The concept of the Earth being round was not proven by a single individual. The understanding that the Earth is spherical dates back to ancient times and was gradually developed through the contributions of numerous civilizations and scholars.
One of the earliest documented proponents of a spherical Earth was the ancient Greek philosopher Pythagoras (6th century BCE), who theorized that the Earth was a sphere based on observations of eclipses and the circular shadow cast by the Earth on the Moon during lunar eclipses.
However, it was the Greek philosopher and mathematician Eratosthenes (3rd century BCE) who made an important contribution to measuring the Earth's circumference and providing evidence for its roundness. Eratosthenes used the angle of the Sun's rays at different locations on Earth to calculate the Earth's circumference with remarkable accuracy.
Later on, in the 2nd century BCE, the Greek astronomer and mathematician Hipparchus further refined the measurements of the Earth's size and shape.
It's important to note that the concept of a spherical Earth was not universally accepted during these ancient times. It took many centuries and additional scientific advancements for the understanding of the Earth's shape to become widely accepted.