Editing Time (section)

== Measurement ==
[[File:Wooden hourglass 3.jpg|thumb|upright=0.8|The flow of sand in an [[hourglass]] can be used to measure the passage of one [[hour]] of time. It also concretely represents the present as being between the past and the future.]]
	
Methods of temporal measurement, or [[chronometry]], generally take two forms. The first is a [[calendar]], a mathematical tool for organising intervals of time on Earth,<ref name="Richards">{{cite book |title=Mapping Time: The Calendar and its History |url=https://archive.org/details/mappingtimecalen00rich_059 |url-access=limited |last=Richards |first=E. G. |date=1998 |pages=[https://archive.org/details/mappingtimecalen00rich_059/page/n23 3]–5 |publisher=[[Oxford University Press]]|isbn=978-0-19-850413-9 }}</ref> consulted for periods longer than a day. The second is a [[clock]], a physical mechanism that indicates the passage of time, consulted for periods less than a day. The combined measurement marks a specific moment in time from a reference point, or ''[[epoch]]''.

=== History of the calendar ===
{{Main|Calendar}}

Artifacts from the [[Paleolithic]] suggest that the moon was used to reckon time as early as 6,000 years ago.<ref name="Rudgley">{{cite book |title=The Lost Civilizations of the Stone Age |last=Rudgley |first=Richard |author-link=Richard Rudgley |date=1999 |pages=86–105 |publisher=Simon & Schuster |location=New York}}
</ref>{{Disputed inline|Talk page section|date=January 2025}} [[Lunar calendar]]s were among the first to appear, with years of either 12 or 13 [[lunar month]]s (either 354 or 384 days). Without [[Intercalation (timekeeping)|intercalation]] to add days or months to some years, seasons quickly drift in a calendar based solely on twelve lunar months. [[Lunisolar calendar]]s 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.

Other early forms of calendars originated in [[Mesoamerica]], particularly in ancient Mayan civilization, in which they developed the [[Maya calendar]], consisting of multiple interrelated calendars. These calendars were religiously and astronomically based; the [[Haabʼ|Haab']] calendar has 18 months in a year and 20 days in a month, plus five [[epagomenal]] days at the end of the year.<ref>{{cite journal | last1 = Van Stone | first1 = Mark | year = 2011 | title = The Maya Long Count Calendar: An Introduction | journal = Archaeoastronomy | volume = 24 | pages = 8–11 }}</ref> In conjunction, the Maya also used a 260-day sacred calendar called the [[Tzolkʼin|Tzolk'in]].<ref>{{Cite web |last=Davies |first=Diane |title=The Maya Calendar Explained |url=https://www.mayaarchaeologist.co.uk/public-resources/maya-world/maya-calendar-system/ |access-date=2025-02-26 |website=Maya Archaeologist |language=en-GB |archive-date=26 February 2025 |archive-url=https://web.archive.org/web/20250226110722/https://www.mayaarchaeologist.co.uk/public-resources/maya-world/maya-calendar-system/ |url-status=live }}</ref>

The reforms of [[Julius Caesar]] in 45&nbsp;BC put the [[Roman Empire|Roman world]] on a [[solar calendar]]. This [[Julian calendar]] was faulty in that its intercalation still allowed the astronomical [[solstice]]s and [[equinox]]es to advance against it by about 11&nbsp;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 it is now by far the most commonly used calendar around the world.

During the [[French Revolution]], a new clock and calendar were invented as part of the [[Dechristianization of France during the French Revolution|dechristianization of France]] and to create a more rational system in order to replace the Gregorian calendar. The [[French Republican Calendar]]'s days consisted of ten hours of a hundred minutes of a hundred seconds, which marked a deviation from the base 12 ([[duodecimal]]) system used in many other devices by many cultures. The system was abolished in 1806.<ref>{{Cite web |title=French republican calendar {{!}} Revolutionary period, decimal system, reform |url=https://www.britannica.com/science/French-republican-calendar |access-date=2025-02-26 |website=Britannica |language=en |archive-date=8 March 2025 |archive-url=https://web.archive.org/web/20250308214916/https://www.britannica.com/science/French-republican-calendar |url-status=live }}</ref>

=== History of other devices ===
[[File:Canberra (AU), Albert Hall, Sundial -- 2019 -- 1740.jpg|thumb|right|Horizontal [[sundial]] in Canberra]]
[[File:Florence-Duomo-Clock.jpg|thumb|24-hour clock face in Florence]]
{{Main|History of timekeeping devices}}
{{See also|Clock}}

A large variety of [[Measuring instrument|devices]] have been invented to measure time. The study of these devices is called [[horology]].<ref>{{Cite web | url=https://nawcc.org/index.php/main-education | title=Education | access-date=1 July 2018 | archive-date=1 May 2019 | archive-url=https://web.archive.org/web/20190501213902/https://nawcc.org/index.php/main-education | url-status=live }}</ref> 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.

