The sky was a tent canopy stretched between mountains at the four corners of the Earth. These fanciful views are Representations rather than Portrayals of reality. The great myths and metaphors make the world seem intelligible and beautiful to the people who invented them. They serve cultural purposes motivated by an aesthetic we can only guess at. The Ancient Greeks were intoxicated by geometry, form, and symmetry.
Ships disappearing over the horizon appear hull-down in the water.
Travelers going south see southern constellations rise higher above the horizon. Shadow of the earth on the moon during a lunar eclipse is round. He also used the curved phases of the moon to argue that the Moon must also be a sphere like the Earth. On that same day, shadows were cast at noon in Alexandria. Therefore, the circumference of the Earth is 50 times the distance from Alexandria to Syene.