It's impossible to say for sure who was the *first* to realize the Earth wasn't flat, as this knowledge likely developed gradually over time. However, we can point to some key figures who contributed to the understanding of the Earth's spherical shape:

* Ancient Greeks:

* Pythagoras (c. 570-495 BC) is credited with being one of the first to propose a spherical Earth based on philosophical and mathematical arguments.

* Aristotle (384-322 BC) provided more concrete evidence, observing the shape of the Earth's shadow during lunar eclipses, and noting that different stars are visible at different latitudes.

* Eratosthenes (c. 276-194 BC) conducted a famous experiment to measure the circumference of the Earth, providing strong evidence for its spherical shape.

While these Greeks were aware of the Earth's sphericity, it wasn't universally accepted at the time, and many people still clung to the flat-Earth model. The idea of a spherical Earth gained more traction during the Renaissance and Age of Exploration, with explorers like Ferdinand Magellan (who circumnavigated the globe) providing irrefutable evidence.

It's crucial to remember that the idea of a flat Earth was a gradual process, not a single moment of discovery. It's the culmination of observations, experiments, and scientific reasoning over centuries.