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It is a system for naming periods of time, particularly days. These names are known as calendar days. The dates are based on the perceived motion of astronomical objects. It is a physical device, generally paper that illustrates the system. It is also used to denote a list of particular set of planned events.
Calendars used on earth are Lunar, Lunisolar, Solar or Arbitary
Lunar Calendar
It is synchronized to the motion of the moon. For e.g. Islamic calendar
Lunisolar Calendar
It is synchronized both to the motion of moon and to the apparent motion of moon. For e.g. Jewish calendar
Solar Calendar
It is based on perceived seasonal changes synchronized to the apparent motion of the Sun. For e.g. Persian calendar
Arbitrary calendar
It is not synchronized either to the moon or sun. For e.g. Julian day used by astronomers
A number of calendars appear to be synchronized to the motion of Venus such as some of the ancient Egyptian calendars.
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