How did the Gregorian calendar replace the Julian calendar?

The Gregorian calendar replaced the Julian calendar through a papal decree in 1582, followed by a patchwork of national adoptions that lasted more than three centuries. Pope Gregory XIII issued the bull Inter Gravissimas on February 24, 1582. It removed ten days from October of that year and reduced the number of leap years. Catholic countries mostly switched within a few years, Protestant and Orthodox countries much later. The last of the countries that had used the Julian calendar, Turkey, changed in 1926. [1] [2]

The Julian calendar, in use from 46 BC, added a leap day every four years. That assumed a year of 365.25 days, which is about eleven minutes too long. Over centuries the calendar drifted against the seasons. The Council of Nicaea in 325 had fixed the spring equinox at March 21, but by the sixteenth century it had slipped to around March 11. This mattered because Easter was defined by the equinox and the full moon that follows it. [3] [4] [1]

The Council of Trent asked the pope in 1563 to oversee reform of the church's liturgical books, which included the calendar. Gregory XIII gave the project to the Jesuit astronomer Christopher Clavius, whose method followed calculations by Aloysius Lilius. Lilius's manuscript is lost, and its contents are known from the writings of Clavius and his commission. [5]

The bull had two main mechanisms. First, Thursday, October 4, 1582 was followed by Friday, October 15, restoring the equinox to March 21. Second, century years became leap years only if divisible by 400, so 1600 stayed a leap year while 1700, 1800 and 1900 did not. That gives 97 leap years every 400 years instead of 100. The University of Nottingham notes that the remaining error would take about 3,700 years to build up to a full day. The bull also called for new lunar tables for calculating Easter. [1] [4] [5]

The pope's legal authority covered only the Church and the Papal States, though he urged Christian princes to follow. Spain, Portugal, Poland and most of Italy changed in October 1582, and France and the Spanish Netherlands in December. Austria, Bohemia, Hungary and Prussia followed within a few decades. Protestant countries such as England and Scotland did not recognize the pope's authority and held out. Sources differ slightly on some dates. The Nottingham table gives Prussia as 1583, while Encyclopedia Virginia says 1610, and timeanddate.com lists Protestant German areas as switching in 1700. [5] [1] [4] [2]

Britain and its colonies, including those that became the United States, switched under the Calendar (New Style) Act, and September 2, 1752 was followed by September 14. By then the gap had grown to eleven days. Japan changed in 1872 and 1873, dropping 12 days. Russia, Greece, Bulgaria and Turkey switched between 1916 and 1926, dropping 13 days. In China the Republic adopted the calendar in 1912, but warlords' use of other calendars meant it was not actually applied. The Nationalist Government decreed it in 1929. [1] [2]

Because countries switched at different times, letters between Britain and France could appear to be answered before they were sent. Documents were sometimes dated in both styles, as in 23/13 December 1688. England also began its year on March 25 until 1752, so dates between January 1 and March 25 were often written with two years. Sweden mishandled its transition and had a February 30 in 1712. Today the Julian calendar is 13 days behind the Gregorian, and the gap will grow to 14 days in 2100. [4] [1] [2]