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Deutsches Technikmuseum - March

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Exhibit of the Month March 2015

Photo: A look at the black watch face with its three white number rings and its four hands.

The idea for the "Rotational Time" wristwatch has been protected by an international patent since 1999. © SDTB / Photo: C. Kirchner

Patented wristwatch "Rotational Time", 2004

Do our clocks run the wrong way around? And what are the time zones all about?

The earth’s rotation is a measure of time. It is depicted on our clocks. The hands of a conventional clock travel, well, clockwise. They trace the apparent course of the sun: from east to west, like the shadow of a horizontal sun clock in the Northern Hemisphere. In this way, clocks embody the earlier conception of the universe in which our sun revolved around the earth.

We know today that the sun is the centre of our solar system and that the earth rotates from west to east beneath the sun. Conventional clocks therefore run the "wrong" way around. The clock shown here has an hour ring (0 to 24) that rotates counterclockwise. This hour dial ring moves in conformity with the rotation of the earth and in doing so gives the time as the earth would have it.

By way of illustration:

Look down from the North Pole on the earth and mentally slice the earth parallel to the line of the equator. Now divide the resulting 360-degree circle into 24 equal parts. Each section corresponds to 15 degrees, 15 lines of longitude or one hour – because the earth takes 24 hours to make one complete rotation. Our circle moves counterclockwise just like the earth. The hour ring on this wristwatch works the same way.

  • A long hand rotates clockwise and points to the outer number ring indicating the seconds – in the photo here: 9 seconds.
  • A second long hand likewise rotates clockwise and indicates the minutes – in the photo here: 20 minutes.
  • Each of the two small hands is set at an arbitrary degree of longitude – in the photo: 60 degrees west (e.g. Bolivia) and 75 degrees east (e.g. Pakistan). The hour at the respective degrees of longitude (the time zone) is readable on the middle ring (0 to 24) – in the photo: between 16 and 17 o’clock as well as 1 and 2 o’clock.
  • The time is therefore 16 hours, 20 minutes and 9 seconds and 1 hour, 20 minutes and 9 seconds respectively.