How can you measure the diameter of the sun using only a meter- stick and a piece of cardboard?
Make a pinhole camera by putting a pinhole in your piece of cardboard. Hold a piece of paper one meter away from the cardboard. Your pin-hole camera will cast an image of the sun on your paper. Measure the diameter of the image cast on your paper. The ratio of the sun's actual diameter to its distance from the earth is the same as the ratio of the diameter of the sun's image to the distance between the cardboard and the image. In other words, the diameter of the sun (X) divided by the distance from the earth to the sun is equal to the diameter of your image divided by the distance from the pinhole to the image. Good luck.