Eratosthenes

The ancient Greek mathematician and geographer Eratosthenes is credited with making the first accurate estimation of the Earth's circumference. In the third century BC, he observed that at noon on the summer solstice, the sun was directly overhead at the city of Syene (present-day Aswan, Egypt). At the same time, in the city of Alexandria, he measured the angle of the sun's rays relative to the ground and found it to be about 7.2 degrees.  Using these observations, Eratosthenes was able to calculate the Earth's circumference by assuming that the sun's rays are parallel (which is not exactly true, but close enough for his purposes) and using simple geometry. His estimate was remarkably accurate, and modern measurements have shown that it was only about 15% too high.  He was also the first to calculate Earth's axial tilt, which also proved to have remarkable accuracy.  He created the first global projection of the world, incorporating parallels and meridians based on the available geographic knowledge of his era.   He is also known for the sieve of Eratosthenes.