Gregorian calendar

Gregorian calendar
Name	Gregorian calendar
Type	Solar
Caption	A page from the 1582 papal bull Inter gravissimas
Num months	12
Week length	7 days
Year length	365 days (common), 366 days (leap)
Epoch	Anno Domini
Introduced by	Pope Gregory XIII
Adopted	1582

Gregorian calendar. The Gregorian calendar is the world's most widely used civil calendar. It was introduced in October 1582 by Pope Gregory XIII as a reform of the Julian calendar, primarily to correct the drift in the date of Easter. Its adoption was a gradual process spanning centuries, with nations like Great Britain and its colonies not implementing it until 1752. Today, it serves as the international standard for civil use, though many cultures and religions maintain parallel traditional calendars for cultural or liturgical purposes.

History and development

The need for reform stemmed from a growing discrepancy between the Julian calendar and the tropical year, causing the vernal equinox to drift earlier each century. This shift threatened the accuracy of calculating Easter, a major concern for the Catholic Church. Commissioned by Pope Gregory XIII, the reform was developed by astronomers including Aloysius Lilius and Christopher Clavius. The changes were promulgated by the papal bull Inter gravissimas in 1582. The immediate correction involved advancing the date by ten days, with Thursday, 4 October 1582, being followed by Friday, 15 October 1582, in territories like Italy, Spain, and Portugal.

Structure and features

This solar calendar organizes the year into twelve months of varying lengths: January (31 days), February (28 or 29), March (31), April (30), May (31), June (30), July (31), August (31), September (30), October (31), November (30), and December (31). Its key innovation is a refined leap year rule: a year is a leap year if divisible by 4, except if divisible by 100 unless also divisible by 400. This omits three leap days every 400 years compared to the Julian calendar. The calendar cycle repeats every 400 years, containing 146,097 days. The seven-day week, with origins in Judaism and Christianity, runs independently of months and years.

Adoption and implementation

Initial adoption was largely confined to Catholic nations under the influence of the Papal States. Protestant and Eastern Orthodox countries resisted for decades or centuries due to religious and political tensions. For instance, Great Britain and its American colonies adopted it with the Calendar (New Style) Act 1750, losing eleven days. Sweden implemented a complex, gradual plan. Japan adopted it during the Meiji Restoration, while Russia only transitioned after the October Revolution of 1917, which occurred in November under the new system. Some Eastern Orthodox Churches, like in Greece and Romania, adopted it for civil use but retain the Julian calendar for calculating Pascha.

Accuracy and astronomical basis

The calendar is aligned with the tropical year, the time between successive vernal equinoxes, approximately 365.2422 days. Its mean year of 365.2425 days is achieved through the 400-year leap cycle. This results in an error of about one day every 3,030 years relative to the tropical year. It is more accurate than the Julian calendar's year of 365.25 days, which had accumulated a ten-day lag by the 16th century. The calendar's epoch is tied to the traditional calculation of the Anno Domini era, established by Dionysius Exiguus. Modern timekeeping is further refined by the occasional insertion of leap seconds by the International Earth Rotation and Reference Systems Service.

Comparison with other calendars

The primary historical competitor was the Julian calendar, from which it diverges in leap year rules. Many world calendars operate on different principles: the Islamic calendar is purely lunar; the Hebrew calendar is lunisolar, coordinating with both the Moon and Sun; and the Chinese calendar is also lunisolar, dictating festivals like Chinese New Year. The Indian national calendar and the Persian calendar (used in Iran and Afghanistan) are solar but anchor their new year to the vernal equinox. Proposals like the World Calendar or the International Fixed Calendar have sought further rationalization but never gained widespread acceptance.

Cultural and religious impact

Its global adoption for commerce and diplomacy has standardized international affairs, influencing treaties like the Treaty of Versailles and the founding of the United Nations. However, many religious observances follow older systems: Eastern Orthodox churches often celebrate Christmas on 7 January, and Judaism calculates Rosh Hashanah via the Hebrew calendar. Cultural events like Lunar New Year in East Asia or Diwali in India remain mobile within its framework. The calendar's association with Western colonialism has also made its adoption a complex symbol of modernization versus tradition in many societies.

Category:Calendars Category:1582 introductions