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Decimal time: the revolution that never was
History & Masterpieces

Decimal time: the revolution that never was

Monday, 13 September 2010
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Christophe Roulet
Editor-in-chief, HH Journal

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5 min read

Looking for Noon at Five O’Clock, a temporary exhibition at the Musée International d’Horlogerie in La Chaux-de-Fonds, Switzerland, tells the story of decimal time. Ushered in with the French Revolution, this idiosyncratic system never succeeded in dislodging the sexagesimal system, inherited from the Babylonians, which had been setting the pace of daily life “since the dawn of time.”

Why is it that we count hours, minutes and seconds, not to mention the months, using a sexagesimal system when weights and measures, in Europe at least, have always been decimalised? No one is complaining, if nothing else from force of habit, yet there is perhaps more to this simple observation than meets the eye. Or so the French believed in the wake of the 1789 Revolution. They concluded that the system passed down from the ancient Babylonians then the Egyptians, where time is divided into units of six, was no longer justified. What’s more, they had the backing of the scientific community: angles, geographic coordinates, time measurement, the revolutionary broom was to sweep clean, as the temporary exhibition at the Musée International d’Horlogerie in La Chaux-de-Fonds, Looking for Noon at Five O’Clock, relates.

Henceforth the year began at midnight at the true autumn equinox as given by the Paris Observatory.
Ten hours to the day

“The French Revolution was conceived by its protagonists as the starting point of history and a break between the old and new eras,” the exhibition’s Journal explains. “This representation transpired in a system of symbols: the promise of a new future, the evocation of a new era, a regenerated population and a new society. This ideology of the past, now ended, and the future which the Revolution proposed gave rise to the Revolutionary or Republican calendar and the introduction of decimal time. The Gregorian calendar came to an end on 1st Vendémiaire Year II (September 22nd 1793, the autumn equinox). Henceforth the year began at midnight at the true autumn equinox as given by the Paris Observatory. The new calendar was based on two principles: that the Republican year should coincide with the movement of the planets, and that it should measure time more accurately and more symmetrically by applying the decimal system wherever possible. Non-religious, it advocated a rational approach and honoured the seasons and work in the fields.”

The year was divided into twelve months, each with thirty days and whose names were inspired by the weather and the seasons. Five extra days, the sansculottides, were added for the Republican holidays. At the end of a four-year cycle known as a franciade, a sixth sansculottide was added to the end of the year to make the Republican calendar year coincide with the sidereal year. Each month was divided according to a decimal system of three ten-day periods known as décades. The reform, in its quest to decimalise the entire system of weights and measures, abandoned the division of the day into 24 hours and sexagesimal subdivisions. It instead decreed that the day, from midnight to midnight, was divided into ten hours, each hour comprising 100 decimal minutes of 100 seconds each. And so decimal time was born, with one decimal hour being equivalent to 2 hours and 24 minutes. Noon was now at 5 o’clock.

Clocks and watches comply

Naturally, clock and watchmakers were expected to conform to these new directives and produce instruments whose dials indicated the day of the décade, the date and the Republican month. “Between 1793 and 1796, the dials of watches and clocks were transformed. Duodecimal hours and their sexagesimal divisions were joined by decimal divisions. This dual numbering was intended to help the public familiarise itself with the new time. These decimal dials gave a host of indications.” Concentric circles, swept by a single pair of hands, indicated either the five hours equivalent to 12 hours or half a day, the ten hours equivalent to the 24 hours of a full day, or displayed ten hours on two circles, one for the day and one for night, above the duodecimal hours shown as I to XII. Only a handful of watchmakers – Louis Berthoud, Robert Robin, Pierre-Basile Lepaute and Antide Janvier – would succeed in making completely decimal chronometers or clocks for scientific observations and measurements.

Skeleton regulator with Gregorian and Republican calendars. Robert Robin, Paris, 1796/1799. Collection of the MIH © MIH
Skeleton regulator with Gregorian and Republican calendars. Robert Robin, Paris, 1796/1799. Collection of the MIH © MIH

Daily life, the importance of foreign trade, and the fact that existing clocks and watches couldn’t be adapted put paid to decimal time. On April 7th 1795, five hundred days after its introduction, the Convention passed a law that indefinitely suspended the decimal division of the day and hour. The idea resurfaced in the late nineteenth century, put forward by scientists at major conferences such as the 1883 International Geodetic Association Conference in Rome, the 1884 International Meridian Conference in Washington, or the International Chronometry Congress held at the Paris Observatory in 1900. All these proponents of decimal time proclaimed the need to align the divisions of time with the circumference of the Earth. In 1898, an international competition to develop timekeeping instruments with decimal graduations drew more than twenty entries. However, given the scope of change it implied, decimal time died a second death though not without leaving some trace: subdivisions of the second are in decimal units… and the Swatch Group’s Internet Time divides the day into 1,000 beats!

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