Calendar, a system of dividing time into convenient periods of days, months, and years. The system used in most parts of the world is the Gregorian calendar.

The day and the year are based on solar (sun) time. One day equals the length of time it takes the earth to make one complete revolution on its own axis. One year is the approximate time it takes the earth to make one complete revolution around the sun. The month was originally the length of time it takes the moon to complete all its phases as it revolves around the earth. This lunar month of approximately 29 ½ days has been replaced in most calendar systems by a slightly longer month so that each month falls during the same season every year.

The week does not depend on movements of the earth or moon, or on anything else in nature. It is an arbitrary division of time developed for convenience.



Earliest Calendars

The Sumerians, Babylonians, and ancient Egyptians had calendars based on the lunar month. The Egyptians, however, adjusted their system so that in each year the same months would fall in the same seasons. They fixed the year at 12 months of 30 days each, with five extra days a year belonging to no month. This gave them a year of 365 days, one quarter of a day short of a full solar year. To make up for this quarter day, they established a leap-year system, providing an extra day every four years.

The Roman calendar under the republic had 12 months, with 355 days in a year. Each new year was begun on the date when the Romans' consul, who was elected annually, took office. Until 153 B.C., consuls took office on March 1; thereafter, on January 1.

The first day of the Roman month was called the Calends, or Kalends. From this word we get calendar. The Ides was the 15th day in months of 31 days and the 13th day in months of fewer days. The Nones was the 9th day before the Ides. The Romans reckoned time backwards, as so many days before the Ides, Nones, or Calends.

The Roman high priests (called pontifexes, or pontiffs) had charge of the official calendar. It was their task to add an extra, or intercalary, month about every two years to make the Roman calendar come out even with the solar year of 365 1/4 days. But the pontiffs abused this power. For political reasons, such as making an official's term shorter or longer, they sometimes would neglect to add the intercalary month or would add too many of them.



Julian Calendar

By 46 B.C. the Roman calendar had run 90 days, or nearly three months, behind the seasons. The spring equinox (first day of spring) came in June instead of in March. This situation was corrected by Julius Caesar, who, with the help of the Alexandrian astronomer Sosigenes, reformed the calendar. Caesar added 90 days to 46 B.C., making that year 445 days long. He decreed that the Roman calendar would thereafter have 365 days and that every fourth year would be a leap year with 366 days. The extra day was to be added to February, the shortest month.

The Senate renamed the month called Quintilis (for fifth, from the old calendar beginning with March) July in honor of Julius Caesar. Later, the month called Sextilis (for sixth) was renamed August for Augustus Caesar.

The Julian calendar went into effect in 45 B.C. The Julian year ran only 11 minutes longer than the solar year; after some minor adjustments, the calendar functioned very well. It was introduced gradually throughout western Europe and in time was adopted, at least in modified form, by most of Rome's eastern provinces.



Gregorian Calendar

The slight inaccuracy in the Julian year amounted to a loss of about three-quarters of a day in a century. By 1582 the spring equinox fell on March 11 instead of March 21. Pope Gregory XIII therefore decreed that the day following October 4, 1582, should be the 15th instead of the fifth of the month. To avoid future difficulty, he decreed further that the last year of each century would be a leap year only if that century can be divided by 400. Accordingly, 1600 was a leap year, but 1700, 1800, and 1900 had only 365 days. The year 2000 in the Gregorian calendar was a leap year, with 366 days.

The Gregorian calendar was readily adopted in Roman Catholic countries. Other countries followed, until it became virtually universal as a measure of civil time. By 1752, when England adopted it, the difference between the Julian and Gregorian calendars was 11 days. An act of Parliament therefore provided that the day following September 2, 1752, should be September 14, rather than September 3. At the same time England changed its New Year's Day from March 25 to January 1.

The Gregorian calendar is referred to as New Style (N.S.), and the Julian as Old Style (O.S.). Thus George Washington was born on February 11, 1732 (O.S.). But because he was a young man when the Gregorian calendar was adopted, his birthday is celebrated on February 22 (N.S.).



Other Calendars

The Jewish Calendar

contains 12 lunar months. In every 19-year cycle there are seven leap years, in which an extra month is inserted, so that the spring equinox always falls in the month Nisan, when the Passover is celebrated. Thus the Jewish calendar is adjusted to the solar year. In the Jewish calendar, years are counted from the date on which, according to Jewish tradition, the universe was created. It corresponds to 3761 B.C.

