How Do We Know the Shape of Earth?

We can tell the Earth is round because it curves around us. Imagine you're on a boat and you watch another ship come toward you, at first, only the top of the ship shows, and then it slowly comes into view as it gets closer. That happens because the Earth curves under the water. Also, when people travel far away, they can see the sun or stars that aren't visible from where you are, like when someone on a plane sees more of the sky than you do.