Question by ClaudeandBuddy: Is there a place in Lee’s Summit to donate old hardback books?
Answer by matt man
This article is about the concept of time. For the magazine, see Time (magazine). For other uses, see Time (disambiguation).
Sunrise shown in time lapse.
The motions of Sun and Moon have demonstrated and symbolized time throughout humanity’s existence.
The flow of sand in an hourglass can be used to keep track of elapsed time. It also concretely represents the present as being between the past and the future.Time is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Time has been a major subject of religion, philosophy, and science, but defining time in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.
In physics and other sciences, time is considered one of the few fundamental quantities. Time is used to define other quantities – such as velocity – and defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, has a high utility value in the conduct of both advanced experiments and everyday affairs of life. The operational definition leaves aside the question whether there is something called time, apart from the counting activity just mentioned, that flows and that can be measured. Investigations of a single continuum called space-time brings the nature of time into association with related questions into the nature of space, questions that have their roots in the works of early students of natural philosophy.
Among prominent philosophers, there are two distinct viewpoints on time. One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence. Sir Isaac Newton subscribed to this realist view, and hence it is sometimes referred to as Newtonian time. The opposing view is that time does not refer to any kind of “container” that events and objects “move through”, nor to any entity that “flows”, but that it is instead part of a fundamental intellectual structure (together with space and number) within which humans sequence and compare events. This second view, in the tradition of Gottfried Leibniz and Immanuel Kant, holds that time is neither an event nor a thing, and thus is not itself measurable.
Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined in terms of radiation emitted by caesium atoms (see below). 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.
1 Temporal measurement
1.1 History of the calendar
1.2 History of time measurement devices
2 Definitions and standards
2.1 World time
2.2 Sidereal time
3 Time in religion and mythology
3.1 Linear and cyclical time
4 Time in philosophy
4.1 Time as “unreal”
5 Time in the physical sciences
5.1 Time in classical mechanics
5.2 Time in modern physics
5.4 Time dilation
5.5 Relativistic time versus Newtonian time
5.6 Arrow of time
5.7 Quantised time
6 Time and the Big Bang
6.1 Speculative physics beyond the Big Bang
7 Time travel
8 Perception of time
8.1 Time in psychology
8.2 Time in altered states of consciousness
9 Use of time
10 See also
10.3 Miscellaneous arts and sciences
10.4 Miscellaneous units of time
11 Notes and references
12 Further reading
13 External links
13.1 Perception of time
14 Navigation templates
 Temporal measurement
Temporal measurement, or chronometry, takes two distinct period forms: the calendar, a mathematical abstraction for calculating extensive periods of time, and the clock, a concrete mechanism that counts the ongoing passage of time. In day-to-day life, the clock is consulted for periods less than a day, the calendar, for periods longer than a day.
 History of the calendar
Main article: Calendar
Artifacts from the Palaeolithic suggest that the moon was used to calculate time as early as 12,000, and possibly even 30,000 BP.
The Sumerian civilization of approximately 2000 BC introduced the sexagesimal system based on the number 60. 60 seconds in a minute, 60 minutes in an hour – and possibly a calendar with 360 (60×6) 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 (with 12 being 1/5 of 60).
The reforms of Julius Caesar in 45 BC put the Roman world on a solar calendar. This Julian calendar was faulty in that its intercalation still allowed the astronomical solstices and equinoxes to advance against it by about 11 minutes per year. Pope Gregory XIII introduced a correction in 1582; the Gregorian calendar was only slowly adopted by different nations over a period of centuries, but is today the one in most common use around the world.
 History of time measurement devices
Horizontal sundial in Taganrog (1833)Main article: History of timekeeping devices
See also: Clock
A large variety of devices have been invented to measure time. The study of these devices is called horology.
An Egyptian device dating to c.1500 BC, 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.
The most accurate timekeeping devices of the ancient world were the waterclock or clepsydra, one of which was found in the tomb of Egyptian pharaoh Amenhotep I (1525–1504 BC). They could be used to measure the hours even at night, but required manual timekeeping to replenish the flow of water. The Greeks and Chaldeans regularly maintained timekeeping records as an essential part of their astronomical observations. Arab engineers in particular made improvements on the use of waterclocks up to the Middle Ages.
A contemporary quartz watchThe 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).
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 English word clock probably comes from the Middle Dutch word “klocke” which is in turn derived from the mediaeval Latin word “clocca”, which is ultimately derived from Celtic, and is cognate with 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.
A chip-scale atomic clockClocks 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.
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.
The most accurate timekeeping devices are atomic clocks, which are accurate to seconds in many millions of years, and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the spin property of atoms as their basis, and since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. SI defines the second as 9,192,631,770 cycles of that radiation which corresponds to the transition between two electron spin energy levels of the ground state of the 133Cs atom.
Today, the Global Positioning System in coordination with the Network Time Protocol can be used to synchronize timekeeping systems across the globe.
 Definitions and standards
Common units of time Unit Size Notes
picosecond 0.000 000 000 001 seconds no way of accurately measuring
nanosecond 0.000 000 001 seconds
microsecond 0.000 001 seconds
millisecond 0.001 seconds
second SI base unit
minute 60 seconds
hour 60 minutes
day 24 hours
week 7 days
fortnight 14 days 2 weeks
month 28 to 31 days
quarter 3 months
year 12 months
common year 365 days 52 weeks + 1 day
leap year 366 days 52 weeks + 2 days
tropical year 365.24219 days average
Gregorian year 3
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