Current
Decimal
Time:

Egypt

The ancient Egyptians divided each daily period of sunlight into ten hours, marked by sundials. They had an additional hour of twilight before sunrise and one after sunset, for a total of twelve hours of light. The night was also divided into twelve hours, marked by certain stars, for a total of 24 hours. The use of sundials to tell time became common in many places, which resulted in hours being variable in length during the year, becoming longer in the summer and shorter in the winter. The invention of mechanical clocks in the Middle Ages, which ran at fairly constant rates, finally led to hours of equal length.

Babylonia

At first the hours were divided roughly into halves and quarters, but the addition of another clock hand allowed the hour to be divided more finely. The first hand revolved around the clock dial twice per day, with the dial divided into twelve hours. The second hand revolved once per hour, dividing the hour into twelve parts. Each twelfth-hour was then divided by five by adding four marks between the hour marks, thus dividing the hour into sixty parts. This mirrored the Babylonian sexigesimal system, which was already used to measure angles. The Babylonians divided the circle into six parts, each equal to the angle of an equilateral triangle, which was divided into 60 degrees. Each degree was divided into 60 smaller parts, and so on. The first division of the degree was called a minute, from the Latin pars minuta, meaning "small part". One sixtieth of a minute was called a second from pars minuta secunda, because it was the 2nd level of division. (Even greater levels of angular division have sometimes been used, called "thirds", etc.) Divisions of the clock hour were based on the ancient Babylonian divisions of the degree, first by adding a minutes hand, and later also a seconds hand.

Standard Time

Originally clocks displayed local solar time, and were set to twelve o'clock when the sun reached its highest point of the day. However, the length of the day varied slightly during the year, so that this might happen as much as a quarter-hour before or after the mean solar noon. In addition, clocks varied as one travelled east or west, according to the curve of the earth. With the invention of instant communication via telegraph and rapid travel via railroad, the time of an entire country was usually synchronized with that of its capital city. For instance, in the United Kindom the time was kept at the Royal Observatory at Greenwich, near London. In 1884 the International Meridian Conference was held in Washington, DC, which established hourly offsets from Greenwich mean time as Standard Time, so that local Standard Time is usually within a half-hour of mean solar time, establshing synchronized zones hundreds of miles wide. This was immediately adopted in the United States, and eventually in the rest of the world, although some countries use intermediate offsets. This was modified in some places during the twentieth century by shifting the time during part of the year by one hour, so that the sun rises and sets later according to the clock when there are more hours of daylight; this is called Daylight Saving Time or Summer Time.

China

The ancient Chinese used clepsydras or water clocks to divide the day into 100 units called ke, each equal to 14 minutes and 24 seconds, which existed alongside a system which divided the day into 12 double-hours, named for the "earthly branches". After European time was introduced, a quarter-hour came to be called a ke.

France

During the French Revolution, the National Convention passed a law on October 5, 1793, establishing the Republican Calendar with 10-day décades instead of 7-day weeks, which also included an article defining decimal time. This article was modified slightly by the law of November 24, 1793, (which also named the months of the new calendar, the first month being named Vendémiaire) with the addition of the underlined words:

VIII. Chaque mois est divisé en trois parties égales de dix jours chacune, et qui sont appelées décades... 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. La centième partie de l'heure est appelée minute décimale; la centième partie de la minute est appelée seconde décimale. Cet article ne sera de rigueur pour les actes publics, qu'à compter du 1er. du premier mois de la troisième année de la république Vendémiaire, l'an trois de la République.

Which translates into English as:

VIII. Each month is divided into three equal parts, of ten days each, which are called decades... XI. The day, from midnight to midnight, is divided into ten parts or hours, each part into ten others, so on until the smallest commensurable portion of the duration. The hundredth part of the hour is called decimal minute; the hundredth part of the minute is called decimal second. This article will not be required for the public acts, until from the 1st of the first month of the third year of the republic Vendémiaire, the year three of the Republic. (September 22, 1794)

Thus, French decimal time was originally defined simply as a decimal fraction of the day, which was later modified with the definitions of decimal time units (decimal hours, minutes and seconds) to resemble traditional timekeeping. Each multiple of ten decimal minutes was sometimes called a décime, because it was one-tenth of a decimal hour, and which was conveniently close to an old-style quarter-hour.

On August 22, 1794, a contest was held for designs to convert clocks and watches to the new style, to be judged by Ferdinand Berthoud, Antide Janvier, and Jean Antoine Lépine. Clocks and watches were built with decimal dials, most displaying both 10 and 24 hours. The hours were numbered 1-10, with 10 for midnight and 5 for noon.

The modern convention of separating units with colons (:) was not in common use then, and even today it is still common in France to state the time in heures (hours) and minutes (if any) and to abbreviate it by separating the hours and minutes with the letter h; e.g. 10 o'clock (10:00) is written as "10 heures" or "10h00". During the Revolution, decimal times were indicated by adding décimales, e.g. one written account from the eighth year of the Republic gives the decimal time as deux heures 10 minutes décimales (two hours 10 minutes decimal) along with the old style equivalent: cinq heures du matin (five hours of the morning).

However, about six months after decimal time became mandatory for public acts, the metric system law of 18 Germinal, an III, (April 7, 1795) declared:

Art. 22. La disposition de la loi du 4 frimaire an 2, qui rend obligatoire l'usage de la division décimale du jour et de ses parties, est suspendue indéfiniment.

Which means:

Article 22. The provision of the law of Frimaire 4 year 2 (November 24, 1793), which rendered obligatory the usage of the decimal division of the day and of its parts, is suspended indefinitely.

However, although it was no longer mandatory, official records, such as births, deaths and marriages, continued to be recorded with decimal times in some parts of France for a number of years. The 10-day décades were replaced with 7-day weeks in April of 1802, and the Gregorian calendar finally reinstated on January 1, 1806.

The French made another attempt at the decimalization of time in 1897, when the Commission de décimalisation du temps was created by the Bureau of Longitude, with the mathematician Henri Poincaré as secretary. The proposed unit of time was the standard hour of 1/24 day, divided into 100 minutes, and each minute into 100 seconds. However, this effort failed to gain any acceptance.

Astronomy

By the nineteenth century astronomers were using fractional days. In 1849, the British astronomer, John Herschel, published Outlines of Astronomy, describing fractional days and also introducing a system of decimal dates, by counting days of the Julian Period. Astronomers began adding fractional days to Julian Days, which together are called Julian Dates. Astronomers then started and ended the day at noon, so Julian Dates also started and ended at noon, as observed in Greenwich, England, where the Prime Meridian was agreed to cross by an international conference in Washington, DC, in 1884. Since 1925, astronomers have started and ended the day at midnight, Greenwich Mean Time, so astronomers introduced Modified Julian Dates and other variations which are synchronized with the Universal Day.

Also at the International Meridian Conference of 1884, the following resolution was passed, at the proposal of the French delegation:

VII.

    “That the Conference expresses the hope that the technical studies designed to regulate and extend the application of the decimal system to the division of angular space and of time shall be resumed, so as to permit the extension of this application to all cases in which it presents real advantages.”

This resolution has yet to be carried out. top