XII. Measure the Diameter of the Sun.

1. You will need to work carefully to get good results. It would probably be good to have a helper. In no case should you ever be looking at the Sun! Do not look at the Sun through the pinhole.
2. On a clear day when the Sun is high in the sky use the pinhole to cast an image of the Sun on your screen. Measure the diameter of the image and the distance from the pinhole to the image. Because you have two similar triangles, geometry says that the ratios distance to diameter are the same. In other words,
3. Be sure that you collect several (at least 3, but preferably more) different measurements for the size of the Sun.  Compare these not only to one another, but also to the known diameter of the Sun. Use the known distance to the Sun from your textbook. Be careful to use the same units of measure - all in meters for example.
4. Your measurements will be most accurate if you project the image onto a screen that is at least 1 foot (30 cm) away from the pinhole.  You may adjust the size of the pinhole in order to get a better projected image.  Smaller pinholes work best for short projections, while larger pinholes may be necessary for longer projections.
5. Again, do not look directly at the Sun itself. Staring at the Sun will cause permanent eye damge!