>For those who don't know it, in brief: Put a vertical pole in flat ground at noon and measure the length of the shadow. The next day, drive an hour or two directly north or south and do the same measurement at noon again. With a bit of trigonometry, you can estimate the earth's circumference - or for our purposes, at least prove to yourself that the earth is not flat, as the shadows will be different lengths with the sun in nearly the same position both times.

Sure, but a flat earther will just argue that the sun is much closer than scientists claim, which explains the difference in shadow length. And again, what's a layman to do?

The best experiment to prove the earth is round that I've thought up involves the Coriolis Effect and a rifle fired at different points both north and south of the equator. Unfortunately that requires international travel with firearms, making it legally difficult to try.