasons. Accordingly, 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 ten days.
The Julian calendar day Thursday, October 4, 1582 was followed by the first day of the Gregorian calendar, Friday, October 15, 1582 (the cycle of weekdays was not affected).
Four Catholic countries; Spain, Portugal, the Polish–Lithuanian Commonwealth, and most of Italy implemented the new calendar on the date specified by the bull, with Julian Thursday, October 4, 1582, being followed by Gregorian Friday, October 15, 1582. The Spanish and Portuguese colonies adopted the calendar later because of the slowness of communication. Other Catholic countries soon followed. France adopted the new calendar with Sunday, December 9, 1582, being followed by Monday, December 20, 1582.
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, September 2, 1752, was followed by Thursday, September 14, 1752.
In Alaska, the change took place when Friday, October 6, 1867, was followed again by Friday, October 18, 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 (although not known by that name in 1867) was shifted from Alaska’s eastern to western boundary along with the change to the Gregorian calendar.
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