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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).
Calendars in use on Earth are most frequently lunar, solar, lunisolar or arbitrary.
A lunar calendar is synchronized to the motion of the Moon ( moon phases); an example is the Islamic calendar.
A solar calendar is based on perceived seasonal changes synchronized to the motion of the Sun; an example is the Persian 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.
Main article: Solar calendar
Solar calendars assign a date to each solar day. A day may consist of the period between sunriseSunrise also called sunup in some American English dialects, is the time at which the first part of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the (variously defined) point at which the sky begins to and sunsetSunset also called sundown in some American English dialects, is the time at which the Sun disappears below the horizon in the west. It should not be confused with dusk, which is the (variously defined) point at which darkness falls (some time after the S, with a following period of nightThis article describes the time of day. For the work by Elie Wiesel, see Night (book . Night is the time when a location is facing away from the Sun, and thus dark. On Earth, it is night on just under half the planet at any time. The atmosphere refracts s, 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.
Under the Roman Republic, the solar Julian calendarThe Julian calendar was introduced by Julius Caesar in 46 BC, taking force in 45 BC or 709 ab urbe condita''. It was chosen after consultation with the Alexandrian astronomer Sosigenes and was probably designed to approximate the tropical year, known sinc 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 yearA leap year (or intercalary year is a year containing an extra day or month in order to keep the calendar year in sync with an astronomical or seasonal year. Seasons and astronomical events do not repeat at an exact number of days, so a calendar which had of 366 days. So the mean calendar year is 365.25 days.
Unfortunately, Earth's tropical yearA tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). The precise length of time depends on which point of the ecliptic one 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.
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