So, you see, Columbus was really the first man to say, I believe the earth is round and I will show you that it is by sailing to the lands that are on the other side of the earth.
He even figured out how far it was around the world.

Your geography, you know, tells you now that what is called the circumference of the earth--that is, a straight line drawn right around it--is nearly twenty-five thousand miles.

Columbus had figured it up pretty carefully and he thought it was about twenty thousand miles.

If I could start from Genoa, he said, and walk straight ahead until I got back to Genoa again, I should walk about twenty thousand miles.

Cathay, he thought, would take up so much land on the other side of the world that, if he went west instead of east, he would only need to sail about twenty-five hundred or three thousand miles.
If you have studied your geography carefully you will see what a mistake he made.
It is really about twelve thousand miles from Spain to China (or Cathay as he called it).