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Calendars
A calendar is a system of organizing days for a social, religious, commercial or administrative purpose. This organization is done by giving names to periods of time - typically days, weeks, months and years. The name given to each day is known as a date. more...
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Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycles of some astronomical phenomenon, such as the cycle of the sun, or the moon.
Many civilizations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs.
A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerized systems, which can be set to remind the user of upcoming events and appointments.
As a subset, 'calendar' is also used to denote a list of particular set of planned events (for example, court calendar).
Calendar systems
A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.
The simplest calendar system just counts days from a reference day. This applies for the Julian day. Virtually the only possible variation is using a different reference day, in particular one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.
Other calendars have one (or multiple) larger units of time.
Calendars that contain one level of cycles:
week and weekday - this system (without year, the week number keeps on increasing) is not very common;
year and ordinal date within the year, e.g. the ISO 8601 ordinal date system;
Calendars with two levels of cycles:
year, month, and day - most systems, including the Gregorian calendar (and its very similar predecessor, the Julian calendar), the Islamic calendar, and the Hebrew calendar;
year, week, and weekday - e.g. the ISO week date;
Cycles can be synchronized with periodic phenomena:
A lunar calendar is synchronized to the motion of the Moon (lunar phases); an example is the Islamic calendar.;
A solar calendar is based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian 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.;
The week cycle is an example of one that is not synchronized to any external phenomenon (although it may have been derived from lunar phases, beginning anew every month).;
Read more at Wikipedia.org
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