Gregorian calendar
The Gregorian Calendar was devised because the mean year in the Julian Calendar was slightly too long, causing the vernal equinox to slowly drift backwards in the calendar year.
History
Invention
The motivation of the Catholic Church in adjusting the calendar was to have Easter celebrated at the time that they thought had been agreed to at the First Council of Nicaea in 325. Although a canon of the council implies that all churches used the same Easter, they did not. The Church of Alexandria celebrated Easter on the Sunday after the 14th day of the Moon that falls on or after the vernal equinox, which they placed on 21 March. However, the Church of Rome still regarded 25 March as the equinox and used a different day of the moon. By the tenth century all churches (except for some on the eastern border of the Byzantine Empire) had adopted the Alexandrian Easter, which still placed the vernal equinox on 21 March, although Bede had already noted its drift in 725 — it had drifted even further by the sixteenth century.
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 one day every 310 years. So by the sixteenth century the lunar calendar was out of phase with the real Moon by four days.
The Council of Trent approved a plan in 1563 for correcting the calendrial errors, 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.
The fix was to come in two stages. First, it was necessary to approximate the correct length of a solar year. The value chosen was 365.2425 days in decimal notation. This is 365;14,33 days in sexagesimal notation - the length of the tropical year, rounded to two sexagesimal positions; this was the value used in the major astronomical tables of the day. Although close to the mean tropical year of 365.24219 days, it is even closer to the vernal equinox year of 365.2424 days; this fact made the choice of approximation particularly appropriate as the purpose of creating the calendar was to ensure that the vernal equinox would be near a specific date (March 21). See Accuracy.
The second stage was to devise a model based on the approximation which would provide an accurate yet simple, rule-based calendar. The formula designed by Aloysius Lilius was ultimately successful. It proposed a 10 day correction to revert the drift since Nicaea and the imposition of a leap day in only 97 years in 400 rather than in 1 year in 4. To implement the model, it was provided that years divisible by 100 would be leap years only if they were divisible by 400 as well. So, in the last millennium, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. In this millennium, 2100, 2200, 2300 and 2500 will not be leap years, but 2400 will be. This theory was expanded upon by Christopher Clavius in a closely argued, 800 page volume. He would later defend his and Lilius's work against detractors.
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 (1700, 1800, 1900, 2100 etc.) that are no longer leap years. In fact, a new method for computing the date of Easter was introduced.
Lilius originally proposed that the 10 day correction should be implemented by deleting the Julian leap day on each of its ten occurrences during a period of 40 years, thereby providing for a gradual return of the equinox to 21 March. However, Clavius's opinion was that the correction should take place in one move and it was this advice which prevailed with Gregory. 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 last day of the Julian calendar was 4 October 1582 and this was followed by the first day of the Gregorian calendar 15 October 1582. Nevertheless, the dates "5 October 1582" to "14 October 1582" (inclusive) are still valid in virtually all countries because even most Roman Catholic countries did not adopt the new calendar on the date specified by the bull, but months or even years later (the last in 1587).
Beginning of the year
During the Middle Ages 1 January was given the name New Year's Day (or an equivalent name) in all Western European countries (those with predominantly Catholic populations), even while most of those countries began their numbered year on 25 December (the Nativity of Jesus), then 25 March (the Incarnation of Jesus), and even Easter, as in France. This name was the result of always displaying the months of the medieval calendar from January to December (in twelve columns containing 28 to 31 days each), just like the Romans did. Furthermore, all Western European countries (except for a few Italian states) shifted the first day of their numbered year to 1 January while they were still using the Julian calendar, before they adopted the Gregorian calendar, many during the sixteenth century. Eastern European countries (most of them with populations showing allegiance to the Orthodox Church) began their numbered year on 1 September (since about 988).
| Country | 1 January NY | Greg Cal |
|---|---|---|
| Venice | 1522 | 1582 |
| Germany | 1544 | from 1583 |
| Spain, Portugal, and Roman Catholic Netherlands | 1556 | from 1582 |
| Prussia | 1559 | 1700 |
| Denmark | Early 14th century | 1700 |
| Sweden | 1559 | 1753 |
| France | 1564 | 1582 |
| Lorraine | 1579 | 1682 |
| Protestant Netherlands | 1583 | from 1700 |
| Scotland | 1600 | 1752 |
| Russia | 1700 | 1918 |
| Tuscany | 1721 | 1750 |
| England | 1752 | 1752 |
Neither the papal bull nor its attached canons explicitly state that the year of the Gregorian calendar is to begin on 1 January, contrary to popular opinion. However, it does imply such a year by including two tables of saint's days, one labeled 1582 which ends on 31 December, and another for any full year that begins on 1 January. It also specifies its epact relative to 1 January, in contrast with the Julian calendar, which specified it relative to 22 March. These would have been the inevitable result of the above shift in the beginning of the Julian year.
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 1751/52", 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.
Adoption outside of Roman Catholic nations
Very few countries implemented the new calendar on 15 October 1582 — only Spain, Portugal, the Polish-Lithuanian Commonwealth, and most of Italy. Non-Catholic countries objected to adopting a Catholic invention. England, Scotland and thereby the rest of the British Empire (including the eastern 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 (2 September 1752 being followed by 14 September 1752) to account for 29 February 1700 (Julian). Four years later, someone running for a seat in Parliament used the campaign slogan "Give us back our eleven days!", which created false stories of riots at the change-over. Britain legislated special provisions to make sure that monthly or yearly payments would not become due until the dates that they originally would have in the Julian calendar. From 1753 until 1799, the tax year in Britain began on 5 April, which was the "old style" new 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 Britain is still 6 April. "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.
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. The day of the week was repeated on successive days because the International Date Line was shifted from east of to west of Alaska along with the change to the Gregorian calendar.
Denmark, Norway and the Protestant states of Germany adopted the solar portion of the new calendar in 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.
Sweden's relationship with the Gregorian Calendar had a difficult birth. Sweden started to make the change from the OS calendar and towards the NS calendar in 1700, but it was decided to make the (now 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 also the difference would not be static 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 should by now have been 8 days behind the Gregorian, but it was still in fact 10 days behind. King Charles XII wisely recognised that the gradual change to the new system was not working and he abandoned it. However, rather than now proceeding directly to the Gregorian calendar (as in hindsight seems to have been the sensible and obvious thing to do), 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 17 February was followed by 1 March.
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 decreed that 31 January 1918 was to be followed by 14 February 1918.
The last country of Eastern Europe to adopt the Gregorian calendar was Greece in 1923. However, these were all civil adoptions — none of the national churches accepted it. Instead, a Revised Julian calendar was proposed in May 1923 which dropped 13 days in 1923 and adopted a different leap year rule that resulted in no difference between the two calendars until 2800. The Orthodox churches of Greece, Bulgaria, Romania, Poland, and a few others around the Eastern Mediterranean (Constantinople, Alexandria, Antioch, and Cyprus) adopted the Revised Julian calendar, so these New calendarists will celebrate the Nativity along with the Western churches on 25 December in the Gregorian calendar until 2800. The Orthodox churches of Russia, Serbia, Jerusalem, and a few bishops in Greece did not accept the Revised Julian calendar. 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 that are not Orthodox churches, like the Coptic, Ethiopic, Nestorian, Jacobite, and Armenian, 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.
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. This system remains in use. The "western calendar" (西暦, seireki) is nonetheless widely accepted by civilians and to a less extent by government agencies.