From Grolier's Multimedia Encyclopedia


A calendar is a system, defined by rules, for designating the year, dividing it into smaller units, and assigning days to those units; it is also used for determining the dates of civil and religious holidays. The rudiments of a calendric system may have been constructed as long ago as 2000 BC, when STONE ALIGNMENTS were used, it is believed, to determine the length of the solar year by marking the progress of the Sun along the horizon.

Modern international society requires that the same civil calendar be used worldwide. The civil calendar so used is the Gregorian calendar, which was introduced in 1582. The average length of a Gregorian YEAR is close to that of the solar year, or TROPICAL YEAR, about 365.2422 mean solar days, so that the seasons begin on about the same dates each year. The Gregorian calendar, which is derived from ancient calendars, is a determinate calendar; that is, it is defined solely by numerical rules and can be formed for any year in advance. This was not generally true for ancient calendars, which depended on observational rules.

Ancient Calendars

The earliest complete calendars were probably based on lunar observations. The Moon's phases occur over an easily observed interval, the month; religious authorities declared a month to have begun when they first saw the new crescent Moon. During cloudy weather, when it was impossible to see the Moon, the beginning of the month was determined by calculation. The interval from new moon to new moon, called a synodic month, is about 29.53 days. Hence, calendar months contained either 29 or 30 days. Twelve lunar months, which total 354.36 days, form a lunar year, almost 11 days shorter than a tropical year.

A lunar year is not suitable for agricultural purposes. To keep in step with the Sun, lunar-solar calendars were formed by adding an additional (leap) month when the observation of crops made it seem necessary. Hundreds of such calendars, with variations, were formed at various times in such different areas as Mesopotamia, Greece, Rome, India, and China. The month was not always based on the phases of the Moon; the Mayan calendar divided the year into 18 20-day months, with a 5-day period at the end.

Romans, during the late republic, used various lunar-solar calendars. These calendars were supposedly based only on observation, but in fact they were influenced by political considerations. The Roman calendar was in error by several months during the reign of Julius Caesar, who recognized the need for a stable, predictable calendar and formed one with the help of an astronomer, Sosigenes. The year 46 BC was given 445 days, to compensate for past errors, and every common year thereafter was to have 365 days. Every fourth year, starting with 45 BC, was to be designated a leap year of 366 days, during which February, which commonly had 28 days, was extended by one day. The rule was not correctly applied, but the calendar was corrected by Augustus Caesar by AD 8.

Gregorian Calendar

The Julian leap-year rule created three leap years too many in every period of 385 years. As a result, the actual occurrence of the equinoxes and solstices 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. Thursday, Oct. 4, 1582 (Julian), was followed by Friday, 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 and, eventually, by every Western country and Japan, Egypt, and China.

Year Beginning

The Roman year began in March; December, whose name is derived from the Latin word for "ten," was the tenth month of the year. 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 Months

In every Western European language, the names of the months retain their Roman origin. English names are January, for Janus, god of beginning and endings; February, derived from Februalia, a time for religious atonement; March, for Mars, the god of war; April, from aperire, Latin for "to open" (as, spring flowers); May, for Maia, the goddess of plant growth; June, from juvenis, "youth"; July, for Julius Caesar; August, for Augustus, first Roman emperor; September, from septem, "seven"; October, Nov ember, and December, from octo, novem, and decem, "eight,", "nine," and "ten." (The earliest Latin calendar had only 10 months, with September as the 7th month and December the 10th.)


The Babylonians used a nonastronomical, seven-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 seven 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). Dionysius had referred the year of Christ's birth to other eras. Modern chronology, however, places the event at about 4 BC. The 1st century of the Christian Era began in AD 1, the 2nd in AD 101; the 21st will begin in 2001.

The 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 seven days, ending with the Sabbath, Saturday; the year consists of 12 lunar months — Tishri, Cheshvan, Kislev, 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.

The 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. There are 12 lunar months of alternate 30 and 29 days, making the year only 354 days long, so the months move backward through all the seasons and complete 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.

Calendar Reform

Many proposals have been made for calendar improvement. They go under such names as "universal calendar" and "perpetual calendar," and even a "fixed calendar" consisting of 13 equal months of 28 days. While they do offer advantages of greater simplicity for statistical handling, cultural resistance to such change is strong. In addition, correcting some problems invariably causes others, especially those connected with religious holidays.

During the French Revolution a reformed calendar rid of religious connections was in fact adopted having a 10-day week and 12 months of 30 days. The days left at year's end were given over to vacations and celebrations. The calendar began on Sept. 22, 1792, the day the republic was proclaimed. The months were called Vendemiaire (vintage), Brumaire (mist), Frimaire (frost), Nivose (snow), Pluviose, Ventose (wind), Germinal (sprouting time), Floreal (blossom), Prairial (meadow), Messidor (harvest), Thermidor (heat), and Fructidor (fruit). France returned to the Gregorian calendar on Jan. 1, 1806, under Napoleon I.

William Markowitz


Copyright 1995 by Grolier Electronic Publishing, Inc.

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