Most of the mass of our Solar System is in the Sun, about 99%. That leaves very little for the planets, moons, and us! It is difficult to imagine how large a star is without seeing it up close. We can't do that but we can do a quick experiment to calculate the size of the Sun. When you do this activity make sure you don't look directly at the Sun, it can cause permanent eye damage!

Materials:

• 8 1/2 x 11 inch sheet of paper
• A yard or meter stick
• Tape
• Pencil
• An index card
• A straight pin
1. On your sheet of paper, draw two parallel (side by side) lines close together, about a pencil lead width apart.
2. Measure the distance between the lines and record it.
3. Pock a small hole in the center of an index card with your straight pin.
4. Fold over one end of the index card about 1 inch, and tape that to the top on the 0 mark of your yard/meter stick.
5. Hold your paper with the lines drawn on it underneath the index card with one edge resting against the yard/meter stick and let the shadow of the card fall on the paper.
6. Slide the paper up and down along the edge of the stick so that the sunlight coming through the index card fills up the space between the lines drawn on the paper.
7. Record where on the yard/meter stick the pin-prick of light fit inside the parallel lines your answer should be around 8.5 inches or 218 mm.

We know the the distance between the two lines and the size of the light spot is the same because we measured the distance, for the sake of simplicity let's assume that this distance is 2mm. We also know that the students recorded how far away from light source, hole in the index card the spot is by the number we recorded in step 7. So they should have an about 2mm spot which is 218mm away from the source hole.

The ratio between the distance to the spot and the width of the spot is the number we get when we divide the recorded distance by the recorded spot size. 218 divided by 2mm is 109. The number 109 is the ratio of the two measurements and will be our ratio to determine the size of the Sun. The Sun is 93,750,000 miles away. So we take our 93,750,000 and divide it by our ratio which is 109 and we get the size of the Sun!