In 1870, Alfred Russell Wallace successfully demonstrated the curvature of the Earth to answer the challenge put forward by a flat-Earther, John Hampden.
To most flat-earthers, the Earth is stationary. Stars are light sources attached to the firmament (for some mysterious reason), and they rotate around Polaris (again, for some mysterious reason).
However, they are missing the fact that Polaris is never visible from the southern hemisphere. And furthermore, the southern stars also rotate around the south celestial pole.