Julian and Gregorian Calendars; Leap Year; Century

from The World Almanac® and Book of Facts 1997

Calendars based on the movements of the sun and moon have been used since ancient times, but none has been perfect. The Julian calendar, under which Western nations measured time until A.D. 1582, was authorized by Julius Caesar in 46 B.C., the year 709 of Rome. His expert was a Greek, Sosigenes. The Julian calendar, on the assumption that the length of the true year was 365¼ days, gave every fourth year 366 days. St. Bede the Venerable, an Anglo-Saxon monk, announced in A.D. 730 that the 365¼-day Julian year was 11 min, 14 sec too long, a cumulative error of about a day every 128 years, but nothing was done about it for more than 800 years.

By 1582 the accumulated error was estimated to amount to 10 days. In that year Pope Gregory XIII decreed that the day following Oct. 4, 1582, should be called Oct. 15, thus dropping 10 days and initiating what became known as the Gregorian calendar.

However, with common years 365 days and a 366-day leap year every fourth year, the error in the length of the year would have recurred at the rate of a little more than 3 days every 400 years. Therefore, 3 of every 4 centesimal years (years ending in 00) were made common years, not leap years. Thus, 1600 was a leap year; 1700, 1800, and 1900 were not, but 2000 will be. Leap years are those years divisible by 4, except centesimal years, which are common unless divisible by 400.

The Gregorian calendar was adopted at once by France, Italy, Spain, Portugal, and Luxembourg. Within 2 years most German Catholic states, Belgium, and parts of Switzerland and the Netherlands were brought under the new calendar, and Hungary followed in 1587. The rest of the Netherlands, along with Denmark and the German Protestant states, made the change in 1699–1700 (German Protestants retained the old reckoning of Easter until 1776).

The British government imposed the Gregorian calendar on all its possessions, including the American colonies, in 1752. The British decreed that the day following Sept. 2, 1752, should be called Sept. 14, a loss of 11 days. All dates preceding were marked O.S., for Old Style. In addition, New Year’s Day was moved to Jan. 1 from Mar. 25 (e.g., under the old reckoning, Mar. 24, 1700, had been followed by Mar. 25, 1701). George Washington’s birthdate, which was Feb. 11, 1731, O.S., became Feb. 22, 1732, N.S. (New Style). In 1753 Sweden too went Gregorian, retaining the old Easter rules until 1844.

In 1793 the French revolutionary government adopted a calendar of 12 months of 30 days each, with 5 extra days in September of each common year and a 6th extra day every fourth year. Napoleon reinstated the Gregorian calendar in 1806.

The Gregorian system later spread to non-European regions, first in the European colonies and then in the independent countries, replacing traditional calendars at least for official purposes. Japan in 1873, Egypt in 1875, China in 1912, and Turkey in 1925 made the change, usually in conjunction with political upheavals. In China, the republican government began reckoning years from its 1911 founding—e.g., 1948 was designated the year 37. After 1949, the Communists adopted the Common, or Christian Era, year count, even for the traditional lunar calendar.

In 1918 the revolutionary government in the Soviet Union decreed that the day after Jan. 31, 1918, O.S., would become Feb. 14, 1918, N.S. Greece followed in 1923. (The Russian Orthodox Church has retained the Julian calendar, as have various Middle Eastern Christian sects.) For the first time in history, all major cultures have one calendar.

To convert from the Julian to the Gregorian calendar, add 10 days to dates Oct. 5, 1582, through Feb. 28, 1700; after that date add 11 days through Feb. 28, 1800; 12 days through Feb. 28, 1900; and 13 days through Feb. 28, 2100.

A century consists of 100 consecutive calendar years. The 1st century A.D. consisted of the years 1 through 100. The 20th century consists of the years 1901 through 2000 and will end Dec. 31, 2000. The 21st century will begin Jan. 1, 2001.


The World Almanac® and Book of Facts 1997 is licensed from K-III Reference Corporation.
Copyright © 1996 by K-III Reference Corporation. All rights reserved.

This page last revised September 02, 1997.

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