Finding the Size of the Sun and Moon

In this activity you'll learn how to build a simple pinhole viewer. This apparatus can be used to project images from a variety of light sources. When used to project an image of the Sun, the pinhole viewer can be used to determine the diameter of Sun.

 Today's image of the Sun taken by the Extreme Ultraviolet imaging Telescope onboard the SOHO satellite. Courtesy of the Solar Data Analysis Center at NASA Goddard Space Flight Center.
 What You'll Need 1 - sheet of cardboard 1 - sheet of white paper 1 - piece of aluminum foil approximately 1 x 1 -inch (3 x 3 centimeters). 1 - pin or sharp point 1 - roll of tape 1 - pair of scissors 1 - ruler 1 - candle

 Did You Know The first person to report a finding on an attempt to measure the distance to the Sun was a Greek astronomer, Aristarchus, who lived in the 3rd century B.C. Unfortunately, the number he came up with was completely wrong, some 20 times too small. It wasn't until the past 100 years that scientists have made more accurate measurements.

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What to Do

Cut a square about 3/4 x 3/4-inch (2 x 2 centimeters) out of the center of the cardboard.

Place the piece of aluminum foil over the opening and tape it in place at the edges.

Using the pin or other sharp point, puncture the foil to produce a small hole. You now have a pinhole viewer.

E X P E R I M E N T     1
Viewing an Image of a Candle

To test your pinhole viewer, set up the candle about 4 inches (10 centimeters) away from one face of the pinhole viewer.
Light the candle.
Turn out the lights in the room.

Hold a white sheet of paper a few inches or centimeters away from the opposite side of the pinhole viewer. You should be able to see an image of the flame projected on the paper.

Discuss with your family what happens to the size of the image as you move the paper farther away. What happens as you move the candle farther away?

E X P E R I M E N T     2
Measuring the Size of the Sun

Hold the pinhole viewer so that the light from the Sun passes through the hole and falls on a sheet of white paper held behind the hole. Try to make the distance between the pinhole and the paper as large as possible.

a. The diameter of the image of the Sun on the paper =

b. The distance from the pinhole to the paper =

You can calculate the diameter of the Sun using the following formula:

 Diameter of the image of the Sun Distance from the pinhole to the paper .X. Distance from Earth to the Sun .=. Diameter of the Sun

Note: The distance from the Earth to the Sun is approximately 93,000,000 miles (149,600,000 kilometers).

E X P E R I M E N T     3
Measuring the Size of the Moon

You can use the same procedure to measure the diameter of the moon. You'll need to pick a night with a full (or near full) moon.

Note: The distance to the Moon is approximately 239,000 miles (384,000 kilometers).

You might want to discuss how many times larger the Sun is than the Moon. Also, why if the Sun is so much larger, doesn't it appear larger in the sky?

What's Going On
The pinhole viewer you built can project a variety of light sources. More complex pinhole viewers and cameras use a dark chamber behind the pinhole. This allows for the projection of images from sources that are not as bright as the ones used in this exercise. In a basic way, this is how cameras work.

 Related Websites Stanford Solar Center http://solar-center.stanford.edu Aristarchus Page at the University of Scotland http://www-groups.dcs.st-andrews.ac.uk/ ~history/Mathematicians/Aristarchus.html
This activity was derived from "Finding the Size of the Sun and the Moon": from Living and Learning in the Space Age, by Jeff Crelinsten. Copyright 1988.

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