Imagine a vast plane with but one tree smack in the middle. If the earth were flat, your vision would extend exactly as far while standing at the base of the tree as it would when at the top of the tree. However, the farther you climb, the farther your line of sight will extend to the horizon. That's because parts of Earth that were concealed from view by its curvature are now revealed because your position has changed. Back to the vast plane. The naked eye can see objects that are millions of miles away in space. Theoretically, with a clear line of sight on a clear night, one would also be able to see bright lights from far-away cities. That this is not possible is further evidence of a round, not flat, Earth.
