The average length of a year in the Julian Calendar is 365.25 days (plus one additional day being added every four years). This is significantly longer than the "real" length of the solar year, 365.2422 days. Tthis error accumulates so that after about 131 years the calendar is out of sync with the equinoxes and solstices by one day.
Pope Paul III recruited several astronomers, principally the Jesuit Christopher Clavius (1537-1612), to come up with a solution. They built upon calendar reform proposals by the astronomer and physician Luigi Lilio (d. 1576). When Pope Gregory XIII was elected he found various proposals for calendar reform before him, and decided in favor of that of Clavius. On 1582-02-24 he issued a papal bull, Inter Gravissimas, establishing what is now called the Gregorian Calendar reform.
The Gregorian reform consisted of the following:
-Ten days were omitted from the calendar, and it was decreed that the day following (Thursday) October 4, 1582 (which is October 5, 1582, in the old calendar) would thenceforth be known as (Friday) October 15, 1582.
-The rule for leap years was changed. In the Julian Calendar a year is a leap year if it is divisible by 4. In the Gregorian Calendar a year is a leap year if either it is divisible by 4 but not by 100, or it is divisible by 400. In other words, a year which is divisible by 4 is a leap year unless it is divisible by 100 but not by 400 (in which case it is not a leap year). Thus the years 1600 and 2000 are leap years, but 1700, 1800, 1900 and 2100 are not.
-New rules for the determination of the date of Easter were adopted.
-The position of the extra day in a leap year was moved from the day before February 25th to the day following February 28th.
In the 16th century, the difference between the Julian calendar and the seasons had accumulated to 10 days. This circumstance led to a reform of the calendar, which was introduced by pope Gregor XIII. Under this reform, 4 October 1582, should be followed by 15 October 1582, while the sequence of the days of the week should not be affected. In addition, the leap year rule was modified by making all century years not divisible by 400 common years, despite they are divisible by 4. Thus the century years 1700, 1800, 1900, and 2100 are common years, while 1600 and 2000 are leap years. With this new rule, 400 Gregorian years have 146,097 days, with an average length of the year of 146,097/400 = 365.2425 days. There still is a slight difference between the Gregorian calendar and the exact length of the year, but this will amount to one day in more than 3,000 years. Finally, the calendar was not to be used to convert any Julian date before 5 October 1582 to the corresponding Gregorian date.
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 the February of the year 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 Julian calendar 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 Julian calendar 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.
With the introduction of the Julian calendar a period of almost complete disorder in the Roman calendar came to an end. Until then, the beginning of the month was announced at the first visibility of the moon's crescent after new moon, though the year was reckoned depending on the sun. The lengths of the years in a four year cycle of this lunisolar calendar were 355, 377, 355, and 378 days, which was almost four days longer than the true length of four years. Soon the calendar derailed because of leap years introduced to keep it aligned with the sun. In 47 BCE, the calendar was in error by about three months.
The astronomer Sosigenes from Alexandria was given, by Julius Caesar, the task of designing an easy-to-use and exact calendar. Sosigenes assumed that the year had a length of 3651/4 days and worked out the leap year rule, by which three common years should be followed by one leap year, the former having 365 days each, while the latter should have 366 days. The months were no longer to be determined by the moon's phases and were given lengths of 30 or 31 days. The length of February was not changed in common years, while in leap years the extra day was to be inserted after 24 February. By the Romans, 24 February was referred to as "day 6 before the calends of March" (ante diem VI Kal. Martii), which led to the extra day being called "second day 6 before the calends of March" (ante diem bis VI Kal. Mar.). Until today the french "année bissextile" reminds us of this curiosity. To align the calendar with the sun, two extra months were inserted, giving this year 445 days, which is why it was called the "annus confusionis". More on the Roman counting of days can be found under The Roman Calendar.
Even the relatively simple leap year rule was, it seems, too complicated for many Romans. Instead of making ever fourth year a leap year, this was done with every third year, which made another reform necessary. This was carried out under Caesars successor Augustus by making three leap years common years until the calendar was aligned with the sun again.
The years were counted "after the foundation of the city" (ab urbe condita), which was thought to have happened in 753 BCE. After August's reform, the Roman year 761 and every fourth year became leap years. Only by coincidence this year corresponds to our year 8, which is divisible by 4.
The counting of the years according to our common era was suggested by the monk Dionysius Exiguus in the 6th century, but only from the 9th century this era became the ordinary counting in Europe.
This Julian calendar was used until as late as the beginning 20th century, some Orthodox Churches still using it for determining some of their feast days. In the Middle Ages already, the days in the months were simply counted instead of hanging on to the rather intricate Roman system.
Beginnings Of The Year
Our New Year's Day, 1 January, was introduced by the Romans, which earlier started their year with 1 March. In the Middle Ages some different days came into use as the Beginning of a year because of some pre-Christian New Year's rituals not liked by the emerging Christian church.
Circumcision style
This is the beginning of the year with 1 January, on which, in the 7th century, the circumcision of Christ, was put, because it was hard to overcome that day as the start of a new year. In civil life, the new year was merely always celebrated on this day, despite different styles in use officially. For official dating, this style was commonly used only since the introduction of the Gregorian calendar. The Papal office has been using this style for brevets since 1621, for bulls since 1691. Other styles have been replaced by 1 January for instance in France in 1563, in the Netherlands and in the diocese Geneva in 1575, in Florence and Pisa in 1749, in England together with the introduction of the Gregorian calendar in whole Britain in 1753, and finally in the diocese Treves during the 30-years-war (1618-1648).