A [[sundial]] is any device that uses the direction of sunlight to cast shadows from a [[gnomon]] onto a set of markings calibrated to indicate the [[local time]], usually to the hour. The idea to separate the day into smaller parts is credited to Egyptians because of their sundials, which operated on a duodecimal system. The importance of the number 12 is due to the number of lunar cycles in a year and the number of stars used to count the passage of night.<ref>{{Cite web |last=Lombardi |first=Michael A. |date=March 5, 2007 |title=Why is a minute divided into 60 seconds, an hour into 60 minutes, yet there are only 24 hours in a day? |url=https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/ |access-date=2025-02-26 |website=Scientific American |language=en |archive-date=20 September 2023 |archive-url=https://web.archive.org/web/20230920130907/https://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/ |url-status=live }}</ref> [[Obelisk|Obelisks]] made as a gnomon were built as early as {{Circa|3500 BC}}.<ref>{{Cite journal |date=2009-08-12 |title=A Walk Through Time - Early Clocks |url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/walk-through-time/walk-through-time-early-clocks |journal=National Institute of Standards and Technology |language=en |archive-date=2 August 2016 |access-date=26 February 2025 |archive-url=https://web.archive.org/web/20160802075828/http://www.nist.gov/pml/general/time/early.cfm |url-status=live }}</ref> An Egyptian device that dates to {{Circa|1500 BC}}, similar in shape to a bent [[T-square]], also measured the passage of time from the shadow cast by its crossbar on a nonlinear 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.<ref>{{cite book |author1=Barnett |first=Jo Ellen |url=https://books.google.com/books?id=-ygDdx73hvcC |title=Time's Pendulum: From Sundials to Atomic Clocks, the Fascinating History of Timekeeping and how Our Discoveries Changed the World |publisher=Harcourt Brace |year=1999 |isbn=978-0-15-600649-1 |edition=reprinted |page=28}}</ref>

Alarm clocks reportedly first appeared in ancient Greece {{Circa|250 BC}} with a water clock made by [[Plato]] that would set off a whistle.<ref>{{Cite book |last=Wagner |first=M. F. |title=The Enigmatic Reality of Time |date=2008 |publisher=Koninklijke Brill |isbn=978-90-474-4360-5 |location=Leiden}}</ref> The hydraulic alarm worked by gradually filling a series of vessels with water. After some time, the water emptied out of a [[siphon]].<ref>{{Cite web |date=2023-11-28 |title=The alarm clock of Plato (5th c. B.C.) - The first awakening device in human history |url=https://kotsanas.com/en/the-alarm-clock-of-plato-5th-c-b-c-the-first-awakening-device-in-human-history/ |access-date=2025-02-25 |website=Museum of the Ancient Greek Technology |language=en-GB}}</ref> Inventor [[Ctesibius]] revised Plato's design; the water clock uses a float as the power drive system and uses a sundial to correct the water flow rate.<ref>{{Cite journal |last1=Hwang |first1=Zheng-Hui |last2=Yan |first2=Hong-Sen |last3=Lin |first3=Tsung-Yi |date=2021-02-19 |title=Historical development of water-powered mechanical clocks |url=https://ms.copernicus.org/articles/12/203/2021/ |journal=Mechanical Sciences |language=English |volume=12 |issue=1 |pages=203–219 |doi=10.5194/ms-12-203-2021 |doi-access=free |bibcode=2021MecSc..12..203H |issn=2191-9151}}</ref>

