I mean, you could say that the earth disk is perpendicular to the sun at the time of the eclipse...
What the ancients did to measure the curvature of the earth is they measured from multiple points angles to stars that were sufficiently far away their distance might as well have been infinite. It requires a bit of astronomical knowledge and precise instruments to measure angles to things, but both are very easy to obtain.
Yes there are other ways to prove it but the eclipse is one of the coolest ways.
What the ancients did to measure the curvature of the earth is they measured from multiple points angles to stars that were sufficiently far away their distance might as well have been infinite. It requires a bit of astronomical knowledge and precise instruments to measure angles to things, but both are very easy to obtain.