Calendar

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.

Calendar systems

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

Related Topics:
Lunar - Solar - Lunisolar

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

Related Topics:
Moon phase - Islamic calendar

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A solar calendar is based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar.

Related Topics:
Seasonal - Persian calendar

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

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

Related Topics:
Week - Julian day

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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.

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Solar calendars

Main article: Solar calendar

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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.

Related Topics:
Solar day - Day - Sunrise - Sunset - Night

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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.

Related Topics:
Julian calendar - Leap year

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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.)

Related Topics:
Tropical year - Gregorian calendar - Russian Empire - Russian Revolution of 1917 - Bolshevik - February - October - Revolution - March - November

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Future reform

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.

Related Topics:
Reform of the calendar - World calendar - International Fixed Calendar - United Nations

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Lunar calendars

Main article: Lunar calendar

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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. An example is the Jewish calendar which uses a 19 year cycle.

Related Topics:
Lunar calendar - Moon phase - Tide - Lunisolar calendar - Jewish

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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.

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Fiscal calendars

Main article: Fiscal calendar

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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.

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