The Ancient Greeks

Championed by Aristotle, the the theory of a Round Earth began in classical antiquity, contradicting the prevailing belief of a Flat Earth. Aristotle founded his theory on three proofs, which are still in use today. As Greek and Roman influence and culture spread throughout the ages the concept of a Round Earth overthrew the ancient knowledge, establishing itself as the cornerstone of Western civilization.

Aristotle's Round Earth Proofs
There are three proofs Aristotle gave for the supposed rotundity of the earth. They are:
 * Ships Appear to Sink as They Recede past the Horizon
 * The Shadow on the Moon During a Lunar Eclipse Is Round
 * The Southern Constellations of the Night Sky Appear to Rise as You Travel Southwards

Eratosthenes' Shadow Experiment
Eratosthenes is famed for his shadow experiment where he determined the circumference of the Round Earth and the distance to the sun. However, this experiment assumes that the earth is a globe and that the sun is far away. The experiment can also be interpreted under a Flat Earth model as well. See:
 * Eratosthenes on Diameter
 * Distance to the Sun

Continuous Universe
The Ancient Greeks believed in a Continuous Universe. This Ancient Greek concept of a perfect universe assumes the following:
 * That perfect circles can exist
 * That one could zoom into a circle forever and see a curve
 * That any length of space can be divided into infinitely smaller parts
 * That the space can be infinitely long
 * Time can likewise be infinitely divided, or infinitely long
 * Light rays travel in perfectly straight lines into infinity
 * The Perspective Lines receded infinitely and continuously into the distance