A calendar is a system for naming periods of time, typically days. These names are known as calendar dates. 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) — this is the most common usage of the word.
As a subset, 'calendar' is also used to denote a list of particular set of planned events (for example, court calendar).
A lunisolar calendar is synchronized both to the motions of the Moon and to the apparent motion of the Sun; an example is the Jewish calendar.
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.
Main article: Solar calendar
Days used by solar calendars
Solar calendars assign a date to each 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 mean solar day. Other types of calendar may also use a solar day.
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 than 365.25 days (it is approximately 365.2422 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. (A famous example of these calendar shifts is seen in the February and October Revolutions, which, from most of the rest of the world's standpoint, took place in March and November, respectively.)
There have been a number of proposals for reform of the calendar, such as the World calendar and International Fixed 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.
Main article: Lunar calendar
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.
Main article: Fiscal calendar
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 to do this (the ISO week). The ISO week runs Monday through Sunday and Week 1 is always the week that contains January 4 Gregorian.
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.
Other 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, which 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.
A pragmatic calendar is one that is based on observation; an example is the religious Islamic calendar and the religious Jewish calendar in the time of the Temple. Such a calendar is also referred to as an observation-based or astronomical calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.
A theoretical calendar is one that is based on a strict set of rules; an example is the Jewish calendar. Such a calendar is also referred to a rule-based or arithmetical calendar. The advantage of such a calendar is the ease of working out when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore if the calendar is very accurate, its accuracy perishes slowly over time owing to changes in Earth's rotation. This limits the lifetime of an accurate theoretical calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar, resulting in a mixed calendar.
A mixed calendar combines the features of both pragmatic and theoretical calendars. Mixed calendars usually begin as theoretical calendars, but are adjusted pragmatically when some type of asynchrony becomes apparent; the shift from the Julian to the Gregorian calendar is such an example.
The Gregorian calendar, as a final example, is complete, solar, and mixed.
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, and Julian calendars 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
- liturgical year
- calendar of saints
- Christian calendar
- Calculating the Day of the Week
- Runic calendar
- French Republican Calendar
- Wall calendar
- Wikipedia:WikiProject Calendars
- Zoroastrian calendar
- Calendrical Calculations; Nachum Dershowitz and Edward M. Reingold; Cambridge University Press, 1997; ISBN 0-521-56474-3; Book Info (http://emr.cs.iit.edu/home/reingold/calendar-book/second-edition/); Online Calculator (http://emr.cs.iit.edu/home/reingold/calendar-book/Calendrica.html)
- A comparative Calendar of the Iranian, Muslim Lunar,and Christian Eras for Three Thousand Years; Ahmad Birashk; Mazda Publishers, 1993; ISBN 0-939214-95-4
- The Comprehensive Hebrew Calendar; Arthur Spier; Feldheim Publishers, 1986; ISBN 0-87306-398-8
- High Days and Holidays in Iceland; Árni Björnsson; Mál og menning, 1995; ISBN 9979-3-0802-8
- Explanatory Supplement to the Astronomical Almanac; P. Kenneth Seidelmann, ed.; University Science Books, 1992; ISBN 0-935702-68-7; Chapter 12: Calendars by L. E. Doggett (http://astro.nmsu.edu/~lhuber/leaphist.html)
- Sun, Moon, and Sothis; Lynn E. Rose; Kronos Press, 1999; ISBN 0-917994-15-9
- Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung; Dieter Schuh; Franz Steiner Verlag GMBH, 1973
- Frequently Asked Questions about Calendars: http://www.tondering.dk/claus/calendar.html
- DMOZ Calendars and Timekeeping category: http://dmoz.org/Science/Astronomy/Calendars_and_Timekeeping/
- DateDex: Calendar Directory (http://www.datedex.com/results.asp)
- A calendar (http://www.timeanddate.com/calendar/).