Purpose
In this activity, students will estimate the diameter of the sun.

Discussion
Take notice of the round spots of light on the shady ground beneath trees. These are sunballs—images of the sun. They are cast by openings between leaves in the trees that act as pinholes. The diameter of a sunball depends on its distance from the small opening that produces it. Large sunballs, several centimeters or so in diameter, are cast by openings that are relatively high above the ground, while small ones are produced by closer "pinholes." The interesting point is that the ratio of the diameter of the sunball to its distance from the pinhole is the same as the ratio of the sun's diameter to its distance from the pinhole.

Knowing that the sun is approximately 150,000,000 km from the pinhole, careful measurement of this ratio tells us the diameter of the sun. That's what this experiment is all about. Instead of finding sunballs under the canopy of trees, you'll make your own easier-to-measure sunballs.