There's no single person who "discovered" that the Earth is not flat. The understanding of Earth's spherical shape developed gradually over centuries through observations and theories from different cultures and civilizations.

Here's a breakdown:

* Ancient Greeks: Philosophers like Pythagoras and Aristotle in the 6th and 4th centuries BC proposed the Earth was a sphere based on observations of lunar eclipses, the changing position of stars, and the disappearance of ships over the horizon.

* Eratosthenes: Around 240 BC, Eratosthenes, a Greek scholar, calculated the Earth's circumference with remarkable accuracy, further supporting the spherical model.

* Medieval and Renaissance: The concept of a spherical Earth was accepted by many scholars and navigators during this period. The voyages of explorers like Ferdinand Magellan in the 16th century provided further evidence by circumnavigating the globe.

* Modern Era: Modern technology, including satellites and space travel, has provided definitive proof of Earth's shape.

Therefore, attributing the discovery to a single person would be inaccurate. It was a collective effort spanning centuries and different cultures.