Although there is a tendency to dwell on those ancients who may have envisioned the Earth as flat, knowledge that the Earth and Moon are spherical was widespread in the Greek world by the fifth century B.C. Aristotle offered three arguments to justify the belief that the Earth was a sphere. First, he noted that when watching a ship sail away from the shore, the last observable part on a clear day was the top of the mast. Second, he argued that the circular shadow projected by the Earth when it eclipsed the Moon was clear evidence of the Earth's spherical nature. The third argument was the changing pattern of stars that could be observed above the northern and southern horizons when one travels north or south. These arguments had been known long before Aristotle's time.
As Hellenistic culture spread throughout the eastern Mediterranean world, a new establishment for science was centered in Alexandria after about 300 B.C. The Museum and its associated Library in Alexandria was one of the most famous centers of learning in the ancient world. The Museum was a center for scientific and mathematical research. One of its geographers, Eratosthenes (273-193 B.C.), knowing that the Earth was a sphere, and from the earlier work of Aristarchus (320?-?250 B.C.), that the Sun was at least 20 times farther away than the Moon (the correct value is nearer 400), reasoned that rays of sunlight ought to be parallel when they reach the Earth, enabling him to measure the Earth's circumference.
Eratosthenes chose observing stations at Alexandria and Syene to the south, where the Aswan Dam is now located on the Nile River. For the time of the experiment he chose local noon on the day of the summer solstice, which comes at the same moment at both sites because they are very nearly on the same meridian of longitude. He probably selected that day, since the Sun was as far north as it would be during the year, meaning that it would pass very near the zenith at local noon at Syene.
At noon an observer in Syene observed that the Sun was directly overhead, while an observer in Alexandria found the Sun to be 7o south of the zenith. Measurers had paced off the distance between the two cities as about 4900 stadia (1 stadium = 0.16 kilometer). Because a straight line cuts two parallel lines at equal angles, the angle at the center of the Earth is equal to the zenith angle, 7o. Working a simple proportion, Eratosthenes found the Earth's circumference as follows: C/4900 stadia = 360o/7o, or C = 252,000 stadia, or about 40,320 kilometers (km). In principle, Eratosthenes' experiment was correct. Although his measuring technique was inaccurate by modern standards, his results were surprisingly close to today's mean value of 40,030 km.