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A '''calendar''' is a system for assigning [[calendar date|dates]] to [[day]]s. The ''dates'' may be based on the perceived motion of [[astronomical]] objects. A '''calendar''' can also be 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'').
==Calendar Systems==
Calendars assign a ''date'' to each [[solar time|solar day]]. A [[day]] may consist of the period between [[sunrise]] and [[sunset]], with a following period of [[night]], or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a [[solar time|mean solar day]].▼
Calendars in use on Earth are most frequently [[Lunar calendar|lunar]], [[Solar calendar|solar]], [[Lunisolar calendar|luni-solar]] or arbitrary. ▼
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Nearly all calendar systems group consecutive days into "[[month]]s" and also into "[[year]]s". In a ''solar calendar'' a ''year'' approximates Earth's [[tropical year]] (that is, the time it takes for a complete cycle of [[season]]s), traditionally used to facilitate the planning of [[agriculture|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 year]]s. 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.▼
An arbitrary calendar is not synchronized to either the Moon or the Sun; an example is the [[Julian day]] used by astronomers.
Not all calendars use the solar year as a unit. A [[lunar calendar]] is one in which days are numbered within each [[Luna|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 [[tide]]s. A [[lunisolar calendar]] is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons.▼
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.▼
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 [[holiday]]s, 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.▼
===Solar Calendars===
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 [[clock]]s has made it possible to record time independently of astronomical events. ▼
▲Solar Calendars assign a ''date'' to each [[solar time|solar day]]. A [[day]] may consist of the period between [[sunrise]] and [[sunset]], with a following period of [[night]], or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a [[solar time|mean solar day]].
Calendars in widespread use today include the [[Gregorian Calendar|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 [[Iranian calendar|Persian calendar]] is used in [[Iran]] and [[Afghanistan]]. The [[Islamic calendar]] is used by [[Moslem]]s the world over. The [[Chinese calendar|Chinese]], Hebrew, and [[Hindu calendar]]s are widely used for religious and/or social purposes. ▼
====Julian Calendar====
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 the 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.
====Future Reform====
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.▼
There have been a number of proposals for reform of the calendar, such as the [[World calendar]] and [[Perpetual calendar]]. The [[United Nations]] considered adopting such a reformed calendar for a while in the 1950s, but these proposals have lost most of their popularity.
===Lunar Calendars===
Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a [[fiscal calendar]].▼
▲Not all calendars use the solar year as a unit. A [[lunar calendar]] is one in which days are numbered within each [[Luna|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 [[tide]]s. A [[lunisolar calendar]] is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons.
===Fiscal Calendars===
A fiscal calendar (such as a 5/4/4 calendar) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 5 weeks (Sunday through Saturday), February has 4 weeks, March has 4 weeks, etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates. There exists an international standard way doing this (the [[ISO 8601|ISO week]]). The ISO week runs Monday through Sunday and Week 1 is always the week that contains January 4 Gregorian.
==Calendar Subdivisions==
▲Nearly all calendar systems group consecutive days into "[[month]]s" and also into "[[year]]s". In a ''solar calendar'' a ''year'' approximates Earth's [[tropical year]] (that is, the time it takes for a complete cycle of [[season]]s), traditionally used to facilitate the planning of [[agriculture|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 year]]s. 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.
▲Calendars in use on Earth are most frequently [[Lunar calendar|lunar]], [[Solar calendar|solar]], [[Lunisolar calendar|luni-solar]] or arbitrary.
▲A lunar calendar is synchronized to the motion of the Moon; an example is the [[Islamic calendar]]. A solar calendar is synchronized to the motion of the Sun; an example is the [[Persian calendar]].
▲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.
▲A luni-solar calendar is synchronized to the motions of both the Moon and the Sun; an example is the [[Hebrew calendar|Jewish calendar]].
▲An arbitrary calendar is not synchronized to either the Moon or the Sun; an example is 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
==Calendar Types==
===Complete and Incomplete Calendars===
Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not. The early Roman calendar, that had no way of designating the days of the winter months other than to lump them together as "winter", is an example of an incomplete calendar, while the Gregorian calendar is an example of a complete calendar.
===Pragmatic, theoretical and Mixed Calendars===
Calendars may be pragmatic, theoretical, or mixed.
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The Gregorian calendar, as a final example, is complete, solar, and mixed.
==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 [[holiday]]s, 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 [[clock]]s has made it possible to record time independently of astronomical events.
==Currently Used Calendars==
▲Calendars in widespread use today include the [[Gregorian Calendar|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 [[Iranian calendar|Persian calendar]] is used in [[Iran]] and [[Afghanistan]]. The [[Islamic calendar]] is used by [[Moslem]]s the world over. The [[Chinese calendar|Chinese]], Hebrew, and [[Hindu calendar]]s are widely used for religious and/or social purposes.
▲Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a [[fiscal calendar]].
==List of calendars==
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