It all started as a philosophical concept developed by the Greeks around the 6th century BC. It wasn’t until the 3rd century BC when Hellenistic astronomy came to the conclusion that the earth was in fact physically spherical.
At first, there was no real explanation of how the Greeks came up with this conclusion, other than their observation on the change of the stars when they traveled.
Pythagoras and Plato taught their students that the world was round but they had no justification or proof. The round Earth theory didn't get a proper explanation until Aristotle (384-322 BC) made three important observations:
- Every portion of the Earth tends toward the center until by compression and convergence they form a sphere.
- Travelers going south see southern constellations rise higher above the horizon; and
- The shadow of Earth on the Moon during a lunar eclipse is round.
This knowledge was then passed from generation to generation. It was then spread to the eastern world where they thought that the earth was a flat disk floating in an ocean surrounded by a spherical sky, as explained by the Mesopotamian mythology.