Editing Metric time

{{Short description|Measure of time intervals using the metric system}}
{{distinguish|Decimal time}}
{{Time sidebar}}

'''Metric time''' is the measure of time intervals using the [[metric system]]. The modern [[SI]] system defines the [[second]] as the [[SI base unit|base unit]] of time, and forms multiples and submultiples with [[metric prefix]]es such as kiloseconds and milliseconds. Other units of time – [[minute]], [[hour]], and [[day]] – are [[Non-SI units mentioned in the SI|accepted for use with SI]], but are not part of it. Metric time is a measure of time intervals, while [[decimal time]] is a means of recording [[time of day]].

==History==
{{Refimprove-section|date=October 2024}}
The second is derived from the [[sexagesimal]] system, which originated with the [[Sumer]]ians and [[Babylonia]]ns. This system divides a base unit into sixty minutes, each minute into sixty seconds, and each second into sixty [[Sexagesimal#Modern usage|tierces]].  The word "minute" comes from the Latin ''pars minuta prima'', meaning "first small part", and "second" from ''pars minuta secunda'' or "second small part".  [[Angular measure]] also uses sexagesimal units; there, it is the [[Degree (angle)|degree]] that is subdivided into minutes and seconds, while in time, it is the hour.

In 1790, French diplomat [[Charles Maurice de Talleyrand-Périgord]] proposed that the fundamental unit of length for the metric system should be the length of a [[pendulum]] with a one-second period, measured at sea level on the 45th parallel (50 grades in the new angular measures), thus basing the metric system on the value of the second. A Commission of Weights and Measures was formed within the French Academy of Sciences to develop the system. The commission rejected the seconds-pendulum definition of the [[metre]] the following year because the second of time was an arbitrary period equal to 1/86,400 day, rather than a decimal fraction of a natural unit. Instead, the metre would be defined as a decimal fraction of the length of the [[Paris Meridian]] between the [[equator]] and the [[North Pole]].<ref>{{cite book |last=Alder |first=Ken |url=https://archive.org/details/measureofallthin00alde |title=The Measure of All Things : The Seven-Year Odyssey and Hidden Error That Transformed the World |publisher=Free Press |year=2002 |pages=291–323|isbn=978-0-7432-1675-3 |location=New York |url-access=registration}}</ref><ref name="Meter">{{Cite web |last=Schwartz |first=Randy K. |date=September 2008 |title=The Birth of the Meter |url=https://www.maa.org/sites/default/files/images/upload_library/22/Schwartz.pdf |access-date=4 April 2023 |website=The Mathematical Association of America}}</ref><ref>{{Cite book |last=Commission des poids et mesures |url=https://gallicaintramuros.bnf.fr/ark:/12148/bpt6k10402036.image |title=Instruction abrégée sur les mesures déduites de la grandeur de la Terre, uniformes pour toute la République, et sur les calculs relatifs à leur division décimale; par la Commission temporaire des poids & mesures républicaines, en exécution des décrets de la Convention nationale. Édition originale. |publisher=de l'imprimerie nationale exécutive du Louvre (A Paris) |year=1793 |editor-last=Haüy |editor-first=René-Just |location=France |language=Fr}}</ref><ref>{{Cite book |last1=Frangsmyr |first1=Tore |url=https://publishing.cdlib.org/ucpressebooks/view?docId=ft6d5nb455;brand=ucpress |title=The Quantifying Spirit in the 18th Century |last2=Heilbron |first2=J. L. |last3=Rider |first3=Robin E. |publisher=University of California Press |year=1990 |location=Berkeley}}</ref><ref>{{Cite book |last1=de Borda |first1=Jean-Charles |url=https://openlibrary.org/books/OL25945285M/Rapport_sur_le_choix_d%27une_unite_de_mesure |title=Rapport sur le choix d'une unité de mesure |last2=de Condorcet |first2=Nicolas |publisher=L'IMPRIMERIE NATIONALE |year=1791 |location=Paris |ol=25945285M |language=fr}}</ref>

The commission initially proposed the decimal time units later enacted as part of the new [[French Republican calendar|Republican calendar]]. In January, 1791, [[Jean-Charles de Borda]] commissioned Louis Berthoud to manufacture a decimal chronometer displaying these units. On March 28, 1794, the commission's president, [[Joseph Louis Lagrange]], proposed using the day (French ''jour'') as the base unit of time, with divisions ''[[Deci|déci]]-jour'' and ''[[centi]]-jour,'' and suggested representing 4 ''déci-jours'' and 5 ''centi-jours'' as "4,5", "4/5", or just "45".<ref>{{cite book |url=https://books.google.com/books?id=JlsoAAAAMAAJ |title=Procès-verbaux du Comité d'instruction publique de la Convention nationale |year=1897 |page=605}}</ref> The final system, as introduced in 1795, included units for length, area, dry volume, liquid capacity, weight or mass, and currency, but not time. [[Decimal time of day]] had been introduced in France two years earlier, but mandatory use was suspended at the same time the metric system was inaugurated, and did not follow the metric pattern of a base unit and prefixed units.

