Decimal time
|
|
Decimal time is the representation of the time of day using units which are decimally related. This term often is used to refer specifically to French Revolutionary Time, which divides the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal seconds, as opposed to the more familiar standard time, which divides the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds.
| Contents |
History
Egypt
Decimal time was said to have been introduced in ancient Egypt by Thoth. The day was divided into 10 parts, each part into 100 subdivisions, each further divided by 100, making 100,000 divisions in a day. Additionally, each Egyptian month was divided into three periods of 10 days called decans, and each of the 36 decans of the year was represented by a different constellation.
China
For its entire recorded history of two or three millennia decimal time had been used in China alongside duodecimal time. The day was divided into both 100 parts called ke and into twelve double hours called shi. To make ke compatible with shi, each ke was subdivided into 60 fen. Jesuits introduced Western time into China during the 17th century, at which time the day was redefined as having 96 ke (as well as 12 shi).
France
In more modern times, decimal time was introduced during the French Revolution in the decree of 5 October 1793:
- XI. Le jour, de minuit à minuit, est divisé en dix parties ou heures, chaque partie en dix autres, ainsi de suite jusqu’à la plus petite portion commensurable de la durée.
To which was added, on 24 November 1793 (4 Frimaire II):
- La centième partie de l'heure est appelée minute décimale; la centième partie de la minute est appelée seconde décimale. (emphasis in original)
All this means, in English:
- XI. The day, from midnight to midnight, is divided into ten parts or hours, each part into ten others, so on until the smallest measurable portion of duration.
- The hundredth part of the hour is called decimal minute; the hundredth part of the minute is called decimal second.
Thus, midnight was reckoned as 10 o'clock, noon as 5 o'clock, etc. Although clocks and watches were produced with faces showing both standard time with numbers 1-24 and decimal time with numbers 1-10, decimal time never caught on and was abandoned in 1795 after less than two years. The French Republican Calendar, which was introduced at the same time and divided the month into three décades of 10 days each, lasted until 1805. The original metric system, also adopted during the Revolution, beginning in 1790, had no unit of time; later versions of the metric system used the second equal to 1/86400 day as the metric time unit.
Conversions
There are 86400 true seconds in a day, but 100000 decimal seconds in the same day, so the decimal second is shorter than its counterpart.
- One decimal second is 86400/100000 = 0.864 true seconds.
- One decimal minute is 1440/1000 = 1.44 true minutes.
- One decimal hour is 24/10 = 2.4 true hours.
- The average adult human heart rate is close to 1 beat per decimal second, or 100 beats per decimal minute.
Fractional days
The most common use of decimal time today is as fractional days used by scientists and computer programmers. Standard 24-hour time is converted into a fractional day simply by dividing the number of hours elapsed since midnight by 24 to make a decimal fraction. Thus, midnight is 0.0 day, noon is 0.5 d, etc., which can be added to any type of date, including:
- Gregorian dates: 2000 January 1.5
- ordinal dates: 00001.5
- Julian dates: 2451545.0
- Excel serial dates: 36526.5
As many decimal places may be used as required for precision, so 0.5 d = 0.500000 d. Fractional days are often reckoned in UTC or TT, although Julian Dates use Astronomical Time (TT+12h) and Excel uses the local time zone of the computer. Using fractional days reduces the number of units in time calculations from four (days, hours, minutes, seconds) to just one (days). Fractional days are often used by astronomers to record observations, and were described in relation to the time of day by the 19th century astronomer John Herschel in his book, Outlines of Astronomy (http://visualiseur.bnf.fr/Visualiseur?Destination=Gallica&O=NUMM-94926), as in these examples:
- Between Greenwich noon of the 22d and 23d of March, 1829, the 1828th equinoctial year terminates, and the 1829th commences. This happens at 0d·286003, or at 6h 51m 50s·66 Greenwich mean time...For example, at 12h 0m 0s Greenwich mean time, or 0d·500000...
Swatch Internet Time
On October 23, 1998, the Swiss watchmaking company, Swatch, introduced a decimal time called Swatch Internet Time, which divides the day into 1000 .beats counted from 000-999, with @000 being midnight and @500 being noon CET (UTC +1), as opposed to UTC. The company sells watches which display Internet Time. Internet Time has been criticized for using an origin different from Universal Time, and for not providing for more precise units, although third-party applications have proposed "centibeats" and "millibeats".
Other decimal times
Numerous individuals have proposed variations of decimal time, dividing the day into different numbers of units and subunits with different names. Most are based upon fractional days, so that one decimal time format may be easily converted into another, such that all the following are equivalent:
- 0.500 fractional day
- 5:00 French decimal time
- @500 Swatch Internet Time
- 500 millidays
- 50.0% Percent Time
Some decimal time proposals are based upon alternate units of metric time. The difference between metric time and decimal time is that metric time defines units for measuring time interval, as measured with a stopwatch, and decimal time defines the time of day, as measured by a clock. Just as standard time uses the metric time unit of the second as its basis, proposed decimal time scales may use alternative metric units.
See also
External links
- Decimal Time (http://www.decimaltime.hynes.net/)
References
- National Convention of the French Republic (1793) LE CALENDRIER RÉPUBLICAIN Textes officiels Décrets Relatifs à l'établissement de l'Ère Républicaine (http://pchapelin.free.fr/calrep/decrets.htm) published by Philippe Chapelin 2002
- Sizes, Inc. (2000) decimal time units (http://hypertextbook.com/physics/foundations/time/) Last revised February 27 2004
- Herschel, John (1849) Outlines of Astronomy (http://visualiseur.bnf.fr/Visualiseur?Destination=Gallica&O=NUMM-94926) published by Gallica 1995de:Dezimalzeit