Lie down on the floor and wait for the sunset exactly when the sun disappears below the horizon, stand up and wait for the next sunset. -Wait a minute, next sunset on the same day? -Yeah, only a while after you stand up, the sun will go away again (about 10s, depending on your height) Is it possible to use that information to calculate the Earth radius? The answer is yes. You can find out the size of the Earth, on the beach, just using your height and a chronometer!