There are a few ways in which the roundness of the earth is directly observable, aside from blasting off in a home made rocket. Ships rising above the horizon has been mentioned, and is perhaps one of the best examples. There are others, even more difficult to refute, but they require some effort, and who would expend any significant effort to challenge their world view? Not a zealot…
I love mentioning to a flat earther that astro nav works perfectly as predicted by the round earth model, or that the distances across the world oceans don’t tally with the way they see things. Their eyes go narrow with suspicion, and they mumble “I see…” before scurrying away from this dangerous agent of the conspiracy.
I wonder what will happen if the Flat Earth International Conference circumnavigates the Antarctic and observes that it takes a surprisingly short time. They will probably conclude that the earth is much smaller than previously thought…