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.

"Appearances vs. Facts

The phenomenon of the disappearance of a ship, hull first, as it recedes from view, is caused by the same law of foreshortening as that which governs the disappearance of the rail, or causes the two rails to appear to approach each other. If we should make calculations on the basis of the appearance instead of on the basis of the fact that the rails do not approach, but only seem to, we necessarily draw false conclusions. This is precisely what the astronomers do. They conclude from appearances rather than from facts." -- The Cellular Cosmogony,  Cyrus Teed