Decimal time

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French decimal clock from the time of the French Revolution
French decimal clock from the time of the French Revolution

Decimal time is the representation of the time of day using units which are decimally related. This term is often 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.

However modern systems of 'decimal time', like the open-source AAT ICAS standards (Integrated Chronological Applications System developed by Alliance for the Advancement of Technology), are expressed in terms of a controlled vocabulary that in many cases specify the use of schematic terms from metric prefixes rather than other non-decimal customary expressions. Thus, some previously used expressions such as 'decimal second' are regarded as non-normative, as normative uses of the term 'second' are reserved in both SI and ICAS frameworks as the 'SI second'.

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[edit] History

[edit] China

Main article: Ke (unit)

For its entire recorded history of two to three millennia, decimal time had been used in China alongside duodecimal time. The day was divided into both 100 parts called ke (Hanzi: 刻; Pinyin: kè) and into twelve double hours called shi (Traditional Chinese: 時辰; Simplified Chinese: 时辰; Pinyin: shíchen). 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). Additionally, each month was divided into three periods of 10 days called xun (Hanzi: 旬; Pinyin: xún). xun are still used in formal documents.

[edit] 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, chaque partie en dix autres, ainsi de suite jusqu’à la plus petite portion commensurable de la durée.
XI. The day, from midnight to midnight, is divided into ten parts, each part into ten others, so on until the smallest measurable portion of duration.

These parts were named on 24 November 1793 (4 Frimaire of the Year II). The primary divisions were called hours, and they added:

La centième partie de l'heure est appelée minute décimale; la centième partie de la minute est appelée seconde décimale. (emphasis in original)
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; it was not officially used until the beginning of the Republican year III, September 22, 1794, and was officially suspended April 7, 1795 (18 Germinal of the Year III), in the same law which introduced the original metric system. Thus, the metric system at first had no time unit, and later versions of the metric system used the second, equal to 1/86400 day, as the metric time unit.

Decimal time was introduced as part of the French Republican Calendar, which, in addition to decimally dividing the day, divided the month into three décades of 10 days each, and was abolished at the end of 1805. The start of each year was determined according to which day the autumnal equinox occurred, in relation to true or apparent solar time at the Paris Observatory. Decimal time would also have been reckoned according to apparent solar time, depending on the location it was observed, as was already the practice generally for the setting of clocks.

The French made another attempt at the decimalization of time in 1897, when the Commission de décimalisation du temps was created by the Bureau des Longitudes, with the mathematician Henri Poincaré as secretary. The commission proposed a compromise of retaining the 24-hour day, but dividing each hour into 100 decimal minutes, and each minute into 100 seconds. The plan did not gain acceptance and was abandoned in 1900.

[edit] Conversions

There are exactly 86,400 standard seconds (see SI for the current definition of the standard second) in a standard day, but in the French decimal time system there are 100,000 decimal seconds in the day, so the decimal second is shorter than its counterpart.

Decimal to Standard

  • One decimal second is 86,400/100,000 = 0.864 standard seconds.
  • One decimal minute is 1,440/1,000 = 1.44 standard minutes, or 1 standard minute and 26.4 standard seconds.
  • One decimal hour is 24/10 = 2.4 standard hours, or 2 standard hours and 24 standard minutes.

One hundredth of a day is 14 standard minutes 24 standard seconds, or approximately 15 minutes.

Standard to Decimal

  • One standard second = 1.15740 decimal seconds
  • One standard minute = 69.44 decimal seconds (or .69 decimal minutes)
  • One standard hour = 4,166.67 decimal seconds (or 41 decimal minutes and 67 decimal seconds)

[edit] Fractional days

The most common use of decimal time of day 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:

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 Microsoft 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, 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 h 51m 50s·66 Greenwich Mean Time...For example, at 12h 0m 0s Greenwich Mean Time, or 0d·500000...

Fractional seconds are arguably more used than fractional days in practice. This is the standard single-unit time representation in many programming languages, most notably C, and part of UNIX/POSIX standards used by Mac OS X, Linux, etc.; To convert fractional days to fractional seconds, multiply the number by 86400. Absolute times are usually represented relative to January 1st, 1970, at midnight. Other systems may use a different zero point, may count in milliseconds instead of seconds, etc.

In practice, most of the time, neither fractional days nor fractional seconds are expressed in decimal, because almost all computer representations of fractional time are made using binary coding. Decimal is sometimes but rarely used for computation, more often for storage, but never as much as for human interaction.

[edit] Swatch Internet Time

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Main article: 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 (each 86.4 s) 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, misrepresenting CET as "Biel Mean Time", and for not providing for more precise units, although third-party applications have proposed "centibeats" (864 ms) and "millibeats" (86.4 ms).

[edit] Decimal times in fiction

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Some science fiction authors use decimal time to reinforce the sense of "otherworldliness", notably Infocom's Planetfall and Stationfall games, which use "1 chronon = 1/10000 day" such that 0000 = midnight and 5000 = noon.

Isaac Asimov also uses and describes the use of decimal time by the humans from the planet Solaria in his novel "The Naked Sun", in which he describes the Solarian hour as been divided into ten decads, each of which is divided into a hundred centads.

Greg Bear's Anvil of Stars tells the story of a starship crew that structure their calendar in "tendays" instead of weeks.

Fritz Lang's science fiction film Metropolis depicts what is often misinterpreted as a decimal clock, as it has ten numerals, but it actually measures a workers' shift of ten conventional hours in one cycle, not decimal hours[citation needed]. Since a normal day cannot be divided into a whole number of such shifts, a 24-hour clock is displayed above the shift clock to give the actual time of day.

In the episode of The Simpsons, "They Saved Lisa's Brain", the members of Springfield's Mensa make changes to their town, including the implementation of using "metric time" to determine the time of day. Dr. Hibbert comments that the time is "eighty past ten". Using "metric time" to indicate decimal time, Principal Skinner comments that the city's trains are not only are running on time, but they are running on metric time, while looking at an analog clock with numbers 1–10.[1]

[edit] 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
  • 5h 0m French decimal time
  • @500 Swatch Internet Time
  • 50.0 centidays
  • 500 millidays
  • 50.0% Percent Time
  • 12:00 Standard 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.

[edit] See also

[edit] External links

[edit] References

  1. ^ "They Saved Lisa's Brain". The Simpsons. FOX. 1999-05-09. No. 22, season 10.