The Church Calendar

refers to the schedule used by the Christian church in indicating events and seasons of the liturgical year. In the Roman Catholic, Anglican, and Protestant churches, the year begins with the Advent season, the four weeks preceding Christmas. Other seasons include Epiphany, Lent, Easter, and Pentecost. The Eastern Orthodox church divides the liturgical year into three partstriodion (the 10 weeks before Easter), pentecostarion (the SO days after Easter), and octoechos (the rest of the year).

The Muslim Calendar

is the only true lunar calendar in everyday use. It consists of a year of 12 lunar months. In every 30-year period there are 19 years of 354 days and 11 leap years of 355 days. (The extra days are needed to make each month begin with a new moon.) Muslims date their calendar from the Hegira, Mohammed's flight to Medina in 622 A.D.

The French Calendar

The French National Convention established a new calendar for France in November of 1793, during the Revolution. The year was divided into 12 months of 30 days each, with five days of merrymaking at the end of each year. The new calendar was dated back to September 22, 1792, when the new republic took form. It was observed until Napoleon abolished it as of January 1, 1806. The Gregorian calendar was then resumed.

The Chinese Calendar

The Chinese calendar uses both lunar and solar dating systems. It is used today primarily to determine the dates of Chinese festivals.

The Chinese year consists of 12 lunar months, alternating between months of 29 and 30 days. The year is 354 days long, or about 12 lunar cycles. An extra, or intercalary, month is added to the lunar year seven times every 19 years to reconcile it with the solar (365-day) year. Each lunar year is named for one of a series of 12 animals.

The Chinese calendar does not count years in a continuous manner. It uses a 60-year cycle and a system of regional years that begin with each emperor. Chinese tradition holds that the first year of the Yellow Emperor was 2637 B.C.E. (before the common era), and that the emperor introduced a counting system based on this date. The calendar is dated from the middle years of the Shang dynasty (about 1300 B.C.).



Calendar Reform

As far as its relation to the seasons is concerned, the Gregorian calendar is very nearly perfect. With 97 years out of every 400 being leap years, the average length of the year is about 26 seconds more than that of the solar yeara difference of one day every 3,323 years. It has been suggested that the year 4000 and future century years that can be divided by 4,000 be made ordinary (365-day) years rather than leap years. If this suggestion were followed, 20,000 years would pass before the calendar was again a day too long.

However, the Gregorian calendar does have shortcomings. The months vary in length, ranging from 28 to 31 days. Furthermore, the six months from January through June have 181 or 182 days, while the second six-month period is 184 days long. The quarterly divisions of the months are also unequalthe first quarter has 90 or 91 days, the second quarter 91 days, and the third and fourth quarters 92 days each. Finally, neither the year nor the months (except February, in common years) can be exactly divided into weeks. These irregular divisions are particularly annoying in the world of business.

Many suggestions have been made for reforming the calendar. These have been most seriously considered:

The 13-month Calendar

This is a revised version of a calendar first proposed in 1849 by the French philosopher Auguste Comte. It divides the year into 13 months of 28 days each. The extra month, named Sol, comes between June and July. Since this system gives a year of only 364 days, a special day ("Year End Day) is placed between December 28 and January 1. This day belongs to no week or month. Every four years a similar Leap Year Day is inserted between June and Sol. Each month begins on Sunday and ends on Saturday. This plan is also called the international fixed calendar.

The World Calendar

This calendar, also first proposed in the 19th century, did not receive much attention until after World War I. It keeps the 12-month scheme. The first month of each quarter contains 31 days and the other two months 30 days each. Each quarter begins on Sunday, and each has the same number of working days. The Year End Day is placed between December and January, and the Leap Year Day between June and July. These two days would be holidays outside the seven-day week, as in the 13-month calendar.

The Perpetual Calendar

First proposed in 1919 by Willard E. Edwards, this calendar is a 12-month system similar to the World Calendar except for certain details. Each quarter has 91 days; the first two months are 30 clays long, the third is 31. The week begins on Monday instead of Sunday, and each quarter begins on a Monday and ends on a Sunday. The extra day each year is New. Year's Day, which falls between December 31 and January 1.