I'm damned impressed with Birunis 1% margin of error. Hard to believe, since it it would require a precision to a hundredth of a degree in the measurement. But maybe. The problem with asking people to go up on a mountain to measure the angle of the horizon is that tall enough mountains are rather inaccessible to most people, and anyway we don't have astrolabes at hand. I propose the following equivalent experiment, to be carried out in an airplane with simple and cheap tools from the hardware store. -------- Experiment: Measure the radius of the earth from a regular passenger air plane. Required materials: Air plane Tape measure Level Social skills An assistant (optional) Procedure: Acquire a window seat. In the diagram, your seat is on the left. Film (or look at) the horizon out of the window at the opposite side of the plane. Measure the horizontal distance from the camera to the opposite window. This is the length a in the diagram. Use your social skills to make the other passengers not get annoyed with you. When the horizon coincides with the bottom edge of the window, measure the vertical distance of the camera lens (or eye) from the level of the window edge. This is the length c in the diagram. Use your level to make sure the plane was level while you did this. Use your social skills to get an altitude reading from the pilot. The radius of the Earth is 2 * altitude * a^2 / c^2 If c = 0, then the earth is flat, and the "radius" is infinite. If c>0, the earth is not flat.