For one, airplane flight times only makes sense if the world is round.

If it were flat, all planes going to the opposite side of the earth would go over the north pole. But they don't.

If the world were flat, the flight time from Argentina to Australia would be double the flight time from America to Russia. And they would both go over the North pole halfway. But instead they're the same flight time, because they both fly east-west, because the earth is round.

It's obvious.