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There are two distinct views on the meaning of '''time'''.

There are two distinct views on the meaning of '''time'''.

One view is that time is part of the fundamental structure of the [[universe]], a [[dimension]] in which events occur in [[sequence]], and time itself is something that can be measured. This is the [[Philosophical realism|realist]]'s view, to which [[Sir Isaac Newton]] subscribed, and hence is sometimes referred to as [[Newtonian time]].<ref>Newton's Views on Space, Time, and Motion - Stanford University http://plato.stanford.edu/entries/newton-stm/</ref>

One view is that time is part of the fundamental structure of the [[universe]], a [[dimension]] in which events occur in [[sequence]], and time itself is something that can be measured. This is the [[Philosophical realism|realist]]'s view, to which [[Sir Isaac Newton]] subscribed, and hence is sometimes referred to as [[Newtonian time]].<ref>Newton's Views on Space, Time, and Motion - Stanford University [http://plato.stanford.edu/entries/newton-stm/]

A contrasting view is that time is part of the fundamental intellectual structure (together with [[space]] and [[number]]). Within this structure, humans sequence events, [[quantity|quantify]] the duration of events and the intervals between them, and compare the [[motion (physics)|motions]] of objects. In this second view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of [[Gottfried Leibniz]]<ref> Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy http://www.iep.utm.edu/l/leib-met.htm#H7</ref> and [[Immanuel Kant]],<ref>Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM</ref><ref>Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy http://www.iep.utm.edu/k/kantmeta.htm#H4</ref> in which time, rather than being an objective thing to be measured, is part of the [[mind|mental]] measuring system.

A contrasting view is that time is part of the fundamental intellectual structure (together with [[space]] and [[number]]). Within this structure, humans sequence events, [[quantity|quantify]] the duration of events and the intervals between them, and compare the [[motion (physics)|motions]] of objects. In this second view, time does not refer to any kind of entity that "flows", that objects "move through", or that is a "container" for events. This view is in the tradition of [[Gottfried Leibniz]]<ref> Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy [http://www.iep.utm.edu/l/leib-met.htm#H7] and [[Immanuel Kant]], Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM</ref><ref>Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy [http://www.iep.utm.edu/k/kantmeta.htm#H4] in which time, rather than being an objective thing to be measured, is part of the [[mind|mental]] measuring system.

In [[physics]], time and space are considered [[fundamental unit|fundamental quantities]] (i.e. they cannot be defined in terms of other quantities because other quantities - such as [[velocity]], [[force]], [[energy]], etc - are already defined in terms of them). Thus the only definition possible is an [[operational definition|operational]] one, in which time is defined by the process of [[measurement]] and by the [[unit]]s chosen. Periodic events and periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, heartbeats, etc. Currently, the unit of time interval (the [[second]]) is defined as a certain number of [[hyperfine]] transitions in [[Cesium]] atoms (see below). All properties of time follow from this definition.

In [[physics]], time and space are considered [[fundamental unit|fundamental quantities]] (i.e. they cannot be defined in terms of other quantities because other quantities - such as [[velocity]], [[force]], [[energy]], etc - are already defined in terms of them). Thus the only definition possible is an [[operational definition|operational]] one, in which time is defined by the process of [[measurement]] and by the [[unit]]s chosen. Periodic events and periodic motion have long served as standards for units of time. Examples are the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, heartbeats, etc. Currently, the unit of time interval (the [[second]]) is defined as a certain number of [[hyperfine]] transitions in [[Cesium]] atoms (see below). All properties of time follow from this definition.

=== Measurement devices ===

=== Measurement devices ===

 

A large variety of [[Measuring instrument|device]]s have been invented to measure time. The study of these devices is called [[horology]].

A large variety of [[Measuring instrument|device]]s have been invented to measure time. The study of these devices is called [[horology]].

An [[Egypt]]ian device dating to c.[[1500 BCE]], similar in shape to a bent [[T-square]], measured the passage of time from the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward in the mornings. At [[noon]], the device was turned around so that it could cast its shadow in the evening direction.<ref>Jo Ellen Barnett, ''Time's Pendulum'' ISBN 0-306-45787-3 p.28</ref>

An [[Egypt]]ian device dating to c.[[1500 BCE]], similar in shape to a bent [[T-square]], measured the passage of time from the shadow cast by its crossbar on a non-linear rule. The T was oriented eastward in the mornings. At [[noon]], the device was turned around so that it could cast its shadow in the evening direction. Jo Ellen Barnett, ''Time's Pendulum'' ISBN 0-306-45787-3 p.28

A [[sundial]] uses a [[gnomon]] to cast a shadow on a set of markings which were calibrated to the [[hour]]. The position of the shadow marked the hour in [[local time]]. [[Pliny the Elder]] records that the first sundial in Rome was looted from [[Catania]], [[Sicily]] ([[264 BCE]]), which gave the incorrect time for a century, until the markings appropriate for the latitude of [[Rome]] were used ([[164 BCE]]).<ref>Jo Ellen Barnett, ''Time's Pendulum'' p.31</ref> Noontime was an event which could be marked by the time of the shortest shadow on a sundial. This was used in Rome to judge when a court of law was open; lawyers had to be at the court by that time.

