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About the Cover

Vol. 79 No. 5 (2003)

Vol. 79 No. 5 (2003)

An Astronomical Clock

  These pictures appeared in the Compendium of the Kansei calendar, Vol. 19, published around 1800 by the Astronomical Observatory of Tokugawa Shogunate.
  The left picture shows the entire view of the clock box. A Japanese astronomer, inspired by a description in a Chinese astronomical book, invented this clock to be used for astronomical observation.
  The picture on the right page shows how the clock works with a pendulum and records its oscillations. The pendulum swings back in 1.4 seconds, or 60,000 oscillations per day. A wheel gear at the bottom, connected to the pendulum, first rotates and numbers of oscillations are written up to 100 on a disk on the top of the right side. The second wheel in the middle, connected to the first one, turns at a rotational speed of 1/10 of the first wheel, and the third one at a speed of 1/10 of the second one. The numbers of the oscillations of each wheel are recorded on separate disks. The clock can record up to 10,000, and can count up to 1,000,000 with these three wheels and additional devices. Errors in the measurement were estimated to be a few seconds per day.
  (Collection of the National Astronomical Observatory of Japan)
  (by courtesy of Dr. Setsuko Ito, Research Associate of the National Astronomical Observatory)

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