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Build a SundialGo Back
Sundial Illustration Sundials are the oldest way to tell time. The position of the sun changes during the day. The sun doesn't move; the Earth rotates around the sun, making it seem like the sun rises in the east and sets in the west. As the sun goes across the sky, the post in the center of the sundial casts a shadow on a circular plate. Marks on the plate tell you what time it is. It's just like reading a clock!

To make your own sundial, you need:

  • A large styrofoam or paper drinking cup with a plastic lid and straw,
  • A wristwatch
  • A permanent marker
  • A pencil
  • Tape
  • Some small pebbles (enough to fill the cup about half full)
  • A compass

Solar Cup Illustration Now you are ready to build:

  1. Use the pencil to poke a hole in the side of the cup approximately 2 inches below the top.
  2. Put the pebbles in the cup so it doesn't tip over.
  3. Put the lid on the cup.
  4. Put the straw through the hole in the cover and the hole in the side of the cup. Let it stick out about half an inch from the side and tape the straw to the cup.

Finished building—Let's give it a try!

Start using your sundial on the morning of a sunny day when you'll be home all day. Find a spot out in the open that is sunny from 10 in the morning until 3 in the afternoon. Look for a good, open view to the South with no trees or buildings in the way. Place the sundial on level ground or on a level table.

Use the compass to find North, and point the straw to the North. Now be careful not to move the sundial. Just in case, you might want to make a mark near the bottom of the cup and on the ground or the table, so you can line them up again if they get moved. Or you can just line up the sundial to the North again using the compass.

At 10 a.m., use the marker to mark where the shadow from the straw falls on the lid of the cup. Repeat this every hour until 3 p.m. Make sure your marks are neat and straight. If you have to leave or if it clouds over, you can always finish doing this a few days later—you don't have to do it all in one day. Once you have marks for all the times, you're ready to use the sundial. Wait until the next day and try reading the time by comparing the shadow to the marks you made. How well can you tell what time it is?

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