The earth turns on its axis every 24 hours — or so we’re taught. But, before we had pictures from space, how would we prove it to someone? The rising and setting of the sun and stars in the sky provide pretty strong evidence, but not absolute proof, since they could be moving and the earth standing still! One piece of evidence from right here on the ground is the behavior of falling objects. Newton‘s laws of gravity and motion predict that a dropped object on a rotating earth should fall slightly toward the east as it drops. Here’s why. Suppose you have a ball suspended from the top of a tower. Because the earth turns, the top of the tower travels just a little faster, in terms of miles per hour, than the ground directly below, just like the edge of a phonograph record travels faster than a point near the center.