JCC wrote:
I miss pre-hiatus JCC. Times were so much simpler then.
ime is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects.[1] The temporal position of events with respect to the transitory present is continually changing; future events become present, then pass further and further into the past. Time has been a major subject of religion, philosophy, and science, but defining it in a non-controversial manner applicable to all fields of study has consistently eluded the greatest scholars.
Time is one of the seven fundamental physical quantities in the International System of Units. Time is used to define other quantities — such as velocity — so defining time in terms of such quantities would result in circularity of definition.[2] 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, is highly useful 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 spacetime bring questions about space into questions about time, questions that have their roots in the works of early students of natural philosophy.
Two contrasting viewpoints on time divide many prominent philosophers. 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.[3][4] Time travel, in this view, becomes a possibility as other "times" persist like frames of a film strip, spread out across the time line. 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[5] and Immanuel Kant,[6][7] holds that time is neither an event nor a thing, and thus is not itself measurable nor can it be travelled.
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 life spans.
Ray Cummings, an early writer of science fiction, wrote in 1922, "Time… is what keeps everything from happening at once",[8] a sentence repeated by scientists such as C. J. Overbeck,[9] and John Archibald Wheeler.[10][11]
Contents [hide]
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 Time conversions
2.3 Sidereal time
2.4 Chronology
3 Religion
3.1 Linear and cyclical time
3.2 Numeric and Divine time
4 Philosophy
4.1 Time as "unreal"
5 Physical definition
5.1 Classical mechanics
5.2 Modern physics
5.3 Spacetime
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 Judgement of time
8.1 Biopsychology
8.2 Alterations
9 Use of time
10 See also
10.1 Books
10.2 Organizations
10.3 Miscellaneous arts and sciences
10.4 Miscellaneous units of time
11 References
12 Further reading
13 External links
13.1 Perception of time
13.2 Physics
13.3 Philosophy
13.4 Timekeeping
13.5 Miscellaneous
[edit]Temporal measurement
Temporal measurement, or chronometry, takes two distinct period forms: the calendar, a mathematical abstraction for calculating extensive periods of time,[12] and the clock, a physical 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. Increasingly, personal electronic devices display both calendars and clocks simultaneously. The number (as on a clock dial or calendar) that marks the occurrence of a specified event as to hour or date is obtained by counting from a fiducial epoch — a central reference point.
[edit]History of the calendar
Main article: Calendar
Artifacts from the Palaeolithic suggest that the moon was used to calculate time as early as 6,000 years ago.[13] Lunar calendars were among the first to appear, either 12 or 13 lunar months (either 346 or 364 days). Without intercalation to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. Lunisolar calendars have a thirteenth month added to some years to make up for the difference between a full year (now known to be about 365.24 days) and a year of just twelve lunar months. The numbers twelve and thirteen came to feature prominently in many cultures, at least partly due to this relationship of months to years.
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 by far the one in most common use around the world.
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