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 realist's view, to which Sir Isaac Newton subscribed, and hence is sometimes referred to as Newtonian time.
A contrasting view is that time is part of the fundamental intellectual structure (together with space and number). Within this structure, humans sequence events, quantify the duration of events and the intervals between them, and compare the 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 and Immanuel Kant, in which time, rather than being an objective thing to be measured, is part of the mental measuring system.
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 units 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.
Time has long been a major subject of science, philosophy, and art. Its measurement has occupied scientists and technologists, 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.
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 units 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 Sumerian 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.
An Egyptian 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.
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). 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). 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.
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). The English word clock 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.
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 watches, 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 calendars, 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.
- Newton's Views on Space, Time, and Motion - Stanford University http://plato.stanford.edu/entries/newton-stm/
- Leibniz on Space, Time, and Indiscernibles - Against the Absolute Theory -- Internet Encyclopedia of Philosophy http://www.iep.utm.edu/l/leib-met.htm#H7
- Critique of Pure Reason - Lecture notes of G. J. Mattey, UC Davis http://www-philosophy.ucdavis.edu/mattey/kant/TIMELEC.HTM
- Kant's Transcendental Idealism - Internet Encyclopedia of Philosophy http://www.iep.utm.edu/k/kantmeta.htm#H4
- Jo Ellen Barnett, Time's Pendulum ISBN 0-306-45787-3 p.28
- Jo Ellen Barnett, Time's Pendulum p.31
- Jo Ellen Barnett, Time's Pendulum p.38
- Jo Ellen Barnett, Time's Pendulum p.37
- 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
- North, J. (2004) God's Clockmaker: Richard of Wallingford and the Invention of Time. Oxbow Books. ISBN 1-85285-451-0
- Watson, E (1979) "The St Albans Clock of Richard of Wallingford". Antiquarian Horology 372-384.