Calendar
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For other uses, see Calendar (disambiguation)
A page from the Hindu calendar 1871–1872.
A page from the Hindu calendar 1871–1872.
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 REMINDER DOWNLOAD name given to each day is known as a date. 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 REMINDER DOWNLOAD 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 REMINDER DOWNLOAD set of planned events (for example, court calendar).
Contents
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* 1 Calendar systems
o 1.1 Mayan Calendar
o 1.2 Solar calendars
+ 1.2.1 REMINDER DOWNLOAD Days used by solar calendars
+ 1.2.2 Calendar reform
o 1.3 Lunar calendars
* 2 Calendar subdivisions
* 3 Other calendar types
REMINDER DOWNLOAD o 3.1 Arithmetic and astronomical calendars
o 3.2 Complete and incomplete calendars
* 4 Uses
* 5 Currently used calendars
o 5.1 Fiscal calendars
REMINDER DOWNLOAD o 5.2 School calendars
* 6 Gregorian Calendar with Easter Sunday
* 7 Physical calendars
* 8 Calendars in computing
o 8.1 Layout
* 9 See also
REMINDER DOWNLOAD o 9.1 List of calendars
* 10 Sources
* 11 References
* 12 External links
[edit] 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 REMINDER DOWNLOAD 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 REMINDER DOWNLOAD 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 REMINDER DOWNLOAD 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:
REMINDER DOWNLOAD * 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 REMINDER DOWNLOAD 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 REMINDER DOWNLOAD anew every month).
Very commonly a calendar includes more than one type of cycle, or has both cyclic and acyclic elements. A lunisolar calendar is synchronized both to the motion of the moon and to the apparent motion of the sun; an example is the Hebrew calendar.
Many calendars incorporate simpler calendars as elements. For example, the rules of the REMINDER DOWNLOAD Hebrew calendar depend on the seven-day week cycle (a very simple calendar), so the week is one of the cycles of the Hebrew calendar. It is also common to operate two calendars simultaneously, usually providing unrelated cycles, and the result may also be considered a more complex calendar. For example, the Gregorian calendar has no inherent dependence on REMINDER DOWNLOAD the seven-day week, but in Western society the two are used together, and calendar tools indicate both the Gregorian date and the day of week.
The week cycle is shared by various calendar systems (although the significance of special days such as Friday, Saturday, and Sunday varies). Systems of leap days usually do not affect the week cycle. The week cycle was not even interrupted when 10, 11, 12, or 13 dates were skipped when the Julian calendar was replaced by the Gregorian calendar by various countries.
[edit] Mayan Calendar
Main article: Mayan calendar
[edit] Solar calendars
Main article: Solar calendar
[edit] 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.
