How was the first man to theorize that the earth was a sphe?

The first man to theorize that the Earth was a sphere was the ancient Greek philosopher Pythagoras, around the 6th century BCE. He based his idea on observations of the moon and stars, noting that they appeared round and suggesting that Earth, too, must be spherical. Later, philosophers like Plato and Aristotle supported this notion with additional observations, such as the shape of the Earth’s shadow during lunar eclipses. This concept was further solidified by the work of astronomers like Eratosthenes, who calculated the Earth's circumference in the 3rd century BCE.