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!

All About Evidence

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.

If you drop the ball, gravity will pull it toward the center of the earth, but will not diminish that extra eastward speed.  So as the ball falls it'll outrun the ground and land slightly east of the straight-down point.

Edwin W. Hall

Edwin H. Hall, a Harvard University physicist, looked for this effect in 1902.  Working in an enclosed tower, he dropped one-inch bronze balls from a height of 75 feet into a pan of tallow.

Hall was careful.  He built a mechanism to release the ball without jiggle or spin, and a cloth tube to protect the path of fall from breezes.  He used high-precision surveying telescopes to measure the position of a plumb line before each drop, and the position of the ball afterward.  He made a total of 948 drops.

The Result?

The average eastward deflection was just as predicted by Newton's laws for a 75-foot tower at the latitude of Cambridge, Massachusetts: about one-seventeenth of an inch.