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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.

As a subset, 'calendar' is also used to denote a list of particular set of planned events (for example, court calendar).

Contents 1 Calendar systems 1.1 Solar calendars 1.1.1 Days used by solar calendars 1.1.2 Future reform 1.2 Lunar calendars 1.3 Fiscal calendars 2 Calendar subdivisions 3 Other calendar types 3.1 Complete and incomplete calendars 3.2 Pragmatic, theoretical and mixed calendars 4 Uses 5 Currently used calendars 6 See also 7 Sources 8 External links

Calendar systems

Calendars in use on Earth are 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 apparent motion of the Sun; an example is the Persian calendar.

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.

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.

Solar calendars

Main article: Solar calendar

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.

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.

Lunar calendars

Main article: Lunar calendar

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.

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.

Fiscal calendars

Main article: Fiscal calendar

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.

Calendar subdivisions

Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.

Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be done with leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.

Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years.

Other calendar types

Complete and incomplete calendars

Calendars may be either complete or incomplete. Complete calendars provide a way of naming each consecutive day, while incomplete calendars do not. The early Roman calendar, which had no way of designating the days of the winter months other than to lump them together as "winter", is an example of an incomplete calendar, while the Gregorian calendar is an example of a complete calendar.

Pragmatic, theoretical and mixed calendars

Calendars may be pragmatic, theoretical, or mixed.

A pragmatic calendar is one that is based on observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based or astronomical calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.

A theoretical calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to a rule-based or arithmetical calendar. The advantage of such a calendar is the ease of working out when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore if the calendar is very accurate, its accuracy perishes slowly over time owing to changes in Earth's rotation. This limits the lifetime of an accurate theoretical calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar, resulting in a mixed calendar.

A mixed calendar combines the features of both pragmatic and theoretical calendars. Mixed calendars usually begin as theoretical calendars, but are adjusted pragmatically when some type of asynchrony becomes apparent; the shift from the Julian to the Gregorian calendar is such an example.

The Gregorian calendar, as a final example, is complete, solar, and mixed.

Uses

The primary practical use of a calendar is to identify days: to be informed about and/or to agree on a future event and to record an event that has happened. Days may be significant for civil, religious or social reasons. For example, a calendar provides a way to determine which days are religious or civil holidays, which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also a calendar may, by identifying a day, provide other useful information about the day such as its season.

Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, written calendars are no longer an essential part of such systems, as the advent of accurate clocks has made it possible to record time independently of astronomical events.

Currently used calendars

Calendars in widespread use today include the Gregorian calendar, which is the de facto international standard, and is used almost everywhere in the world for civil purposes, including in China and India (along with the Indian national calendar). The Hebrew calendar is the official calendar of Israel's government, but the Gregorian calendar is much more widely used in Israel's business and day-to-day affairs. The Persian calendar is used in Iran and Afghanistan. The Islamic calendar is used by Muslims the world over. The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and/or social purposes.

Even where there is a commonly used calendar such as the Gregorian calendar, alternate calendars may also be used, such as a fiscal calendar.

See also

• List of calendars
• Gregorian calendar
• Julian calendar
• liturgical year
• calendar of saints
• Christian calendar
• Eastern Orthodox Church calendar
• Calculating the day of the week
• Cycles
• Runic calendar
• French Republican Calendar
• Wall calendar
• Wikipedia:WikiProject Calendars
• Zoroastrian calendar
• iCalendar
• hCalendar
• Calendar reform

Sources

• Calendrical Calculations; Nachum Dershowitz and Edward M. Reingold; Cambridge University Press, 1997; ISBN 0-521-56474-3; Book Info; Online Calculator
• Mapping Time, the calendar and its history; E G Richards; Oxford University Press, 1998; ISBN 0-19-850413-6
• A comparative Calendar of the Iranian, Muslim Lunar,and Christian Eras for Three Thousand Years; Ahmad Birashk; Mazda Publishers, 1993; ISBN 0-939214-95-4
• The Comprehensive Hebrew Calendar; Arthur Spier; Feldheim Publishers, 1986; ISBN 0-87306-398-8
• High Days and Holidays in Iceland; Árni Björnsson; Mál og menning, 1995; ISBN 9979-3-0802-8
• Explanatory Supplement to the Astronomical Almanac; P. Kenneth Seidelmann, ed.; University Science Books, 1992; ISBN 0-935702-68-7; Chapter 12: Calendars by L. E. Doggett
• Sun, Moon, and Sothis; Lynn E. Rose; Kronos Press, 1999; ISBN 0-917994-15-9
• Untersuchungen zur Geschichte der Tibetischen Kalenderrechnung; Dieter Schuh; Franz Steiner Verlag GMBH, 1973

