Gregorian calendar

Gregorian calendar

The Gregorian calendar is the internationally predominant civil calendar instituted in 1582 by Pope Gregory XIII through the papal bull Inter gravissimas. It refined the earlier Julian calendar to better align the civil year with the astronomical tropical year and the equinox used to determine the date of Easter. The reform involved a revised leap-year rule and an immediate date adjustment, provoking varied responses from Catholic Church authorities, Protestant rulers such as Elizabeth I of England, and Orthodox institutions like the Ecumenical Patriarchate of Constantinople.

History

The reform emerged from astronomical and ecclesiastical concerns involving figures and institutions such as the Council of Trent, astronomers like Christopher Clavius, and mathematicians connected to the Roman Curia, alongside earlier proposals from Aloysius Lilius. Pressure came from inaccuracies that affected the timing of observances fixed by the First Council of Nicaea and by monastic chronologies maintained at places like Saint Peter's Basilica. The papal bull Inter gravissimas implemented a correction by omitting ten days in October 1582 to realign the calendar with the vernal equinox as used at the time of Easter. The change was accepted quickly in Italy, Spain, Portugal, and most of the Polish–Lithuanian Commonwealth, while states such as the Kingdom of England and the Tsardom of Russia delayed adoption for political or religious reasons.

Structure and rules

The system retained a 12-month framework with months like January, February, March, and December and varied month lengths rooted in Roman practice from the era of Julius Caesar and the Roman Republic. Leap years occur in years divisible by 4, except those divisible by 100 unless divisible by 400; thus 1600 and 2000 were leap years while 1700, 1800, and 1900 were not. This rule reduced the average year length to 365.2425 days, closer to the tropical year measured by astronomers at institutions such as the Royal Observatory, Greenwich and the Observatoire de Paris. The calendar preserves conventions used in documents from legal institutions like the Holy Roman Empire and influenced dating systems in mercantile centers such as Amsterdam and Venice.

Adoption and global spread

Initial adoption was concentrated in Roman Catholic polities: the Spanish Empire and the Papacy's territories in 1582. Protestant and Orthodox states adopted the reform over subsequent centuries: the Kingdom of France followed rapidly, while the Kingdom of England and its colonies switched in 1752 after parliamentary action by the British Parliament, resulting in the omission of eleven days and changes to fiscal year reckoning used by institutions like the Bank of England. The Russian Empire adopted the calendar after the Russian Revolution of 1917, impacting civil datekeeping and synchronizing with diplomatic partners including the United States and United Kingdom. Colonialism and international law promoted worldwide civil use, with countries such as Japan, China, Turkey, and Brazil adopting it during periods of modernization or reform associated with governments like the Meiji government and the Republic of Turkey under Mustafa Kemal Atatürk. Today it is used for civil purposes in virtually all states represented at the United Nations and by international organizations such as the International Organization for Standardization.

Accuracy and leap-year reform comparisons

Compared to the Julian calendar, with its simple quadrennial leap rule, the Gregorian reform reduced drift against the tropical year from about one day per 128 years to about one day per 3,226 years. Alternative proposals arose: the 19th- and 20th-century reformers at scientific bodies like the Royal Society considered schemes such as the World Calendar and the International Fixed Calendar, while astronomers at institutions such as the US Naval Observatory and the Max Planck Institute for Astronomy modelled lunisolar and atomic-time based systems. Civil proposals like the Fiscal Year Reform or perennial-calendar ideas were debated in legislatures including the United States Congress but foundered due to impacts on religious observance calendars maintained by entities such as the Anglican Communion, Eastern Orthodox Church, and Islamic world institutions.

Impact on society, religion, and economy

The reform affected the dating of religious festivals like Easter, administrative records in archives such as those of the Vatican Secret Archives, and daily life in cities such as Rome and London. Financial systems—stock exchanges in Amsterdam and banking institutions like the Bank of England—adjusted fiscal schedules and interest reckoning after national adoptions. Diplomacy and treaty dating, exemplified by documents at the Treaty of Versailles archives or correspondence between the French Republic and the United States of America, required harmonization. Resistance sometimes sparked riots, as reported in contemporary accounts from the Kingdom of England and Irish provinces, and interconfessional disputes persisted in calendars used for liturgical cycles in the Eastern Orthodox Church and Oriental Orthodox Churches. The Gregorian calendar's widespread adoption facilitated global coordination for commerce, navigation charts used by explorers associated with ports like Lisbon and Hamburg, scientific collaboration across observatories, and standardization enacted through bodies such as the International Telecommunication Union.

Category:Calendars