Decimal time

Because a new decan also appears heliacally every ten days (that is, every ten days, a new decanic star group reappears in the eastern sky at dawn right before the Sun rises, after a period of being obscured by the Sun's light), the ancient Greeks called them dekanoi (δεκανοί; pl.

[3] Chinese decimal time ceased to be used in 1645 when the Shíxiàn calendar, based on European astronomy and brought to China by the Jesuits, adopted 96 ke per day alongside 12 double hours, making each ke exactly one-quarter hour.

As early as the Bronze-Age Xia dynasty, days were grouped into ten-day weeks known as xún (旬).

The distance the twilight zone travels in one such tierce at the equator, which would be one-billionth of the circumference of the earth, would be a new unit of length, provisionally called a half-handbreadth, equal to four modern centimetres.

Jean-Charles de Borda made a proposal for decimal time on 5 November 1792.

The National Convention issued a decree on 5 October 1793, to which the underlined words were added on 24 November 1793 (4 Frimaire of the Year II): Thus, midnight was called dix heures ("ten hours"), noon was called cinq heures ("five hours"), etc.

In "Methods to find the Leap Years of the French Calendar", Jean-Baptiste-Joseph Delambre used three different representations for the same decimal time: Sometimes in official records, decimal hours were divided into tenths, or décimes, instead of minutes.

[11] In some places, decimal time was used to record certificates of births, marriages, and deaths until the end of Year VIII (September 1800).

On the Palace of the Tuileries in Paris, two of the four clock faces displayed decimal time until at least 1801.

Decimal time was part of a larger attempt at decimalisation in revolutionary France (which also included decimalisation of currency and metrication) and 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; this calendar was abolished at the end of 1805.

The start of each year was determined according to the day of the autumnal equinox, in relation to true or apparent solar time at the Paris Observatory.

This was to include units for length, weight, area, liquid capacity, volume, and money.

[13] On March 28, 1794, Joseph-Louis Lagrange proposed to the Commission for Republican Weights and Measures on dividing the day into 10 decidays and 100 centidays, which would be expressed together as two digits, counting periods of 14 minutes and 24 seconds since midnight, nearly a quarter hour.

A third hand on a smaller dial would further divide these into 10, which would be 1/100,000 day, or 864 milliseconds, slightly less than a whole second.

Prieur (of the Côte-d'Or), read at the National Convention on Ventôse 11, year III (March 1, 1795): Thus, the law of 18 Germinal An III (April 7, 1795) establishing the metric system, rather than including metric units for time, repealed the mandatory use of decimal time, although its use continued for a number of years in some places.

At the International Meridian Conference of 1884, the following resolution was proposed by the French delegation and passed nem con (with 3 abstentions): In the 1890s, Joseph Charles François de Rey-Pailhade, president of the Toulouse Geographical Society, proposed dividing the day into 100 parts, called cés, equal to 14.4 standard minutes, and each divided into 10 decicés, 100 centicés, etc.

The Toulouse Chamber of Commerce adopted a resolution supporting his proposal in April 1897.

[15] 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 adopted a compromise, originally proposed by Henri de Sarrauton of the Oran Geographical Society, of retaining the 24-hour day, but dividing each hour into 100 decimal minutes, and each minute into 100 seconds.

[18] Although endorsed by the Bureau des Longitudes, this proposal failed, but using decimal fractions of an hour to represent the time of day instead of minutes has become common.

This is intended to make accounting easier by eliminating the need to convert between minutes and hours.

Fractional days are often used by astronomers to record observations, and were expressed in relation to Paris Mean Time by the 18th century French mathematician and astronomer Pierre-Simon Laplace, as in these examples:[20] ... et la distance périhélie, égale à 1,053095; ce qui a donné pour l'instant du passage au périhélie, sept.29j,10239, temps moyen compté de minuit à Paris.

Les valeurs précédentes de a, b, h, l, relatives à trois observations, ont donné la distance périhélie égale à 1,053650; et pour l'instant du passage, sept.29j,04587; ce qui diffère peu des résultats fondés sur cinq observations.

For instance, the 19th century British astronomer John Herschel gave these examples:[21]

Fractional seconds are represented as milliseconds (ms), microseconds (μs) or nanoseconds (ns).

In principle, time spans greater than one second may be given in units such as kiloseconds (ks), megaseconds (Ms), gigaseconds (Gs), and so on.

In the fictional Star Trek universe, each stardate increment represents one milliyear, with 78 years in 2401, counted from 2323.