External links

• Frequently Asked Questions about Calendars
• 1911 Encyclopaedia Britannica entry
• Event Calendar based on Wikipedia Content
• Perpetual Calendar 1800 - 2400
• Perpetual Calendar
• Current calendar.
• DateDex: Selected events on selected dates

This article is based on the article "Calendars" from Wikipedia - the free encyclopedia created and edited by online user community. This article is distributed under the terms of GNU Free Documentation License. Here you find the list of authors of this article. The article can only edited within Wikipedia. Edit this article in Wikipedia.

For other uses, see Time (disambiguation).

Attempting to understand Time has long been a prime occupation for philosophers, scientists and artists. There are widely divergent views about its meaning, hence it is difficult to provide an uncontroversial and clear definition of time. The Oxford English Dictionary defines it as "the indefinite continued progress of existence and events in the past, present, and future, regarded as a whole". Another standard dictionary definition is "a non-spatial linear continuum wherein events occur in an apparently irreversible order." This article looks at some of the main philosophical and scientific issues relating to time.

The measurement of time has also occupied scientists and technologists, and was a prime motivation in astronomy. Time is also a matter of significant social importance, having economic value ("time is money") as well as personal value due to an awareness of the limited time in each day and in our lives. Units of time have been agreed upon to quantify the duration of events and the intervals between them. Regularly recurring events and objects with apparently periodic motion have long served as standards for units of time - such as the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum.

Contents 1 Philosophy of time 1.1 Contemporary theses in the philosophy of time 2 Time in physics 3 Measurement 3.1 Present day standards 3.2 Chronology 4 Psychology 4.1 Use of time 5 See also 5.1 General units of time 5.2 Special units of time 5.3 Time measurement and horology 5.4 Theory and study of time 6 References 7 External links 7.1 Perception of time 7.2 Physics 7.3 Timekeeping 7.4 Miscellaneous 8 Further reading

Philosophy of time

Main article: Philosophy of space and time; Ontology

In ancient thought, Zeno's paradoxes challenged the conception of infinite divisibility, and eventually led to the development of calculus. Parmenides (of whom Zeno was a follower) believed that time, motion, and change were illusions, basing this on a rather interesting argument. More recently, McTaggart held a similar belief.

Newton believed time and space form a container for events, which is as real as the objects it contains. In contrast, Leibniz believed that time and space are a conceptual apparatus describing the interrelations between events.

Leibniz and others thought of time as a fundamental part of an abstract conceptual framework, together with space and number, within which we sequence events, quantify their duration, and compare the motions of objects. In this view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events.

The bucket argument proved problematic for Leibniz, and his account fell into disfavour, at least amongst scientists, until the development of Mach's principle. Modern physics views the curvature of spacetime around an object as much a feature of that object as are its mass and volume.

Immanuel Kant, in the Critique of Pure Reason, described time as an a priori notion that allows us (together with other a priori notions such as space) to comprehend sense experience. With Kant, neither space nor time are conceived as substances, but rather both are elements of a systematic framework necessarily structuring the experiences of any rational agent. Spatial measurements are used to quantify how far apart objects are, and temporal measurements are used to quantify how far apart events occur.

Nietzsche, inspired by the concept of eternal return in his book Thus Spoke Zarathustra, argued that time possesses a circular characteristic. Postulating an infinite past, "all things" must have come to pass therein; the same for an infinite future.

In Existentialism, time is considered fundamental to the question of being, in particular by the philosopher Martin Heidegger.

Contemporary theses in the philosophy of time

In contempoary philosophy there has been a very active debate over the nature of time, especially in light of the big changes in physics since the 1920s. Contributors include Ned Markosian, Ted Sider, Quentin Smith, and L. Nathan Oaklander. Two major theses have been developed, along with some hybrids. There is no real consensus among philosophers about which, if any, is correct. The two major theories can be summed up as follows:

1. A-theory of time: Presentism: Oaklander writes: "[A] version of the pure A-theory, known as "", purports to avoid… the problem of change... According to presentism, only the present exists. Thus, it is not the case that, say, O is green and [then] O is red [if, for example, O is a tomato]." (Oaklander, L. Nathan. In Smith, Quentin, and Oaklander, L. Nathan. 1995. Time, Change, and Freedom. New York: Routledge. 2004, 27.)

