One of the standards of the past, that shows that the earth isn't flat, but that it is rather spherical, is that of sailing ships.

On a rather calm yet breezy day, get out to the ocean. Through a telescope, watch a sailing ship coming in towards land. At a great distance away, only the topmost parts of the masts are visible on the horizon. As the ship approaches, more of the masts become visible. Finally, as the ship is reasonably close, the whole of the ship that is above water becomes visible.