Eli5: How did people in olden days (such as Galileo) come to the conclusion that the earth was spherical without very advanced technology?

Question

Eli5: How did people in olden days (such as Galileo) come to the conclusion that the earth was spherical without very advanced technology?

In: Physics

Answer 1

Astronomy, shadows, math.

Using data collected from available sources like stars, moon, sun, shadows, distance between shadows, then do math.