If you’re visiting Paris, a good place to see some interesting clocks is the Musée des arts et métiers.

In 1793, the Revolutionary Government in France decreed that the day should be divided into 10 hours of 100 minutes, and the year into 10 months of 30 days. For a few years, clock and watch makers designed some unusual pieces to help the population learn and adopt to the new decimal time system. Here’s a good example:

The single long hand with a circle at one end and a point at the other probably shows the hours in both old and new systems. One end points to the decimal time, with 10 (0) being midnight, and 5 being midday, the other end to the equivalent old-style time. Presumably, therefore, old-style midnight (XII) is at the bottom of the dial, so that the other end can point to 10 (0). 1 o’clock (decimal) is about 02:20 old style, and the position of the hour hand in this photograph suggests that the time is 0.90 (d), or 02:10 old style.

If the other brass hand is the new minute hand, it shows 80 decimal minutes past 0. In theory a time of 0.80 (d) corresponds with about 01:55, so perhaps the hands are not quite adjusted correctly – or my assumptions are incorrect.

The grey pointer indicates the calendar day – the 12th. Every day in the new Revolutionary calendar had a object or plant associated with it, so if the current month had been Pluviôse (Jan 20 ~ Feb 18), today would be Broccoli day. (For a full list, see here.)

Here’s a watch with, I expect, midnight at the bottom:



At about 3 o’clock (decimal), or 07:15, it’s time for croissants and café. Again it’s not clear whether the minute hand is showing old or new minutes.

This next example is more radical. Only half the dial is organized for the new decimal time, the other half is defiantly quadrovigesimal (duodecimal times two):  Presumably the long dual-purpose hour hand is used again here, but I’m puzzled as to why only the roman numerals I to V are used – converting to a new time format is hard enough without having to add 5 to the hour after midday. The shorter hand pointing to the 12 might be another old-style hour hand (how could it work?), but the other hand pointing to the 2 could be an old or new style minute hand. Some more research is needed.

Intriguingly, the museum shows only 10% of its holdings at a time, with the remaining items held in a big storage facility on the outskirts of Paris.

If you can’t get to this great museum in person – to see these and other wonderful scientific objects from the past (and to eat in the excellent cafe) – the museum’s web site is worth a look.

I’ve been looking at the Zeitladen site and, being far from fluent in German, I can’t decide whether they’re selling standard 24 hour clocks and watches or sidereal (sternzeit or star time) versions.

If you watch a particular star and make a note of when it is highest in the sky (its transit time) on two successive days, you’ve measured a sidereal day. A sidereal day is about 23 hours and 56 minutes.

If you set two clocks, one showing sidereal time, the other 24 hour time, to tell the same time as each other, starting at September 22 or so (the Autumn equinox), the sidereal clock would run faster than a 24 hour clock, gaining 4 minutes in the course of every 24 hour day. After 6 months, the sidereal clock would be 12 hours faster. After a year, the sidereal clock would be exactly a day faster: there are 366 sidereal days in a year.

I presume that astronomers like to use sidereal clocks because it helps them locate stars.

Do they also use sidereal watches? Is this a sidereal watch?

What about this clock?

I can’t decode the German text enough to find out.

Visually there seems to be no difference between the design of the 24 hour and sidereal dials. So you would be well advised to place one of these sidereal clocks next to an ordinary 24 hour clock, or else clearly label it as sidereal. As for watches, you could wear the sidereal watch on your right wrist, the ordinary 24 hour watch on the left…

I’ve had watches and clocks that seem to gain 4 minutes a day. Perhaps they were sidereal and I didn’t know it.

Zeitladen also sells radio-controlled sidereal clocks.

The Zeitladen site was interesting, particularly if your German was good!

The site is now closed, unfortunately.