2. B-theory of time: Eternalism: the following passage from L. Nathan Oaklander sums this up

…[T]ime [involves] events strung out along a series united to one another by the relations of earlier than, later and simultaneity… The events in the temporal series are fixed in that they never change their position relative to each other… It has become customary to call the entire series of events spread out along the time-line from earlier to later, the “B-series.” When viewed solely in terms of the B-series, time is thought of as static or unchanging for there is nothing about temporal relations between events that changes... Time not only has a static aspect, it also has a transitory aspect. In addition to conceiving of time in terms of events standing in temporal relations, we also conceive of time and the events in time as moving or passing from the far future to the near future, from the near future to the present, and then from present they recede into the more and more distant past… When events are ordered in terms of the notions of past, present, or future they form what is called an “A-series.” It should be noted, of course, that the A- and B-series are not really “two” different series of events, but the same series ordered in two different ways. (Oaklander 2004,Page 69)

Time in physics

Main article: Time in physics

Time is currently one of the few fundamental quantities (quantities which cannot be defined via other quantities because there is nothing more fundamental known at present). Thus, similar to definition of other fundamental quantities (like space and mass), time is defined via measurement. Currently, the standard time interval (called conventional second, or simply second) is defined as 9 192 631 770 oscillations of a hyperfine transition in the ¹³³Cs atom.

Prior to Albert Einstein's relativistic physics, time and space had been treated as distinct dimensions; Einstein linked time and space into spacetime. Einstein showed that people traveling at different speeds will measure different times for events and different distances between objects, though these differences are minute unless one is traveling at a speed close to that of light. Many subatomic particles exist for only a fixed fraction of a second in a lab relatively at rest, but some that travel close to the speed of light can be measured to travel further and survive longer than expected. According to the special theory of relativity, in the high-speed particle's frame of reference, it exists for the same amount of time as usual, and the distance it travels in that time is what would be expected for that velocity. Relative to a frame of reference at rest, time seems to "slow down" for the particle. Relative to the high-speed particle, distances seems to shorten. Even in Newtonian terms time may be considered the fourth dimension of motion; but Einstein showed how both temporal and spatial dimensions can be altered (or "warped") by high-speed motion.

Einstein (The Meaning Of Relativity - 1968): "Two events taking place at the points A and B of a system K are simultaneous if they appear at the same instant when observed from the middle point, M, of the interval AB. Time is then defined as the ensemble of the indications of similar clocks, at rest relatively to K, which register the same simultaneously."

Measurement

Present day standards

The standard unit for time is the SI second, from which larger units are defined like the minute, hour, and day. Because they do not use the decimal system, and because of the occasional need for a leap-second, the minute, hour, and day are "non-SI" units, but are officially accepted for use with the International System. There are no fixed ratios between seconds (or days) on the one hand and months and years on the other hand -- months and years having significant variations in length. Despite its great social importance, the week is not mentioned even as a "non-SI" unit. (See external pdf file: The International System of Units.)

The measurement of time is so critical to the functioning of our modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on atomic clocks around the world, known as International Atomic Time (TAI). This is the yardstick for other time scales including Coordinated Universal Time (UTC) which is the basis for civil time.

The 60 base used for seconds, minutes and hours is all the remains of the ancient Phoenician counting base, using 60 as the equivalent of 10, or 100 in modern times. A 60 base is known as sexagesimal.

Chronology

Another form of time measurement consists of studying the past. Events in the past can be ordered in a sequence (creating a chronology), and be put into chronological groups (periodization). One of the most important systems of periodization is Geologic time, which is a system of periodizing the events that shaped the Earth and its life. Chronology, periodization, and interpretation of the past are together known as the study of history.

Psychology

Different people may judge identical lengths of time quite differently. Time can "fly"; that is, a long period of time can seem to go by very quickly. Likewise, time can seem to "drag," as in when one performs a boring task. The psychologist Jean Piaget called this form of time perception "lived time".

Time appears to go fast when sleeping, or, to put it differently, time seems not to have passed while asleep. Time also appears to pass more quickly as one gets older. For example, a day for a child seems to last longer than a day for an adult. One possible reason for this is that with increasing age, each segment of time is an increasingly smaller percentage of the person's total experience.

