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'''Time''' is the never-ending continued [[sequence|
To measure time, we can use anything that repeats itself regularly. One [[example]] is the start of a new [[day]] (as [[Earth]] [[rotation|rotates]] on its axis). Two more are the phases of the [[moon]] (as it orbits the Earth), and the seasons of the [[year]] (as the Earth orbits the [[Sun]]).
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Even in ancient times, people developed [[calendar]]s to keep track of the number of days in a year. They also developed [[sundial]]s that used the moving shadows cast by the sun through the day to measure times smaller than a day. Today, highly accurate [[clock]]s can measure time in less than a billionth of a second. The study of time [[measurement]] is known as [[horology]].
The [[SI]] (International Systems of Units) unit of time is one [[second]], written as '''s'''.<ref>{{Cite web|title=Current definitions of the SI units|url=https://physics.nist.gov/cuu/Units/current.html#:~:text=The%20second,%20symbol%20s,%20is,is%20equal%20to%20s-1.|access-date=2020-08-16|website=physics.nist.gov|archive-date=2020-09-20|archive-url=https://web.archive.org/web/20200920012307/https://www.physics.nist.gov/cuu/Units/current.html#:~:text=The%20second,%20symbol%20s,%20is,is%20equal%20to%20s-1.|url-status=live}}</ref> When used as a variable in mathematics, time is often represented by the symbol <math>t</math>.<ref>{{Cite web|date=2020-03-01|title=Compendium of Mathematical Symbols|url=https://mathvault.ca/hub/higher-math/math-symbols/|access-date=2020-08-16|website=Math Vault|language=en-US|archive-date=2020-04-28|archive-url=https://web.archive.org/web/20200428101652/https://mathvault.ca/hub/higher-math/math-symbols/|url-status=live}}</ref>
In [[Einstein]]ian physics, time and space can be combined into a single concept. For more on the topic, see [[space-time continuum]].
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* 1 [[femtosecond]] = 1/1,000,000,000,000,000 of a second
* 1 [[attosecond]] = 1/1,000,000,000,000,000,000 of a second
* 1 [[Planck time]] =
=== Things used to measure time ===
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* [[Chronology]]
* [[Chronos]]
* [[24-hour clock]]
* [[Time zone]]
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== References ==
<references />
{{Metaphysics}}
[[Category:Basic English 850 words]]
[[Category:Philosophy]]
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