Ask the Astronomer
Astronomer William Georgevich replies:
There are many proofs of the Earth's roundness. They began with the ancient Greek philosophers who knew that the shadow of the Earth caused lunar eclipses. Because the the Earth's shadow racing across the moon was curved this was one proof that the earth was round.
Another proof was offered by Galileo with his new invention the telescope. When looking out to sea at the horizon, Galileo noticed that at first he noticed the top of the sailing ship (the crow's nest and mast), and then he saw the rest of ship rising into view. This meant that the ocean was slightly curved. You can do the same experiment with a basket or beach ball at home by keeping your eye fixed in one position on the ball while tracing an object such as pencil along the curve of the ball. First you will see the eraser followed by the rest of the pencil as it comes into view.