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Wednesday, June 9, 2010


We "tell time" with clocks. Clocks need to use some kind of regular, repeating event to keep time. What kind of event might that be? Lots of different things have been used throughout history, and as the month goes on, you'll get instructions on how to make many of these kinds of clocks!
  • The sun comes up, moves across the sky, then sets every day, so it can be used for a clock. Have you ever seen a sundial in a garden? It uses a shadow cast by the sun to tell time. People have been using sundials for over 2500 years!
  • In water clocks, a stream of dripping water was used to mark off time. They've been around for a few thousand years, too.
  • Around 1200 CE, mechanical clocks began to be used. They were based on some kind of simple machine, like a gear, a spring, or a swinging pendulum. These clocks had to be wound, but they were much more accurate than sundials or water clocks. This kind of clock is still made today - do you know anyone who has one?
  • In the 1920's, scientists figured out how to use crystals of the mineral quartz to measure time. When these crystals are placed in an electric field, they can be made to vibrate steadily. They are much more exact than any earlier type of clock, and are powered by electricity.
  • In 1949, scientists began to use the vibrations of atoms themselves to tell time. The cesium atomic clock is the official clock for the entire world. In the U.S., the official time is broadcast from Fort Collins, Colorado. Devices like portable atomic watches or alarm clocks, or GPS navigation units, can receive this radio signal and make sure that the time they're measuring is correct.

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