In medieval philosophical writings, the atom was a unit of time referred to as the smallest possible division of time. The earliest known occurrence in English is in [[Byrhtferth]]'s ''Enchiridion'' (a science text) of 1010–1012,<ref>{{cite encyclopedia |year=2008 |title=Byrhtferth of Ramsey |encyclopedia=[[Encyclopædia Britannica]] |url=http://search.eb.com/eb/article-9438957 |access-date=15 September 2008 |url-status=live |archive-url=https://web.archive.org/web/20200614055906/https://academic.eb.com/?target=%2Flevels |archive-date=14 June 2020}}</ref> where it was defined as 1/564 of a ''momentum'' (1{{frac|1|2}}&nbsp;minutes),<ref>"atom", [[Oxford English Dictionary]], Draft Revision September 2008 (contains relevant citations from Byrhtferth's ''Enchiridion'')</ref> and thus equal to 15/94 of a second. It was used in the ''[[computus]]'', the process of calculating the date of Easter. The most precise timekeeping device of the [[ancient world]] was the [[water clock]], or ''clepsydra'', one of which was found in the tomb of Egyptian pharaoh [[Amenhotep I]]. They could be used to measure the hours even at night but required manual upkeep to replenish the flow of water. The [[Ancient Greece|ancient Greeks]] and the people from [[Chaldea]] (southeastern Mesopotamia) regularly maintained timekeeping records as an essential part of their astronomical observations. Arab inventors and engineers, in particular, made improvements on the use of water clocks up to the Middle Ages.<ref>Barnett, ''ibid'', p. 37.</ref> In the 11th century, [[List of Chinese inventions|Chinese inventors]] and [[History of science and technology in China|engineers]] invented the first mechanical clocks driven by an [[escapement]] mechanism.

[[File:Swatch Irony angle below.jpg|thumb|A contemporary [[quartz watch]], 2007 ]]
Incense sticks and candles were, and are, commonly used to measure time in temples and churches across the globe. Water clocks, and, later, mechanical clocks, were used to mark the events of the abbeys and monasteries of the Middle Ages. The passage of the hours at sea can also be marked by [[Ship's bell|bell]]. The hours were marked by bells in abbeys as well as at sea. [[Richard of Wallingford]] (1292–1336), abbot of St. Alban's abbey, famously built a mechanical clock as an astronomical [[orrery]] about 1330.<ref>North, J. (2004) ''God's Clockmaker: Richard of Wallingford and the Invention of Time''. Oxbow Books. {{isbn|1-85285-451-0}}</ref><ref>Watson, E., (1979) "The St Albans Clock of Richard of Wallingford". ''Antiquarian Horology,'' pp. 372–384.</ref> The [[hourglass]] uses the flow of sand to measure the flow of time. They were also used in navigation. [[Ferdinand Magellan]] used 18 glasses on each ship for his circumnavigation of the globe (1522).<ref>Bergreen, Laurence. ''Over the Edge of the World: Magellan's Terrifying Circumnavigation of the Globe'' (HarperCollins Publishers, 2003), {{isbn|0-06-621173-5}}{{page needed|date=October 2018}}</ref> The English word [[clock]] probably comes from the Middle Dutch word ''klocke'' which, in turn, derives from the medieval Latin word ''clocca'', which ultimately derives from Celtic and is cognate with French, Latin, and German words that mean [[Bell (instrument)|bell]].

Great advances in accurate time-keeping were made by [[Galileo Galilei]] and especially [[Christiaan Huygens]] with the invention of pendulum-driven clocks along with the invention of the minute hand by Jost Burgi.<ref name="Clocks 2016">"History of Clocks." About.com Inventors. About.com, n.d. Web. 21 February 2016.</ref> There is also a clock that was designed to keep time for 10,000 years called the [[Clock of the Long Now]]. Alarm clock devices were later mechanized. [[Levi Hutchins]]{{'s}} alarm clock has been credited as the first American alarm clock, though it can only ring at 4 a.m. [[Antoine Rédier|Antoine Redier]] was also credited as the first person to patent an adjustable mechanical alarm clock in 1847.<ref>{{Cite web |last=Tikkanen |first=Amy |date=15 January 2009 |title=22 Questions About Time and Timekeeping Answered |url=https://www.britannica.com/list/22-questions-about-time-and-timekeeping-answered |access-date=2025-02-25 |website=Britannica |language=en |archive-date=12 March 2025 |archive-url=https://web.archive.org/web/20250312000400/https://www.britannica.com/list/22-questions-about-time-and-timekeeping-answered |url-status=live }}</ref> Digital forms of alarm clocks became more accessible through digitization and integration with other technologies, such as [[Smartphone|smartphones]].

