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Metric time is the measure of time intervals using themetric system. The modernSI system defines thesecond as thebase unit of time, and forms multiples and submultiples withmetric prefixes such as kiloseconds and milliseconds. Other units of time –minute,hour, andday – areaccepted for use with SI, but are not part of it. Metric time is a measure of time intervals, whiledecimal time is a means of recordingtime of day.
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The second is derived from thesexagesimal system, which originated with theSumerians andBabylonians. This system divides a base unit into sixty minutes, each minute into sixty seconds, and each second into sixtytierces. The word "minute" comes from the Latinpars minuta prima, meaning "first small part", and "second" frompars minuta secunda or "second small part".Angular measure also uses sexagesimal units; there, it is thedegree that is subdivided into minutes and seconds, while in time, it is the hour.
In 1790, French diplomatCharles Maurice de Talleyrand-Périgord proposed that the fundamental unit of length for the metric system should be the length of apendulum 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 themetre 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 theParis Meridian between theequator and theNorth Pole.[1][2][3][4][5]
The commission initially proposed the decimal time units later enacted as part of the newRepublican 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 (Frenchjour) as the base unit of time, with divisionsdéci-jour andcenti-jour, and suggested representing 4déci-jours and 5centi-jours as "4,5", "4/5", or just "45".[6] 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 theSong dynasty, a day was divided into smaller units, calledkè (刻). Onekè was usually defined as1⁄100 of a day until 1628, though there were short periods before then where days had 96, 108 or 120kè.[7] 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 usingcentijours, but abbreviatedcé, and divided into 10decicés, 100centicés, 1,000millicés,[8] and 10,000dimicés.[9][10]
James Clerk Maxwell andElihu Thomson (through theBritish Association for the Advancement of Science, or BAAS) introduced theCentimetre gram second system of units in 1874 to derive electric and magnetic metric units, following the recommendation ofCarl Friedrich Gauss in 1832.
In 1897, theCommission de décimalisation du temps was created by the FrenchBureau of Longitude, with the mathematicianHenri Poincaré as secretary. The commission proposed making the standardhour the base unit of metric time, but the proposal did not gain acceptance and was eventually abandoned.[11]
When the modernSI system was defined at the 10thGeneral 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 theground state of thecaesium-133 atom. The international standardatomic clocks use caesium-133 measurements as their main benchmark.
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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'sNTFS FILETIME as multiples of 100 ns since January 1, 1601.VAX/VMS uses the number of 100 ns since November 17, 1858, andRISC OS the number of centiseconds since January 1, 1900.Microsoft Excel uses number of days (with decimals,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 atleap seconds.
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:
| Multiple | Name of unit | Seconds | In common units |
|---|---|---|---|
| 101 | decasecond | 10 | 10 seconds |
| 102 | hectosecond | 100 | 1 minute 40 seconds |
| 103 | kilosecond | 1000 | 16 minutes and 40 seconds |
| 106 | megasecond | 1000000 | 11.6 days |
| 109 | gigasecond | 1000000000 | 31.7 years |
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