Christmas style
The birth of Christ being celebrated on Christmas day, 25 December, the beginning of the new year was put on this day. In this style 24 December 851 was followed by 25 December 852. This style was used widely in Germany. Documents of the royal or imperial office were dated according to this style without exception until the beginning 13th century. During the reigns of the kings from Philipp (r. 1198-1208) to Konrad IV. (1237/50-1254), the Annunciation style was used sometimes, but since Rudolf I. (r. 1273-1291) the Christmas and Circumcision styles were both used.
Other territories in which the Christmas style was in use were the territories of the Order of the Teutonic Knights, the spanish Netherlands (large parts of what is now Belgium), where the Circumcision style was officially introduced in 1575, and England, where the Christmas style was replaced by the Annuctiation style during the 13th century. In Spain, the Annuntiation style was abolished and the Christmas style adopted in 1350 (Aragon) and 1383 (Castilla) until 1556, when the Circumcision style was introduced.
In the German dioceses the Christmas style was used, with some exceptions (e. g. Treves, Münster).
Annunciation style
This use makes the year begin on 25 March, the day of the annunciation of the Incarnation. The Incarnation having to have taken place before the birth of Christ, the year should have started on 25 March of the preceding year by means of our "normal" calculation. Thus, the year 1405 of the Incarnation must begin on 25 March 1404 and end on 24 March 1405, Julian. But, only in Pisa and the territories influenced by this city this calculation, the calculus pisanus, was adopted. In Florence the year 1405 of the Incarnation started on 25 March 1405 and ended on 24 March 1406, Julian. This was called the calculus florentinus. A great disadvantage of this style is the possibility of Easter occurring not at all, once, or twice in a year.
In Germany, the diocese of Treves used the Annunciation style until the 30-year-war (1618-1648), in Luxemburg and Lotharingia it was used until 1575 and 1579, respectively.
In Britain, the calculus florentinus was in use until the year 1751, which began on 25 March 1751, Julian. The succeeding year 1752 was decreed to begin on 1 January 1752, Julian, and in September that same year, the Gregorian calendar was adopted. Thus, the year 1751 had a length of only 282 days, and 1752 was shortened again, by 11 days, 2 September being followed by 14 September.
The Papal office dated documents according to the Annunciation style from the 10th century until the 13th century, using the calculus Florentinus. Under the popes from Urban II. to Lucius II., the calculus Pisanus was observed sometimes. In France, the Annunciation style could be found from the end of the 10th century until the 12th century, when the Easter style followed. Finally, the Swiss diocese Lausanne used the Annuntiation style until the 16th century.
Easter style
This style lets the year begin on Easter saturday, but there were different uses, which made the year begin on Good Friday already (e. g. in Flanders and Brabant). The feast of Easter falls on a day somewhere between 22 March and 25 April, and this led to the possibility of a date occuring twice a year. The two dates had to be distinguished by marking them "after Easter" and "before Easter".
The Easter style was mainly used in France since the reign of king Philipp I (r. 1059/60-1108). Only as late as 1563 the Circumcision style was adopted officially. In Germany, documents of the diocese of Cologne were dated in the Easter style, which was then replaced by the Christmas style in 1310. In Flanders, Brabant, and Hennegau, which are all parts of the Netherlands, and in the Swiss dioceses of Geneva (from around 1220 until 1305) and Sitten (from around 1200 until around 1250), the Easter style was observed, too. In the Swiss dioceses mentioned, the Christmas style was adopted then.
1 March
The original Roman beginning of the year on 1 March was in use in the Republic of Venice until 1797, when the republic broke down. In Russia, the year began on 1 March until the 14th century, when the beginning of the year was moved to 1 September.
The original Roman Beginning of the year on 1 March was in use in Russia until the 14th century. In the Republic of Venice this was the official style until it was swept away by the French in 1797.
1 September
Since the 13th century, this style became common in Russia, where it was used until the adoption of the Julian calendar. The years were counted "after the creation of the world", so 31 August 1522 Julian was designated 31 August 7030 in Russia. The succeeding day was 1 September 7031.
Sosigenes' leap year rule made four years have 1461 days, resulting in an average length of one year of 365.25 days, which is about 11 minutes longer than the exact length of the year. This difference had amounted to 10 days in the 16th century, which meant that the beginning of the year (1 January) had slowly moved towards spring.
Today, the difference between the Julian and the Gregorian calendar is 13 days. With some Orthodox churches celebrating several feast days according to the Julian calendar, their Christmas Day falls on 7 January of the following year, according to the Gregorian calendar.
The Gregorian calendar is the calendar that is used nearly everywhere in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull Inter gravissimas. Its years are numbered based on the traditional birth year of Jesus Christ, which is labeled the "anno Domini" era [1].
The Gregorian Calendar was devised both because the mean year in the Julian Calendar was slightly too long, causing the vernal equinox to slowly drift backwards in the calendar year, and because the lunar calendar used to compute the date of Easter had grown conspicuously in error as well.
Tomb of Pope Gregory XIII in St. Peter's Basilica.The Gregorian calendar system dealt with these problems by dropping a certain number of days to bring the calendar back into synchronization with the seasons, and then slightly shortening the average number of days in a calendar year, by omitting three Julian leap-days every 400 years.