1. The Blue Marble
The most straightforward way to check the shape of the Earth is to look at it. There's one small problem, though. To see the shape of the Earth as a whole, you need to be far enough away from it. For most of human history, this has not been possible. It was only with the advent of the space age that our technology has allowed us to send a human being, or a camera, more than a few kilometres from the surface.