Its the ancient greeks who had figured out that earth is round ( sphere ). Its a 2000 years old story. Some believe that it was Pythagoras who first come up with his observation that the Earth was round in 500 B.C. Anaxagoras believed that the cause of solar and lunar eclipses , and then the shape of the Earth’s shadow on the Moon during a lunar eclipse is an evidence that the Earth was round.
Later Over 2000 years ago around 350 B.C. he Greek philosopher Aristotle Declared that earth is round . After That a Greek thinker and mathematician, Eratosthenes and Aristarchus, took it forward and measured the Earth’s circumference.