# How Pendulum Clocks Work

## 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.