Star time

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?zeitladen1.jpg

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.


One comment

  1. The lower clock is definitely a star time clock – it says “Sternzeit” on the dial which is German for star time. I don’t know about the other clock.

    Btw, I heard of a star time clock outside an observatory in Berlin. There is no indication that the clock is a star time clock, so people sometimes complain that the clock was not set correctly. 🙂

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