The Gregorian calendar is the most adopted in the world. It takes its name from Pope Gregory XIII, who introduced it in 1582. It's based largely on the previous Julian calendar, with some changes in the duration, as proposed by the scientist Lilio Louis. It is a solar calendar of 365 or 366 days depending on leap years, and it is organized into 12 months from 28 up to 31 days. The Gregorian calendar was introduced to reduce the phase shift introduced by the Julian one. The two main changes are the following:
The trigger for this change was the observation that the vernal equinox had moved and fell no more on March 25, as it was in the year of Caesar’s reform. From 46 BC to 1582 AD, counting one day every 128 years, the equinox has moved to 11th March. Also the winter solstice has moved to December 11th in 1582. In that year the commission headed by Cardinal Sirleto decided to skip 10 days, so to restore the equinox to March 21. The operation took place on October 4, 1582, so the next day was October 15. The period was chosen because it was quite free of liturgical celebrations.
The importance of the equinox date is due to the calculation of the Easter date, which must fall on the first Sunday after the first full moon after the spring equinox. The lack of a fixed reference for the equinox caused errors in the calculation date.
It was decided to move the equinox exactly ten days to match the equinox date in 325 AD, the year of Nicea Council, when the rule for the calculation of Easter was defined.
2) The rule of the leap year
The second change was to fix the previous change for the equinox date. It was decided to modify the average length of the year, changing the rule of leap year. A year multiple of 100 is a leap one whether it is divisible by 4. Leap years are 1600, 2000, 2400, but the years 1700, 1800.1900, 2100, 2200 and 2300 are no more leap ones. The year multiple of 4 but not of 100 remain leap as 2012. Obviously the change is not retroactive, so the years 1500, 1400 and 1300 are leap years. With the new rule, every 400 years there are 97 leap years instead of 100.The average Gregorian year is therefore every day 3/400 of a day, ie 10 minutes and 48 seconds, shorter than the year in Julian calendar. We know that Julian year is longer of 11 minutes and 14 seconds than the solar year, so the delay was reduced to 26 seconds.
The Gregorian calendar was introduced immediately on 15th October 1582 in Italy, Spain and Portugal. France introduced it after two months, Belgium, Austria, Bohemia, the Catholic cantons of Switzerland and the German Catholic lands between 1583 and 1584. Poland, Lithuania and Hungary followed in 1586, while the non-Catholic countries did not accept the reform in opposition to the Pope. In 1610 the Prussian adopted it and in the sixteenth century even the Protestant countries of Germany, Holland, Denmark and Norway chose the Gregorian calendar, followed in 1750 by Britain and Sweden.
Even later followed the Orthodox countries: Russia only in 1918 with the Communist revolution, which took place in October because the Julian calendar, but in November according to Gregorian one. Yugoslavia and Romania introduced it in 1919, Greece in 1928, Japan already in 1873, China in 1911 and Turkey in 1927.
Picture: the Stamp released in Germany by the Deutsche Bundespost on occasione of the four hundred years anniversary of the Gregorian Calendar.