Q & A

Here's a set of questions from readers:

  • Watches obviously do not use pendulums, so how do they keep time? A pendulum is one periodic mechanical system with a precise period. There are other mechanical systems that have the same feature. For example, a weight bouncing on a spring has a precise period. Another example is a wheel with a spring on its axle. In this case, the spring causes the wheel to rotate back and forth on its axis. Most mechanical watches use the wheel/spring arrangement.
  • What is the difference between a weight-driven and a spring-driven clock? Nothing, really. Both a weight and a spring store energy. In a spring-driven clock you wind the spring and it unwinds into the same sort of gear train found on a weight-driven clock.
  • What can you do to make a clock more accurate? There is an excellent book entitled "Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time", by Dava Sobel, that discusses the creation of extremely accurate mechanical clocks to find a ship's longitude. Creating accurate mechanical clocks that can live on a ship (unlike a pendulum clock...) was a real challenge!
  • How does the moon phase dial on a grandfather clock work? The moon phase dial works just like the hands of the clock do. The minute hand on a clock moves at the rate of one revolution every hour. The hour hand moves at one revolution every 12 hours. The moon phase dial moves at a rate of one revolution every 56 days or so. The moon's cycle is 28 days, and the moon phase dial generally has two moons painted on it.

For more information on pendulums, timekeeping and related topics, check out the links on the next page.