Altered states of consciousness are sometimes characterised by a different estimation of time. Some psychoactive substances--such as entheogens--may also dramatically alter a person's temporal judgement.

In explaining his theory of relativity, Albert Einstein is often quoted as saying that although sitting next to a pretty girl for an hour feels like a minute, placing one's hand on a hot stove for a minute feels like an hour. This is intended to introduce the listener to the concept of the interval between two events being perceived differently by different observers.

Use of time

The use of time is an important issue in understanding human behaviour, education, and travel behaviour. The question concerns how time is allocated across a number of activities (such as time spent at home, at work, shopping, etc.). Time use changes with technology, as the television or the Internet created new opportunities to use time in different ways. However, some aspects of time use are relatively stable over long periods of time, such as the amount of time spent traveling to work, which despite major changes in transport, has been observed to be about 20-30 minutes one-way for a large number of cities over a long period of time. This has led to the disputed time budget hypothesis.

Time management is the organization of tasks or events by first estimating how much time a task will take to be completed, when it must be completed, and then adjusting events that would interfere with its completion so that completion is reached in the appropriate amount of time. Calendars and day planners are common examples of time management tools.

Arlie Russell Hochschild and Norbert Elias have written on the use of time from a sociological perspective.

See also

• Event
• Duration
• Change
• Rate
• Sense of time
• Causality
• Present (time)
• Cycles and List of cycles

General units of time

• Second
• Minute
• Hour
• Day
• Week
• Fortnight
• Month
• Quarter
• Year
• Decade
• Century
• Millennium

Special units of time

• Geologic timescale
• Season
• Eon
• Era
• Period
• Epoch
• Stage
• Cosmological decade
• Tithi
• Fiscal year
• Ship's bells
• Half-life
• Periodization and list of time periods
• Unix epoch
• Swatch Internet Time
• Hexadecimal Time
• Shake (time)

Light-year is the distance light can travel in an Earth year and so is a unit of distance rather than time.

Time measurement and horology

• Calendar
• Lunar calendar
• Solar calendar
• Chronometer
• Railroad chronometers
• Clock
• Water clock
• Hourglass
• Sundial
• Time zone
• Time scales and time standards
• Watch
• Network Time Protocol (NTP)

Theory and study of time

• Philosophy of physics
• Spacetime
• Time travel
• Exponential time
• Planck time
• Orders of magnitude (time)
• Eternity
• Peter Lynds
• A Brief History of Time
• Periodization
• Chronology
• History
• Time discipline
• Time management
• Wikibooks:English:Time
• Wheel of time
• Timescapes

References

• Oxford English Dictionary - [1]

External links

Perception of time

• The Experience and Perception of Time
• Subjective Perception of Time and a Progressive Present Moment: The Neurobiological Key to Unlocking Consciousness
• Time and Its Discontents
• Time and Learning
• Time Perception I and II
• The Order of Time: Platform for an Alternative Time Consciousness
• What is Time? An elucidation of the Lubavitcher Rebbe's comments on the topic.

Physics

• A walk through Time
• Time Travel and Multi-Dimensionality
• Time and classical and quantum mechanics: Indeterminacy vs. discontinuity
• Time as a universal consequence of quanta

Timekeeping

• Different systems of measuring time
• non-SI units
• UTC/TAI Timeserver
• Leapsecond
• Hex Time
• Florencetime.net
• BBC article on shortest time ever measured
• American Watchmakers-Clockmakers Institute
• The World Clock - Time Zones

Miscellaneous

• Boost Date-Time Library -- Powerful C++ Library for date-time manipulation
• Cycles Research Institute
• TimeTicker and the time tickers...
• World Time and Zones
• Time Servers NTP Time Servers provide accurate timing for computers and computer networks.

Further reading

• Hightfield, Roger (1992). Arrow of Time: A Voyage through Science to Solve Time's Greatest Mystery, Random House. ISBN 0449907236.
• Peter Galison, Einstein's Clocks and Poincaré's Maps: Empires of Time (2003).
• Seize the Daylight: The Curious and Contentious Story of Daylight Saving Time by David Prerau (Thunder’s Mouth Press; $23.00; ISBN 1-56025-655-9)

This article is based on the article "Timekeeping" from Wikipedia - the free encyclopedia created and edited by online user community. This article is distributed under the terms of GNU Free Documentation License. Here you find the list of authors of this article. The article can only edited within Wikipedia. Edit this article in Wikipedia.