Calendar, a system of measuring time for the needs of civil life, by dividing time into days, weeks, months, and years. Calendar divisions are based on the movements of the earth and the regular appearances of the sun and the moon. A day is the average time required for one rotation of the earth on its axis. The measurement of a year is based on one revolution of the earth around the sun and is called a seasonal, tropical, or solar year. A solar year contains 365 days, 5 hr, 48 min, and 45.5 sec. A month was originally calculated by ancient peoples as the time between two full moons, or the number of days required for the moon to circle the earth (29.5 days). This measurement, called a synodic, or lunar month, resulted in a lunar year of 354 days, 113 days shorter than a solar year. In modern calendars, however, the number of days in a month is not based on the phases of the moon. The length of the months is approximately one-twelfth of a year (28 to 31 days) and is adjusted to fit the 12 months into a solar year. The week was derived from the Judeo-Christian tradition requiring rest from labor every seventh day. It is not based on a natural phenomenon. The Romans named the days of the week in honor of the sun, moon, and various planets.
The variations among the many calendars in use from ancient to modern times have been caused by the inaccuracy of the earliest determinations of the duration of the year, together with the fact that a year cannot be divided evenly by any of the other time units: days, weeks, or months. The earliest calendars based on lunar months eventually failed to agree with the seasons. A month occasionally had to be intercalated, or added, to reconcile lunar months with the solar year. A calendar that makes periodic adjustments of this kind is a lunisolar calendar.
The ancient Babylonians had a lunisolar calendar of 12 lunar months of 30 days each, and they added extra months when necessary to keep the calendar in line with the seasons of the year. The ancient Egyptians were the first to replace the lunar calendar with a calendar based on the solar year. They measured the solar year as 365 days, divided into 12 months of 30 days each, with 5 extra days at the end.
About 238 BC King Ptolemy III ordered that an extra day be added to every fourth year, similar to the modern leap year. In ancient Greece a lunisolar calendar was in use, with a year of 354 days. The Greeks were the first to intercalate extra months into the calendar on a scientific basis, adding months at specific intervals in a cycle of solar years.
The Roman Calendar
The original Roman calendar, introduced about the 7th century BC, had 10 months with 304 days in a year that began with March. Two more months, January and February, were added later in the 7th century BC, but because the months were only 29 or 30 days long, an extra month had to be intercalated approximately every second year. The days of the month were designated by the awkward method of counting backward from three dates: the calends, or first of the month; the ides, or middle of the month, falling on the 13th of some months and the 15th of others; and the nones, or 9th day before the ides. The Roman calendar became hopelessly confused when officials to whom the addition of days and months was entrusted abused their authority to prolong their terms of office or to hasten or delay elections.
In 45 BC Julius Caesar, upon the advice of the Greek astronomer Sosigenes (flourished 1st century BC), decided to use a purely solar calendar. This calendar, known as the Julian calendar, fixed the normal year at 365 days, and the leap year, every fourth year, at 366 days. Leap year is so named because the extra day causes any date after February in a leap year to "leap" over one day in the week and to occur two days later in the week than it did in the previous year, rather than just one day later as in a normal year. The Julian calendar also established the order of the months and the days of the week as they exist in present-day calendars. In 44 BC Julius Caesar changed the name of the month Quintilis to Julius (July), after himself. The month Sextilis was renamed Augustus (August) in honor of the Roman emperor Caesar Augustus, who succeeded Julius Caesar. Some authorities maintain that Augustus established the length of the months we use today.
The Gregorian Calendar
The Julian year was 11 min and 14 sec longer than the solar year. This discrepancy accumulated until by 1582 the vernal equinox (see ECLIPTIC) occurred 10 days early and church holidays did not occur in the appropriate seasons. To make the vernal equinox occur on March 21, as it had in AD 325, the year of the First Council of Nicaea, Pope Gregory XIII issued a decree dropping 10 days from the calendar. To prevent further displacement he instituted a calendar, known as the Gregorian calendar, that provided that century years divisible evenly by 400 should be leap years and that all other century years should be common years. Thus, 1600 was a leap year, but 1700 and 1800 were common years.
