How Can You Use the Sun to Tell Time?

Have students read through the activity and then discuss the following:

• Discuss the importance of the direction that the sundial points.
• Why is it important to calibrate scientific instruments? For example, what could happen if you do not calibrate a balance scale?

Materials:

• cardboard
• scissors
• protractor
• directional compass
• copy of observation table
• latitude of where you live

1. Cut out a square piece of cardboard, about 12 inches on each side.
2. Make a cardboard triangle with a base 6 inches long. The angle at one end of the base should be 90 degrees. The angle at the other end should be the same number of degrees as the latitude of where you live. The triangle is the gnomon (NOH mahn) of your sundial, the part that will cast the shadow.
3. Mount the gnomon on the cardboard with two strips of tape, one on each side. Make sure the the gnomon is vertical.
4. Take the sundial outside and find a place where it will be undisturbed. Using a compass to fund north, position the sundial so that the gnomon points north.
5. Check the sundial at various times of the day. If possible, do it every hour on the hour. Each time you check the shadow, use a pencil to mark the leading edge of the shadow with a line. Each time, label the line with the time. Record your observations in the data table.
6. Repeat the procedure over a few days.

1. Did the shadow fall at the same place at the same time each day? (The shadow should fall at the same point each day.)
2. Once you have labeled the sundial with all of the hours, how accurate would it be for telling time? (Answers may vary. The sundial would be as accurate as the marks made. The sharpness of the shadow also affects the accuracy.)
3. Why do you have to be sure that the gnomon is pointing north when you make your readings? (The gnomon needs to point in the same direction as it idd when it was first calibrated. North is convenient because at noon it is straight overhead the the shadow is easiest to read during most of the day.)
4. What does your sundial tell you about how the earth moves? (It shows that the Earth rotates at a constant rate in an easterly fashion.)

Write a paragraph explaining how your sundial works. If you were to take it with you to another part of the country, would you have to make any adjustments? Explain.