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, a device used to keep time]] There are two distinct views on the meaning of the word ''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 Realist 's view, to which Sir Isaac Newton subscribed, and hence is sometimes referred to as Newtonian Time .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 ,Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTMKant'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 measuring system used by humans. In Physics , time and space are considered 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 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). Time has long been a major subject of Science , Philosophy , and Art . Its measurement has occupied scientists and Technologist s, and was a prime motivation in Astronomy . Time is also 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 human lifespans. MEASUREMENT Time is currently one of the few Fundamental Quantities . These are quantities which cannot be defined via other quantities because there is nothing more fundamental that is presently known. Thus, similar to definitions of other fundamental quantities (like Space and Mass ), time is defined by the Unit s used to measure it and the Method of its measurement. In essence, this definition defines time itself which otherwise is left undefined. The origins of our current measurement system go back to the Sumer ian civilization of approximately 2000 BC . This is known as the Sumerian Sexagesimal System based on the number 60. 60 seconds in a minute, 60 minutes in an hour - and possibly a calendar with 360 (60x6) days in a year (with a few more days added on). Twelve also features prominently, with roughly 12 hours of day and 12 of night, and 12 months in a year. Measurement devices in Taganrog ( 1833 ).]] A large variety of 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.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 ).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 ).Jo Ellen Barnett, ''Time's Pendulum'' p.38 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 actually comes from French, Latin, and German words that mean 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 Mechanical Clock as an astronomical Orrery about 1330.North, J. (2004) ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. Oxbow Books. ISBN 1-85285-451-0Watson, E (1979) "The St Albans Clock of Richard of Wallingford". ''Antiquarian Horology'' 372-384. .]] 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. A "chronometer" is a portable timekeeper that meets certain precision standards. Initially, the term was used to refer to the Marine Chronometer , a timepiece used to determine Longitude by means of Celestial Navigation . More recently, the term has also been applied to the Chronometer Watch , a Wristwatch that meets precision standards set by the Swiss agency COSC . Over 1,000,000 "Officially Certified Chronometer" certificates, mostly for mechanical wrist-chronometers (wristwatches) with sprung balance oscillators, are being delivered each year, after passing the COSC's most severe tests and being singly identified by an officially recorded individual Serial Number . According to COSC, a chronometer is a high-precision watch capable of displaying the seconds and housing a movement that has been tested over several days, in different positions, and at different temperatures, by an official, neutral body (COSC). Each movement is individually tested for several consecutive days, in five positions and at three temperatures. Any watch with the denomination "chronometer" is provided with a certified movement. The most accurate type of timekeeping device is currently the Atomic Clock , which are used to calibrate other clock and timekeeping instruments. Today, the GPS global positioning systems in coordination with the NTP network time protocol can be used to synchronize timekeeping systems across the globe. DEFINITIONS AND STANDARDS See Also: Time standard Orders of magnitude (time) The SI Base Unit for time is the SI Second . From the second, larger units such as the Minute , Hour and Day are defined, though they are "non-SI" units because they do not use the decimal system, and also because of the occasional need for a Leap-second . They are, however, officially accepted for use ''with'' the International System. There are no fixed ratios between seconds and Month s or Year s as months and years have significant variations in length.1 The official SI definition of the second is as follows:23 Previous to 1967, the second was defined as: The definition of time is extremely important in Science and for our everyday life. All known properties of time follow directly from this definition. For example, this definition of time coupled with current definition of Space makes our space and time to be Minkowski Space-time and makes Special Relativity theory absolutely correct ( True ) by definition. World time The measurement of time is so critical to the functioning of modern societies that it is coordinated at an international level. The basis for scientific time is a continuous count of seconds based on Atomic Clock s around the world, known as the International Atomic Time (TAI) . This is the yardstick for other time scales, including Coordinated Universal Time (UTC) , which is the basis for civil time. Earth is split up into a number of Time Zone s. Most time zones are exactly one hour apart, and by convention compute their local time as an offset from UTC or Greenwich Mean Time . In many locations these offsets vary twice yearly due to Daylight Saving Time transitions. 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 . INTERPRETATIONS See Also: Philosophy of space and time Many ancient philosophers wrote lengthy essays on time. A famous analogy compared the time of life to the passing of sand through an hourglass (a common measuring device for time in the past). The sand at the top is associated with the future, and, one tiny grain at a time, the future flows through the present into the past (associated with the sandpile at the bottom of hourglass). The past: ever expanding, the future: ever decreasing, but the future grains become amassed into the past through the present. This was widely discussed in around the 3rd century CE. The earliest recorded philosophy of time was expounded by Ptahhotep , who lived c.2650–2600 BCE said: ''"Do not lessen the time of following desire, for the wasting of time is an abomination to the spirit."'' In the Old Testament book Ecclesiastes , thought to have been written by Solomon (970–928 BCE), time (as the Hebrew word עת ''’êth'' is often translated, as well as "season") was traditionally regarded as a medium for the passage of Predestined events. (Another word, זמן ''zman'', was current as meaning ''time fit for an event'', and is used as the modern Hebrew equivalent to the English word "time".)
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