The Gregorian calendar, or New Style calendar, was slowly adopted throughout Europe. It is used today throughout most of the Western world and in parts of Asia. When the Gregorian calendar was adopted in Great Britain in 1752, another correction of an 11-day discrepancy was made; the day after September 2, 1752, became September 14. The British also adopted January 1 as the day when a new year begins. The Soviet Union adopted the Gregorian calendar in 1918, and Greece adopted it in 1923 for civil purposes, but many countries affiliated with the Greek church retain the Julian, or Old Style, calendar for the celebration of church feasts.
The Gregorian calendar is also called the Christian calendar because it uses the birth of Jesus Christ as a starting date. Dates of the Christian era (see CHRONOLOGY) are often designated AD (Latin anno domini, "in the year of our Lord") and BC (before Christ). Although the birth of Christ was originally given as December 25, 1 BC, modern scholars now place it about 4 BC.
Because the Gregorian calendar still entails months of unequal length, so that dates and days of the week vary through time, numerous proposals have been made for a more practical, reformed calendar. Such proposals include a fixed calendar of 13 equal months and a universal calendar of 4 identical quarterly periods. Thus far, none has been adopted (see CALENDAR REFORM).
As indicated, the Gregorian calendar is basically a Christian calendar. The official Christian church calendar is a table containing the holy days, saints' days, and festivals of the church, with the dates of the civil calendar on which they occur. These include the fixed feasts, such as Christmas, and the movable feasts, which depend on the date of Easter. The most important early church calendar was compiled by Furius Dionisius Philocalus about 354. After the Reformation, the German Lutheran church retained the Roman calendar, as did the Church of England and some other Anglican churches. The calendar of the Protestant Episcopal church retains only those festivals that have a scriptural origin. The principal seasons of the church calendar observed by most Christians are, in order, Advent, Christmas, Epiphany, Lent, Easter, Ascension, Pentecost, and Trinity.
Several other calendars based on religious doctrine can also be described. For example, the Jewish calendar, derived from the ancient Hebrew calendar, has remained unchanged since about AD 900. It is the official calendar of the modern state of Israel and is used by Jewish people throughout the world as a religious calendar. The starting point of Hebrew chronology is the year 3761 BC, the date for the creation of the world as described in the Old Testament. The Jewish calendar is lunisolar, based on lunar months of 29 days alternating with 30 days. An extra month is intercalated every 3 years, based on a cycle of 19 years. Dates of the Jewish calendar are designated AM (Latin anno mundi, "the year of the world") and BCE (before the common era).
Another major religious calendar is the Islamic calendar, a lunar one used in most Muslim countries. It is reckoned from AD 622, the day after the Hegira, or flight of Mohammed from Mecca to Medina. The Islamic year consists of 12 lunar months. Thirty years constitute a cycle in which the 2nd, 5th, 7th, 10th, 13th, 16th, 18th, 21st, 24th, 26th, and 29th years are leap years of 355 days; the others are common years of 354 days. The Islamic date corresponding to a date in the Gregorian calendar can be computed by the following rule, with a maximum error of one day: multiply 970,224 by the Islamic year, divide by 6 decimal places, and add 621.5774. The figure to the left of the decimal point is the year AD, and the decimal fraction multiplied by 365 is the day of the year.
For information concerning the Aztec calendar and Mayan calendar, see AZTECMAYA.
Further Reading: "Calendar," Microsoft (R) Encarta. Copyright (c) 1994 Microsoft Corporation. Copyright (c) 1994 Funk & Wagnall's Corporation.
Why is the year 2000 a leap year?
Years ending on double zeros that are divisible by 400 are exceptions to the exception says Neil deGrasse Tyson, acting director of the Hayden Planetarium in New York. There are even more exceptions to come as astronomers tinker with the calendar to make it come out even with the Earth's trip around the Sun and the passage of the seasons.
The Julian calendar, with its leap day every four years, set up a 365.25-day year, just a little too long for the approximately 365.2422-day trip. So every century a leap year was dropped.
But this made the years too short, with the shortfall adding up to nearly a day every 400 years. By 1582, this difference had built noticeably. So Pope Gregory XIII skipped 10 days and decreed that every 400 years a leap year should be restored.
But this makes the years ever so slightly too long again, and the "extra" leap day is to be dropped every 4,000 years. But this makes the year too short again, solvable by keeping the leap day in every century year that leaves a remainder of 200 or 600 when divided by 900.
-- New York Times News Service