The Gregorian Calendar

Early Reform Projects

The length of the Julian year exceeds the true length of the tropical year by about 11 minutes. Therefore the vernal equinox slowly moved backwards within the calendar year. This fact was noticed in the early Medieval, and first reform proposals were made in the 13th century already. John Holiwood, or Sacrobosco, proposed to leave out a leap year every 288 years. In a paper for pope Urban VI (pope 1261-1264), Giovanni Campano favoured the astronomical determination of equinoxes and full moons.

The problem was also considered within Orthodox churches. The Byzantine Nikiforos Grigorius told emperor Andronikos II (r 1282-1328) about the shifting equinox and called for a reform of the calendar. Others deemed a reform unnecessary since the shift of the equinox and with it of Easter bore the advantage of shifting Easter away from Passah. In 1373, the scholar Isaak Argyros stated that a reform was not necessary since Judgement day would come in 1491, the Byzantine year 7000 beginning on 1 September that year.

In March 1437 the council of Basle considered a proposal of Nikolaus von Kues (Cusanus) to omit the last seven days of May 1439 and thus bring back the vernal equinox to 21 March. Pope Sixtus IV (pope 1471-1484) began preparations for a calendar reform and invited the astronomer and mathematician Johannes Müller (Regiomontanus, after his home city of Königsberg in Franconia) to come to Rome. Regiomontanus died shortly after his arrival in 1476.

The calendar question was discussed on both the Lateran council (1512-1517) and the council of Trent (1545-1563), but without a result. The Polish astronomer Mikołaj Kopernik (Nicolaus Copernicus) was invited to the council of Trent. Copernicus refused and stated that the true length of the tropical year was not known exactly enough. Eventually a reform was decreed by pope Gregory XIII (pope 1572-1585).

Gregorian Reform of the Calendar

In a bull called "Inter gravissimas" pope Gregory XIII published the reform details. 4 October 1582 was to be followed by 15 October 1582 without interrupting the sequence of the days of the week. The leap year rule was modified so that a centennial year remained a common year unless divisible by 400. Thus, the years 1700, 1800, 1900 and 2100 were made common years, whereas 1600 and 2000 remained leap years as in the Julian calendar. With these new rules, 400 Gregorian years have 146,097 days, with an average length of the year of 146,097/400 = 365.2425 days = 365 d 5 h 49 min 12 s. This is only four seconds short of the length of the tropical year taken from astronomical tables published by Erasmus Reinhold in Königsberg (in Prussia) in 1552 and used as the true year length in the reform. The remaining difference to the true length of a tropical year wil amount to one day in more than 3,000 years.

The new calendar was not to be applied for dates before the reform, and the rules for determining the date of Easter were modified and expanded, see The Calculation of Easter.

Introduction of the Gregorian Calendar

Only Spain, Portugal, and parts of Italy began to use the new calendar on 15 October 1582, the other Catholic European countries introducing it only until 1584. The Protestants refused to take over the new style and hung on to the Julian calendar until as late as 1700, when the "Improved Calendar" was adopted. The only difference between the "Improved" and the Gregorian calendar was the determination of Easter but finally, in 1775, the Protestants threw over board their "astronomical" Easter calculation and, since 1776, celebrated Easter together with the Catholic church. A detailed list of the adoption of the new calendar shows when and where the Gregorian or the "Improved" calendar was introduced.

In Sweden the switch from the old to the new calender was performed under weird circumstances. First, the year 1700 was declared a common year instead of a leap year, thus putting the calendar in Sweden one day ahead of the Julian calendar and leaving a difference between the Gregorian and the Swedish calendar of ten days. The re-adoption of the Julian calendar was done by adding a 30th day to February in 1712. The Swedish waited another 41 years before introducing the Gregorian calender in 1753.

Most of the eastern European countries used the Julian calendar until the beginning 20th century, Russia having territories in which the Gregorian calendar had been adopted before they fell to Russia (Finland became a Russian province in 1809, Poland in 1815). Russia adopted the Christian era as late as 1700, until then counting the years from the "creation of the world" which was assumed to have taken place in 5509 BCE, and beginning the year on 1 September. The Christian era was switched to by declaring that 31 December 7208 should be followed by 1 January 1700.

In Turkey the Islamic calendar was in use until 1926.

Adoption of the Gregorian or Improved or New Julian Calendar

The following table shows the dates of the introduction of the new calendar for several territories. The days given are the last day of the Julian calendar and the first day of the new calendar.

abbreviations: dc.-diocese, adc.-arch diocese, prp.-principality (Fürstentum), dch.-duchy, mg.-margraviate

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