Base units equivalent to decimal divisions of the day, such as 1/10, 1/100, 1/1,000, or 1/100,000 day, or other divisions of the day, such as 1/20 or 1/40 day, have also been proposed, with various names. Such alternative units did not gain any notable acceptance. In China, during the [[Song dynasty]], a day was divided into smaller units, called ''kè'' ({{zh|labels=no|c={{linktext|刻}}}}). One ''kè'' was usually defined as {{frac|100}} of a day until 1628, though there were short periods before then where days had 96, 108 or 120 ''kè''.<ref name="Units">{{Cite journal|last1=Sôma|first1=Mitsuru|last2=Kawabata|first2=Kin-aki|last3=Tanikawa|first3=Kiyotaka|date=2004-10-25|title=Units of Time in Ancient China and Japan|url=http://pasj.oxfordjournals.org/content/56/5/887|journal=Publications of the Astronomical Society of Japan|language=en|volume=56|issue=5|pages=887–904|doi=10.1093/pasj/56.5.887|issn=0004-6264|bibcode=2004PASJ...56..887S|url-access=subscription}}</ref>  A kè is about 14.4 minutes, or 14 minutes 24 seconds. In the 19th century, Joseph Charles François de Rey-Pailhade endorsed Lagrange’s proposal of using ''centijours,'' but abbreviated ''cé'', and divided into 10 ''decicés'', 100 ''centicés'', 1,000 ''millicés'',<ref>{{usurped|1=[https://web.archive.org/web/20150522083043/http://heuresdumonde.com/histoire-dheure.html?start=2 Histoire d'heure - Fractionnement du temps]}}</ref> and 10,000 ''dimicés''.<ref>{{cite web| url = https://books.google.com/books?id=FTsCAAAAYAAJ&pg=PA189| title = AJB, Volume 9, 1907| date = 1908}}</ref><ref>{{cite web| url = https://books.google.com/books?id=xMgBAAAAYAAJ&pg=PA255| title = Report of the Sixth International Geographical Congress: Held in London, 1895| date = 1896}}</ref>

[[James Clerk Maxwell]] and [[Elihu Thomson]] (through the [[British Association for the Advancement of Science]], or BAAS) introduced the [[Centimetre gram second system of units]] in 1874 to derive electric and magnetic metric units, following the recommendation of [[Carl Friedrich Gauss]] in 1832.

In 1897, the ''Commission de décimalisation du temps'' was created by the French [[Bureau of Longitude]], with the mathematician [[Henri Poincaré]] as secretary. The commission proposed making the standard [[hour]] the base unit of metric time, but the proposal did not gain acceptance and was eventually abandoned.<ref>{{cite web| url = https://books.google.com/books?id=ISBN0393326047| title = Einstein's Clocks, Poincare's Maps: empires of time By Peter Louis Galison}}</ref>

When the modern [[SI]] system was defined at the 10th [[Conférence Générale des Poids et Mesures|General Conference on Weights and Measures]] (CGPM) in 1954, the ephemeris second (1/86400 of a mean solar day) was made one of the system's base units. Because the Earth's rotation is slowly decelerating at an irregular rate and was thus unsuitable as a reference point for precise measurements, the SI second was later redefined more precisely as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the [[ground state]] of the [[caesium]]-133 atom. The international standard [[atomic clock]]s use caesium-133 measurements as their main benchmark.

==In computing==
{{Unreferenced section|date=October 2024}}
In computing, at least internally, metric time gained widespread use for ease of computation. [[Unix time]] gives date and time as the number of seconds since January 1, 1970, and Microsoft's [[NTFS]] FILETIME as multiples of 100&nbsp;ns since January 1, 1601. [[OpenVMS#Timekeeping|VAX/VMS]] uses the number of 100&nbsp;ns since November 17, 1858, and [[RISC OS]] the number of centiseconds since January 1, 1900. [[Microsoft Excel]] uses number of days (with decimals, [[Floating-point arithmetic|floating point]]) since January 1, 1900.

All these systems present time for the user using traditional units. None of these systems is strictly linear, as they each have discontinuities at [[leap second]]s.

==Prefixes==
{{Unreferenced section|date=October 2024}}
Metric prefixes for subdivisions of a second are commonly used in science and technology. Milliseconds and microseconds are particularly common. Prefixes for multiples of a second are rarely used:
{| class="wikitable" style="text-align: center; margin-left: 2.5%; table-layout: fixed"
! Multiple !! Name of unit !! Seconds !! In common units
|-
| 10<sup>1</sup> || decasecond || 10 || 0.17 minutes
|-
| 10<sup>2</sup> || hectosecond || 100 || 1.67 minutes (or 1 minute 40 seconds)
|-
| 10<sup>3</sup> || kilosecond || {{val|1000|fmt=gaps}} || 16.7 minutes (or 16 minutes and 40 seconds)
|-
| 10<sup>6</sup> || megasecond || {{val|1000000}}  || 11.6 days (or 11 days, 13 hours, 46 minutes and 40 seconds)
|-
| 10<sup>9</sup> || gigasecond || {{val|1000000000}} || 31.7 years (or 31 years, 252 days, 1 hour, 46 minutes, 40 seconds, assuming that there are 7 leap years in the interval)
|}

==See also==
* [[List of unusual units of measurement#Time]], under which prefixed multiples of the second are included
* [[Soviet calendar]]

==References==
{{reflist}}

==External links==
* [http://www.bipm.org/en/si/si_brochure/chapter2/2-1/second.html Metric unit of time (second)] Official text of SI brochure from International Bureau of Weights and Measures
* [http://van.physics.uiuc.edu/qa/listing.php?id=811 Metric Time?] University of Illinois Physics Department

{{Time Topics}}
{{Time measurement and standards}}

{{DEFAULTSORT:Metric Time}}
[[Category:International System of Units]]
[[Category:Time measurement systems]]
[[Category:Decimal time]]

[[de:Dezimalzeit]]