Materials: One 8.5'' x 11'' sheet of white paper, one 8.5'' x 11' piece of cardboard, tape, ruler, watch and pencil.

1. First, build a pinhole camera. Break the sheet into two identical pieces. Do the same with the cardboard. Tape one piece of white paper to one of the cardboard pieces. As for the other cardboard, cut a 2''x2'' window in the middle, then tape the white piece on it. Now use a thin paper clip to puncture a neat hole going through the white sheet and the window (be sure the hole is round).

2. On a clear day, aim the pinhole directly at the Sun (please, do not stare directly at it. You may damage your retina!). Observe the image of the Sun projected onto the other piece of clear paper at a distance x from the pinhole. Adjust the distance x until you have a clear, circular image of reasonable size.

3. Measure the distance x (in meters) between pinhole and image and the diameter d (in meters) of the image of the sun by placing pencil marks on the image. You may have to experiment a few times in order to find the distance that gives the best image, size and configuration to measure x. Get at least three values of each variable, preferably at different times of the day or on different days.

4. The little triangle with base d and height x is a scale model of the big triangle with base D (diameter of the Sun) and height X (distance earthen). The ratio d/x = D/X, is the same for both triangles. Make a drawing of these triangles.

5. The distance Earth to Sun is known to be X  = 150 billion meters = 1.5 x 10^11 m. Find the diameter of the Sun from D = Xd/x (average value and error). Report all the measured values, D,  and the day and time when the measurement was taken.

6. The diameter of Earth is 12800 km = 1.28 x 10^4 km = 1.28 x 10^7 m. How many of our planets placed side by side would span one Sun's diameter?

Discuss sources of error. Are they significant?

Useful information about the solar system and Moon tides can be found in the web page, Astronomy . The positions of planets at the moment you read this sentence can be found in the site Solar System Live . In another beautiful NASA program, Solar Exploration , you can view planetary orbits at different scales of magnification and at different angles.