[[File:ChipScaleClock2 HR.jpg|thumb|Chip-scale [[atomic clock]]s, such as this one unveiled in 2004, are expected to greatly improve [[Global Positioning System|GPS]] location.<ref>{{cite journal |url=https://www.nist.gov/public_affairs/releases/miniclock.cfm |title=NIST Unveils Chip-Scale Atomic Clock |journal=NIST |date=27 August 2004 |access-date=9 June 2011 |url-status=live |archive-url=https://web.archive.org/web/20110522125607/http://www.nist.gov/public_affairs/releases/miniclock.cfm |archive-date=22 May 2011  }}</ref>]]

The most accurate timekeeping devices are [[atomic clock]]s, which are accurate to seconds in many millions of years,<ref>{{cite news |url=http://www.canada.com/vancouversun/news/story.html?id=e24ccfa7-44eb-40b7-8b67-daf8263569ff |archive-url=https://web.archive.org/web/20120211190407/http://www.canada.com/vancouversun/news/story.html?id=e24ccfa7-44eb-40b7-8b67-daf8263569ff |url-status=dead |archive-date=11 February 2012 |title=New atomic clock can keep time for 200 million years: Super-precise instruments vital to deep space navigation |date=16 February 2008 |work=Vancouver Sun |access-date=9 April 2011}}</ref> and are used to calibrate other clocks and timekeeping instruments. Atomic clocks use the frequency of [[electronic transition]]s in certain atoms to measure the second. One of the atoms used is [[caesium]]; most modern atomic clocks probe caesium with microwaves to determine the frequency of these electron vibrations.<ref>{{cite web |url=https://www.nist.gov/public_affairs/releases/n99-22.cf |title=NIST-F1 Cesium Fountain Clock |access-date=24 July 2015 |archive-date=25 March 2020 |archive-url=https://web.archive.org/web/20200325220323/https://www.nist.gov/public_affairs/releases/n99-22.cf |url-status=dead }}</ref> Since 1967, the International System of Measurements bases its unit of time, the second, on the properties of caesium atoms. [[International System of Units|SI]] defines the second as 9,192,631,770 cycles of the radiation that corresponds to the transition between two electron spin energy levels of the ground state of the <sup>133</sup>Cs atom. A portable timekeeper that meets certain precision standards is called a [[chronometer watch|chronometer]]. Initially, the term was used to refer to the [[marine chronometer]], a timepiece used to determine [[longitude]] by means of [[celestial navigation]], a precision first achieved by [[John Harrison]]. More recently, the term has also been applied to the [[chronometer watch]], a watch that meets precision standards set by the Swiss agency [[COSC]].

In modern times, the [[Global Positioning System]] in coordination with the [[Network Time Protocol]] can be used to synchronize timekeeping systems across the globe. {{As of|May 2010}}, the smallest time interval uncertainty in direct measurements is on the order of 12 [[attosecond]]s (1.2&nbsp;×&nbsp;10<sup>−17</sup> seconds), about 3.7&nbsp;×&nbsp;10<sup>26</sup> [[Planck time]]s.<ref>{{cite web |url=http://www.physorg.com/news192909576.html |title=12 attoseconds is the world record for shortest controllable time |date=12 May 2010 |access-date=19 April 2012 |url-status=live |archive-url=https://web.archive.org/web/20110805173204/http://www.physorg.com/news192909576.html |archive-date=5 August 2011  }}
</ref> The time measured was the delay caused by out-of-sync electron waves' [[Wave interference|interference patterns]].<ref>{{Cite web |last=Andrewes |first=William J. H. |date=February 1, 2006 |title=A Chronicle Of Timekeeping |url=https://www.scientificamerican.com/article/a-chronicle-of-timekeeping-2006-02/ |access-date=2025-02-26 |website=Scientific American |language=en}}</ref>

=== Units ===
{{See also|Time (Orders of magnitude)|Unit of time#List}}

The second (s) is the [[SI]] base unit. A [[minute]] (min) is 60 seconds in length (or, rarely, 59 or 61 seconds when leap seconds are employed), and an [[hour]] is 60 minutes or 3600 seconds in length. A day is usually 24 hours or 86,400 seconds in length; however, the duration of a calendar day can vary due to [[daylight saving time]] and [[leap second]]s.