The ancient Greeks determined the Earth was a sphere and calculated its diameter over 1700 years before Columbus sailed to America:
“The Greeks had noticed that on occasion, Earth blocks the sunlight from hitting the Moon, causing what is called a lunar eclipse. By observing the shadow of Earth cast upon the Moon during a lunar eclipse, they could see that Earth was also a round body, a sphere, just like the Moon and the Sun.
“Eratosthenes, a Greek scholar and the chief of the famous ancient library of Alexandria, Egypt, around 240 BCE, knew that in a town far to the south, Syene, there was a deep water well. On the summer solstice, the longest day of the year–June 21–the full image of the Sun could be seen reflecting, for a brief moment, in the water of the deep well in Syene precisely at noon. Therefore, the Sun at noon must be passing exactly overhead in Syene. He noticed, however, that on this same day, the Sun did not pass directly overhead in his hometown of Alexandria, which was 800 km (500 mi) due north of Syene. Instead, it missed the zenith, the point directly overhead in the sky, by about seven degrees. Eratosthenes concluded that the zenith direction was different by seven degrees in Alexandria from that in Syene. Using some elementary geometry, he could determine the diameter of Earth and found it to be 12,800 km (8,000 mi).
“Earth’s true diameter, as we know it today, depends slightly upon where you measure it, since Earth is oblate, that is, wider through the equator than through the poles, and it also has mountains, tides, and so on, that require us to quote only an ‘average value.’ The average diameter of Earth through the equator is about 12,760 km (7,929 mi), and through the polar axis, about 12,720 km (7,904 mi). This means that Eratosthenes derived the correct result for Earth’s diameter to an astounding precision of better that 1 percent, assuming Earth was a sphere.
Leon M. Lederman and Christopher T. Hill, Symmetry and the Beautiful Universe, Prometheus, Copyright 2004 by Leon M. Lederman and Christopher T. Hill, pp. 18-19.