Tell Time with the Sun

You can make your own sundial very easily. First, you need to scout out a good spot for it. It should be outside, in a location that is lit all day long. Pick your spot, then check it several times during the day to make sure that it is never in shadow from a nearby building, tree, or other large object. The spot should also be somewhere that your sundial won't be disturbed.

When you're ready to make the sundial, take a sheet of white paper and draw a circle about 8" in diameter on it. You might be able to trace around a plate or bowl to draw a nice circle. Place it in your chosen spot. Put a wad of clay or chewing gum in the middle of the circle, then stick an unsharpened pencil or other straight stick into the wad. Make sure that it stands up straight, and doesn't fall over. You should see the shadow of the stick. Make a mark on the circle where you see the shadow, and write the time next to the mark. Come back in another hour or so - the shadow will have moved! Mark the time again. Keep doing this until it gets too dark to see the shadow. Leave the sundial in the same location, and check back the next day at about the same times to see if the shadow is in the same place. How could you use this sundial to tell time? What are some of the problems with a sundial? (Some hints - Does it work at night? Does it work on cloudy days?)

Clockwork

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.