A [[sundial]] uses a [[gnomon]] to cast a shadow on a set of markings which were calibrated to the [[hour]]. The position of the shadow marked the hour in [[local time]]. [[Pliny the Elder]] records that the first sundial in Rome was looted from [[Catania]], [[Sicily]] ([[264 BCE]]), which gave the incorrect time for a century, until the markings appropriate for the latitude of [[Rome]] were used ([[164 BCE]]). Jo Ellen Barnett, ''Time's Pendulum'' p.31 Noontime was an event which could be marked by the time of the shortest shadow on a sundial. This was used in Rome to judge when a court of law was open; lawyers had to be at the court by that time.

The most accurate timekeeping devices of the ancient world were the [[waterclock]] or ''clepsydra'', first found in Egypt. A waterclock was found in the tomb of [[pharaoh]] [[Amenhotep I]] (1525 - 1504 BCE). Waterclocks were used in [[Alexandria]], and then worldwide, for example in Greece, from c.[[400 BCE]]. They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. [[Plato]] is said to have invented a water-based alarm clock. It depended on the nightly overflow of a vessel containing lead balls, which would float in a columnar vat. The vat would hold an increasing supply of water supplied by a cistern. Eventually the vessel would float high enough to tip over. The lead balls would then cascade onto a copper platter. The resultant clangor would then awaken his students at the Academy ([[378 BCE]]).<ref>Jo Ellen Barnett, ''Time's Pendulum'' p.38</ref> The [[Greeks]] and [[Chaldeans]] regularly maintained timekeeping records as an essential part of their astronomical observations. In particular, Arab engineers improved on the use of waterclocks up to the Middle Ages.<ref>Jo Ellen Barnett, ''Time's Pendulum'' p.37</ref>

The most accurate timekeeping devices of the ancient world were the [[waterclock]] or ''clepsydra'', first found in Egypt. A waterclock was found in the tomb of [[pharaoh]] [[Amenhotep I]] (1525 - 1504 BCE). Waterclocks were used in [[Alexandria]], and then worldwide, for example in Greece, from c.[[400 BCE]]. They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. [[Plato]] is said to have invented a water-based alarm clock. It depended on the nightly overflow of a vessel containing lead balls, which would float in a columnar vat. The vat would hold an increasing supply of water supplied by a cistern. Eventually the vessel would float high enough to tip over. The lead balls would then cascade onto a copper platter. The resultant clangor would then awaken his students at the Academy ([[378 BCE]]).Jo Ellen Barnett, ''Time's Pendulum'' p.38</ref> The [[Greeks]] and [[Chaldeans]] regularly maintained timekeeping records as an essential part of their astronomical observations. In particular, Arab engineers improved on the use of waterclocks up to the Middle Ages.Jo Ellen Barnett, ''Time's Pendulum'' p.37

The [[hourglass]] uses the flow of sand to measure the flow of time. They were used in navigation. [[Ferdinand Magellan]] used 18 glasses on each ship for his circumnavigation of the globe ([[1522]]).<ref>Laurence Bergreen, ''Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe'', HarperCollins Publishers, 2003, hardcover 480 pages, ISBN 0-06-621173-5</ref> The English word [[clock]] actually comes from French, Latin, and German words that mean [[Bell (instrument)|bell]]. The passage of the hours at sea were marked by bells, and denoted the time (see [[ship's bells]]). The hours were marked by bells in the abbeys as well as at sea.

The [[hourglass]] uses the flow of sand to measure the flow of time. They were used in navigation. [[Ferdinand Magellan]] used 18 glasses on each ship for his circumnavigation of the globe ([[1522]]). Laurence Bergreen, ''Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe'', HarperCollins Publishers, 2003, hardcover 480 pages, ISBN 0-06-621173-5</ref> The English word [[clock]] actually comes from French, Latin, and German words that mean [[Bell (instrument)|bell]]. The passage of the hours at sea were marked by bells, and denoted the time (see [[ship's bells]]). The hours were marked by bells in the abbeys as well as at sea.

Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. [[Richard of Wallingford]] (1292–1336), abbot of St. Alban's abbey, famously built a [[Clock#Early mechanical clocks|mechanical clock]] as an astronomical [[orrery]] about 1330.<ref>North, J. (2004) ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. Oxbow Books. ISBN 1-85285-451-0</ref><ref>Watson, E (1979) "The St Albans Clock of Richard of Wallingford". ''Antiquarian Horology'' 372-384.</ref>

Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Waterclocks, and later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. [[Richard of Wallingford]] (1292–1336), abbot of St. Alban's abbey, famously built a [[Clock#Early mechanical clocks|mechanical clock]] as an astronomical [[orrery]] about 1330.<ref>North, J. (2004) ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. Oxbow Books. ISBN 1-85285-451-0</ref><ref>Watson, E (1979) "The St Albans Clock of Richard of Wallingford". ''Antiquarian Horology'' 372-384.</ref>

The most common devices in day-to-day life are the [[clock]], for periods less than a day, and the [[calendar]], for periods longer than a day. Clocks can range from [[watch]]es, to more exotic varieties such as the [[Clock of the Long Now]]. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a [[pendulum]]. There are also a variety of different [[calendar]]s, for example the [[Lunar calendar]] and the [[Solar calendar]], although the [[Gregorian calendar]] is the most commonly used.

The most common devices in day-to-day life are the [[clock]], for periods less than a day, and the [[calendar]], for periods longer than a day. Clocks can range from [[watch]]es, to more exotic varieties such as the [[Clock of the Long Now]]. They can be driven by a variety of means, including gravity, springs, and various forms of electrical power, and regulated by a variety of means such as a [[pendulum]]. There are also a variety of different [[calendar]]s, for example the [[Lunar calendar]] and the [[Solar calendar]], although the [[Gregorian calendar]] is the most commonly used.