It Is Probably a Sphere

Before any celestial distances or sizes could be calculated, the
ancient Greeks first had to establish that the Earth is a sphere. This view gained acceptance in ancient Greece as philosophers became familiar with the notion that ships gradually disappear over the horizon until only the tip of the mast could be seen. This made sense only if the surface of the sea curves and falls away. If the sea has a curved surface, then presumably so too does the Earth, which means it is probably a sphere.

Simon Singh
Big Bang: The Origin of the Universe