General Information

Many different societies have used their own special calendar during recorded history. Most have been based on the apparent motion in the sky of the Sun or Moon. Early in the Roman Empire, around what we would now call 400 BC, a calendar with a year of 365 days was instituted. Over time, the calendar got out of step with the seasons, and the Emperor Julius Caesar declared every fourth year to be a 'leap year' (with an extra day) and, to solve the past problems, the year we would call 46 BC was made 445 days long!

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This Julian calendar greatly improved the situation, but there was still a small error, where the calendar would get about 3 days off for each 400 years. As a result, the actual occurrence of the equinoxes and solstices slowly drifted away from their assigned calendar dates. As the date of the spring equinox determines that of Easter, the church was concerned, and Pope Gregory XIII, with the help of an astronomer, Christopher Clavius (1537-1612), introduced what is now called the Gregorian calendar. Wednesday, Oct. 4, 1582 (Julian), which was followed by Thursday, Oct. 15, 1582 (Gregorian); leap years occur in years exactly divisible by four, except that years ending in 00 must be divisible by 400 to be leap years. Thus, 1600, 1984, and 2000 are leap years, but 1800 and 1900 are not.

The Gregorian civil calendar is a solar calendar, calculated without reference to the Moon. However, the Gregorian calendar also includes rules for determining the date of Easter and other religious holidays, which are based on both the Sun and the Moon. The Gregorian calendar was quickly adopted by Roman Catholic countries. Other countries adopted it later, sometimes choosing only the civil part. It was not adopted by the Soviet Union until 1918; Turkey did not adopt it until 1927.

Year Beginning

The year used to begin at different times in different localities. The Roman year originally began in March; December, whose name is derived from the Latin word for "ten," was the tenth month of the year. (Similarly, September was the seventh month, October, the eighth, and November, the ninth.) In 153 BC, Roman consuls began taking office on January 1, which became the beginning of the year. This practice was retained in the Julian and Gregorian calendars, although other starting dates continued to be used; England and its colonies, for example, used March 25 and the Julian reckoning until 1752. Thus, George Washington was officially born on Feb. 11, 1731, Old Style (O.S.); this is Feb. 22, 1732, Gregorian, or New Style (N.S.).


The Babylonians used a nonastronomical, 7-day interval, the week, which was adopted by the Jews. The seventh day, the Sabbath, was given a religious significance. Independently, the Romans associated a cycle of 7 days with the Sun, the Moon, and the five known planets. Their names became attached to the days of the week: Sunday (dies solis, "Sun's day"), Monday (dies lunae, "Moon's day"), and Saturday (dies Saturni, "Saturn's day") retain their names derived directly from the Roman culture, and Tuesday ("Tiw's day"), Wednesday ("Woden's day"), Thursday ("Thor's day"), and Friday ("Frigg's day") are derived from the Germanic equivalents of Mars, Mercury, Jupiter, and Venus, respectively.


In ancient calendars, years were generally numbered according to the year of a ruler's reign. About AD 525, a monk named Dionysius Exiguus suggested that years be counted from the birth of Christ, which was designated AD (anno Domini, "the year of the Lord") 1. This proposal came to be adopted throughout Christendom during the next 500 years.

The year before AD 1 is designated 1 BC (before Christ). (There was no "year zero!") Dionysius had referred the year of Christ's birth to other eras. Modern chronology, however, suggests that Dionysius had been off in his calculations that now firmly places the event of Jesus' Birth at about 4 BC.

The 1st century of the Christian Era began in AD 1, the 2d in AD 101; the 21st began in 2001.


Hebrew Calendar

The Hebrew calendar in use today begins at the Creation, which is calculated to have occurred 3,760 years before the Christian era.

The week consists of 7 days, beginning with Saturday, the Sabbath.

The year consists of 12 lunar months -- Tishri, Heshvan, Kislav, Tebet, Shebat, Adar, Nisan, Iyar, Sivan, Tammuz, Ab, and Elul -- which are alternately 29 and 30 days long. Because a year is some 11 days longer than 12 lunar months, a 13th month ve-Adar, is added seven times during every 19-year cycle.

Therefore, the Hebrew calendar stays fairly synchronized with the seasons.


Islamic Calendar

Muslims begin their calendar at the day and year (July 16, 622, by the Gregorian calendar) when Muhammad fled from Mecca to Medina. (The Hegira actually occurred around two months after their beginning of that official lunar year.) There are 12 lunar months of alternate 30 and 29 days, making the year 354 days long. Because of the shortness of the year, the months move backward through all the seasons, completing a cycle every 32 1/2 years.

The months are Muharram, Safar, Rabi I, Rabi II, Jumada I, Jumada II, Rajab, Shaban, Ramadan, Shawwal, Zulkadah, and Zulhijjah.

(The result of all this is that the Holy month of Ramadan occurs in different seasons in different years.)

Wm. Markowitz

Archer, Peter, The Christian Calendar and the Gregorian Reform (1941); Asimov, Isaac, The Clock We Live On (1963); Keane, Jerryl, Book of Calendars (1981); Michels, A. K., The Calendar of the Roman Republic (1967; repr. 1978); Monaco, James, The French Revolutionary Perpetual Calendar (1982); Philips, Alexander, The Calendar: Its History, Structure, and Improvement (1921); Schocken, W. A., The Calendar of the Mayas (1986); Watkins, Harold, Time Counts: The Story of the Calendar (1954).

The individual articles presented here were generally first published in the early 1980s. This subject presentation was first placed on the Internet in December 1997.

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