Metric time
From Wikipedia, the free encyclopedia
Metric time is the measure of time interval using the metric system, which defines the second as the base unit of time, and multiple and submultiple units formed with metric prefixes, such as kiloseconds and milliseconds. It does not define the time of day, as this is defined by various time scales, which may be based upon the metric definition of the second. Other units of time, the minute, hour, and day, are accepted for use with the modernized metric system, but are not part of it.
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[edit] History
When the metric system was introduced in France in 1795, it included units for length, area, dry volume, liquid capacity, weight or mass, and even currency, but not for time. Decimal time of day had been introduced in France two years earlier, but was set aside at the same time the metric system was inaugurated, and did not follow the metric pattern of a base unit and prefixed units. James Clerk Maxwell and Elihu Thomson (through the British Association for the Advancement of Science - BAAS) introduced the Centimetre gram second system of units (cgs) in 1874, in order to derive electric and magnetic metric units, following the recommendation of Carl Friedrich Gauss in 1832.
The ephemeris second (defined as 1/86400 of a mean solar day) was made one of the original base units of the modernized metric system, or International System of Units (SI), at the 10th General Conference on Weights and Measures (CGPM) in 1954. 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.
[edit] Alternative units
Numerous proposals have been made for alternative base units of metric time. An obvious unit is the day, with multiple and submultiple units such as decidays, centidays, and millidays. Base units equivalent to decimal divisions of the day, such as 1/10, 1/100, 1/1000, or 1/100,000 day, or other divisions of the day, such as 1/20 or 1/40 day, have also been proposed, with names such as tick, meck, chi, chron, moment, etc., and multiple and submultiple units formed with metric prefixes. Such alternative units have not gained any notable acceptance, however, mostly from sheer lack of acquaintance and familiarity.
A modified second = 1/100 000 of a day = 0.864 s could be a viable alternative. Any redefinition of the second, however, creates conflicts with anything based on its precise current definition. Another unit for time, more familiar than some other suggestions, could be 14.4 minutes, i.e. a shorter quarter of an hour, or a centiday. The centiday was used in China (called ke in Chinese) for thousands of years, until the Jesuits had it redefined from 1/100th of a day to 1/96th of a day in the 17th century.
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.[1]
[edit] Alternative meaning
Metric time is sometimes used to mean decimal time. Metric time properly refers to measurement of time interval, while decimal time refers to the time of day. Standard time of day is defined by various time scales, such as UTC, which are now usually based upon the metric base unit of time, the second. Some proposals for alternative units of metric time are accompanied by decimal time scales for telling the time of day based upon these alternative units. Other proposals called "metric time" refer only to decimal time, and therefore are not truly metric.
French decimal time is sometimes called "metric time" because it was introduced around the same time as the metric system and both were decimal, but it was not part of the decree creating the original metric system and its units were named for the hour, minute and second, instead of using metric prefixes. Other decimal time standards, such as Swatch Internet Time, are not considered metric time.
[edit] In computing
In computing, at least internally, metric time gained widespread use for ease of computation. Unix time gives date and time as number of seconds since January 1, 1970, Microsoft's FILETIME as multiples of 100ns since January 1, 1601 [2]. VAX/VMS uses the number of 100ns since November 17, 1858.
[edit] References
- ^ Einstein's Clocks, Poincare's Maps: empires of time By Peter Louis Galison
- ^ FILETIME documentation on MSDN
[edit] External links
- Metric unit of time (second) Official text of SI brochure from International Bureau of Weights and Measures
- Metric Time? University of Illinois Physics Department
- Decimal Time - Metric Time explains the difference between decimal time and metric time
