Calendopaedia - The Gregorian Calendar

The Gregorian calendar is the one commonly used today. It was proposed by Aloysius Lilius, a physician from Naples, and adopted by Pope Gregory XIII in accordance with instructions from the Council of Trent (1545-1563) to correct for errors in the older Julian Calendar. It was decreed by Pope Gregory XIII in a papal bull in February 1582.

In the Gregorian calendar, the tropical year is approximated as 365 97/400 days = 365.2425 days. Thus it takes approximately 3300 years for the tropical year to shift one day with respect to the Gregorian calendar.

The approximation 365 97/400 is achieved by having 97 leap years every 400 years.

These are calculated as follows : Every year divisible by 4 is a leap year. However, every year divisible by 100 is not a leap year. However, every year divisible by 400 is a leap year after all.

So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600, 2000, and 2400 are leap years.

(Destruction of a myth: There are no double leap years, i.e. no years with 367 days. See, however, the note on Sweden lower down this page.)

The 4000-year rule.

It has been suggested (by the astronomer John Herschel (1792-1871) among others) that a better approximation to the length of the tropical year would be 365 969/4000 days = 365.24225 days. This would dictate 969 leap years every 4000 years, rather than the 970 leap years mandated by the Gregorian calendar. This could be achieved by dropping one leap year from the Gregorian calendar every 4000 years, which would make years divisible by 4000 non-leap years.

This rule has, however, not been officially adopted.

The change-over from the Julian to the Gregorian calendar.

The papal bull of February 1582 decreed that 10 days should be dropped from October 1582 so that 15 October should follow immediately after 4 October, and from then on the reformed calendar should be used.

This was observed in Italy, Poland, Portugal, and Spain. Other Catholic countries followed shortly after, but Protestant countries were reluctant to change, and the Greek orthodox countries didn't change until the start of this century.

The following list contains the dates for changes in a number of countries.

 
Albania:        December 1912 
 
Austria: Different regions on different dates 
                 5 Oct 1583 was followed by 16 Oct 1583 
                14 Dec 1583 was followed by 25 Dec 1583 
 
Belgium: Different authorities say 
                14 Dec 1582 was followed by 25 Dec 1582 
                21 Dec 1582 was followed by 1 Jan 1583 
 
Bulgaria: Different authorities say 
                Sometime in 1912 
                Sometime in 1915 
                18 Mar 1916 was followed by 1 Apr 1916 
 
China: Different authorities say 
                18 Dec 1911 was followed by 1 Jan 1912 
                18 Dec 1928 was followed by 1 Jan 1929 
 
Czechoslovakia (i.e. Bohemia and Moravia): 
                 6 Jan 1584 was followed by 17 Jan 1584 
 
Denmark (including Norway): 
                18 Feb 1700 was followed by 1 Mar 1700 
 
Egypt:          1875 
 
Estonia:        January 1918 
 
Finland: Then part of Sweden. (Note, however, that Finland later 
                became part of Russia, which then still used the 
                Julian calendar. The Gregorian calendar remained 
                official in Finland, but some use of the Julian  
                calendar was made.) 
 
France:         9 Dec 1582 was followed by 20 Dec 1582 
 
Germany: Different states on different dates: 
                Catholic states on various dates in 1583-1585 
                Prussia: 22 Aug 1610 was followed by 2 Sep 1610 
                Protestant states: 18 Feb 1700 was followed by 1 Mar 1700 
 
Great Britain and Dominions (including what is now the USA): 
                2 Sep 1752 was followed by 14 Sep 1752 
 
Greece:         9 Mar 1924 was followed by 23 Mar 1924 
 
Hungary:        21 Oct 1587 was followed by 1 Nov 1587 
 
Italy:          4 Oct 1582 was followed by 15 Oct 1582 
 
Japan: Different authorities say: 
                 19 Dec 1872 was followed by 1 Jan 1873 
                 18 Dec 1918 was followed by 1 Jan 1919 
 
Latvia:         During German occupation 1915 to 1918 
 
Lithuania:      1915 
 
Luxembourg:     14 Dec 1582 was followed by 25 Dec 1582 
 
Netherlands: 
        Brabant, Flanders, Holland, Artois, Hennegau: 
                14 Dec 1582 was followed by 25 Dec 1582 
        Geldern, Friesland, Zeuthen, Groningen, Overysel: 
                30 Nov 1700 was followed by 12 Dec 1700 
 
Norway: Then part of Denmark. 
 
Poland:         4 Oct 1582 was followed by 15 Oct 1582 
 
Portugal:       4 Oct 1582 was followed by 15 Oct 1582 

Prussia :       22 Aug 1610 was followed by 2 Sept 1610
 
Romania:        31 Mar 1919 was followed by 14 Apr 1919 
 
Russia:         31 Jan 1918 was followed by 14 Feb 1918 
 
Spain:          4 Oct 1582 was followed by 15 Oct 1582 
 
Sweden (including Finland): 
                17 Feb 1753 was followed by 1 Mar 1753 (see note below) 
 
Switzerland: 
        Catholic cantons: 1583 or 1584 
        Zurich, Bern, Basel, Schafhausen, Neuchatel, Geneva: 
                31 Dec 1700 was followed by 12 Jan 1701 
        St Gallen: 1724 

Transylvania :  14 Dec 1590 was followed by 25 Dec 1590
 
Turkey:         18 Dec 1926 was followed by 1 Jan 1927 

Tyrol :         5 Oct 1583 was followed by 16 Oct 1583
 
USA: See Great Britain, of which it was then a colony. 
 
Yugoslavia:     14 January 1919 was followed by 28 January 1919
                but parts of the country had changed over earlier.

Sweden has a curious history. Sweden decided to make a gradual change from the Julian to the Gregorian calendar. By dropping every leap year from 1700 through 1740 the eleven superfluous days would be omitted and from 1 Mar 1740 they would be in sync with the Gregorian calendar. (But in the meantime they would be in sync with nobody!)

So 1700 (which should have been a leap year in the Julian calendar) was not a leap year in Sweden. However, by mistake 1704 and 1708 became leap years. This left Sweden out of synchronisation with both the Julian and the Gregorian world, so they decided to go ‘back’ to the Julian calendar. In order to do this, they inserted an extra day in 1712, making that year a double leap year! So in 1712, February had 30 days in Sweden.

Later, in 1753, Sweden changed to the Gregorian calendar by dropping 11 days like everyone else.

It should be noted that the Gregorian Calendar is useless for astronomy because it has a ten-day hiatus in it. For the purpose of calculating positions backward in time, astronomers use the Julian Date Calendar.

Thanks to Claus Tondering for most of this information.

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