# How to Tell Time by the Sun

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#### Updated on May 01, 2014

Have you ever noticed how the Sun moves across the sky during the day? It rises in the East and reaches its highest point in the sky around lunchtime. Then, it descends, setting in the West. Although it is hard to believe, it is actually we on Earth who are travelling—not the Sun!

The Sun sits in the middle of our solar system. Our Earth revolves, or orbits, around the sun. That means the Earth orbits around the Sun. It takes one full year for the Earth to go around the Sun once. The Earth also rotates, or spins around. It takes one full day for the Earth to rotate around once. The parts of the Earth facing the sun experience day, and the parts of the Earth facing away from the Sun experience night. It’s because of this rotation that the Sun appears to travel across the sky.

People didn’t always have watches or (or cell phones) to tell what time it was. Thousands of years ago, early civilizations invented sundials, devices which use the apparent movement of the Sun to determine how much time has passed. In this activity, you will construct a sundial and revisit it throughout the year to see how the revolution of the Earth affects what time the sundial shows.

### Problem

Why does a sundial’s position change throughout the year?

### Materials

• A long sunny, summer day to start your project on (a day in late August would still work)
• Wooden board, at least 12 inches square
• Hammer
• Nail
• Scissors
• Plastic straw
• Watch
• Ruler
• Tacks
• Pencil
• Permanent marker

### Procedure

1. Hammer a nail ¼ inch into the center of the board. Why do you think it’s a good idea to make a sundial out of wood and nails rather than a paper plate?
2. Cut the straw to a length of six inches.
3. Place your straw over the nail.
4. In the morning, find a bright, level space outside and set the board on the ground at the top of the hour. Make sure no other shadows (from things like trees) will cover your sundial later in the day.
5. Observe the end of the nail’s shadow. Use the pencil to mark the shadow’s end, and gently push a tack into this part of the board to mark this location.
6. Write the hour next to the mark indicating the end of the shadow.
7. Make sure the sundial will not be disturbed the rest of the day. If it gets moved, your measurements will become inaccurate!
8. Visit the sundial again at the top of the next hour, again placing a tack and noting the hour with a pencil.
9. Continue visiting the sundial every hour until sundown, placing a tack and marking.
10. If your sundial was not disturbed and you are happy with your marks, replace the pencil numbers with numbers written in permanent marker.
11. Get to know your sundial the next couple days. In what conditions can’t you use it?
12. Use your sundial on several days throughout the year (make sure your sundial faces the same direction each time you use it! For best results, never move your sun dial). This table includes some suggestions, rather than specific dates, since you can only use your sundial on sunny days. In the observations column, note how the shadows and hours varied from the previous readings.
 Date Observations Early October Mid December Early February Late March Mid May