Time once again for the mail bag. Let's see . . .
Dear Moment of Science: Here's an idea I had for proving that the earth is round. Why not spread out a huge sheet of tinfoil, press it down, and then lift it up and see if it's bent? Would this work?
Well, listener, the answer to your question is: yes. To simplify things, let's imagine that the earth had a perfectly smooth surface. If we take a smooth, spherical object like a bowling ball, and press a sheet of tinfoil down on it so the foil touches the ball's surface at all points, the curvature of the ball would be easily seen in the curved shape of the tinfoil when removed. Notice, though, that if you only examine a small section of the foil--say the size of a postage stamp--you wouldn't see the curvature. Each little piece by itself looks flat!
It's the same with the earth. Any realistically sized piece of tinfoil laid out on the earth will look entirely flat -- the curvature is just too faint to be perceived at that level. But that doesn't mean the idea is wrong; just that our scale of measurement is off. As a matter of fact, if you could do this with a tinfoil sheet two miles wide, your tinfoil would bend by a little over five inches--owing entirely to the curvature of the earth!