Calendar

A calendar is a system for assigning calendar dates to days. The dates may be based on the perceived motion of astronomical objects. A calendar is also a physical device (often paper) that illustrates the system (for example, a desktop calendar). The term is also used to indicate a particular set of planned events (for example, court calendar).

Uses

The primary practical use of a calendar is to identify days: to be informed about and/or to agree on a future event and to record an event that has happened. Days may be significant for civil, religious or social reasons. For example, a calendar provides a way to determine which days are religious or civil holidays, which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also a calendar may, by identifying a day, provide other useful information about the day such as its season.

Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.

Currently used calendars

Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes, including in China and India. The Hebrew calendar is the official calendar of Israel's government, but the Gregorian calendar is much more widely used in Israel's business and day-to-day affairs. The Persian calendar is used in Iran and Afghanistan. The Islamic calendar is used by Muslims the world over. The Chinese, Hebrew, Hindu calendar, and Julian calendars are widely used for religious and/or social purposes.

Calendar systems

Calendars in use on Earth are most frequently solar, lunar, lunisolar or arbitrary.

A solar calendar is based on perceived seasonal changes synchronized to the motion of the Sun; an example is the Gregorian calendar, the modern international standard civil calendar.

A lunar calendar is synchronized to the motion of the Moon (moon phases); an example is the Islamic calendar.

A lunisolar calendar is synchronized to the motions of both the Moon and the Sun; an example is the Jewish calendar.

An arbitrary calendar is not synchronized to either the Moon or the Sun; examples are the week and the Julian day used by astronomers.

There are some calendars that appear to be synchronized to the motion of Venus, such as some of the ancient Egyptian calendars; synchronization to Venus appears to occur primarily in civilizations near the Equator.

Solar Calendar

A solar calendar is a calendar whose dates indicate the season and so are synchronized to the declination of the sun. The mean calendar year approximates the tropical year.

The Julian (old Christian) calendar and Gregorian (modern civil) calendar are solar calendars, with a year of 365 days, which is occasionally extended by adding an extra day to form a leap year.

Julian and Gregorian calendars

Under the Roman Republic, the solar Julian calendar was adopted. It numbers days within months that are longer than the lunar cycle, so it is not convenient for tracking phases of the moon, but it does a better job of tracking the seasons. Each calendar year has 365 days, except every 4th year which is a leap year of 366 days. So the mean calendar year is 365.25 days.

Unfortunately, Earth's tropical year is a little less 365.25 days (it is approximately 365.242 days), so this calendar, too, slowly drifted out of sync with the seasons. For such reasons, the Gregorian calendar was later adopted by most of the West, starting in 1582, and it has since become the world's dominant civic calendar. The Russian Empire notably refused to change from the Julian calendar until the Russian Revolution of 1917, when the Bolsheviks switched Russia to the Gregorian calendar. Consequently, dates in Russian history are frequently given as both "(O.S.)" (Old Style) and "(N.S.)", (New Style), or Julian and Gregorian, respectively, to avoid confusion.

Lunar calendars

Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each moon phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons. It does, however, stay constant with respect to other phenomena, notably tides.

A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons.

Lunar calendars are believed to be the oldest calendars invented by mankind. Cro-Magnon people are claimed to have invented one around 32,000 BC.

Calendar subdivisions

Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.

Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be done with leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.

Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years.

This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia and from ShiningRise.com

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