The man who first measured the world, the Greek astronomer Eratosthenes (c. 276-196 B.C.), lived in Alexandria during the 3rd century B.C. He noticed that on the first day of summer in Syene (now Aswan), Egypt, the Sun appeared directly overhead at noon. At the same time in Alexandria, however, the Sun appeared slightly south (about 7 degrees) of the zenith. Knowing the distance between Syene and Alexandria and assuming that the Sunís rays were parallel when they struck the curved Earth, he calculated the size of our planet using simple geometry. His result, about 25,000 miles for the circumference, proved remarkably accurate.
Eratosthenes wasnít the only Greek who tried to measure the Earth. About a century later, Posidonius copied this feat, using the star Canopus as his light source and the cities of Rhodes and Alexandria as his baseline. Although his technique was sound, he had the wrong value for the distance between Rhodes and Alexandria, so his circumference came out too small. Ptolemy recorded this smaller figure in his geography treatise, where it was seized upon by Renaissance explorers looking for a quicker way to the Indies. Had Ptolemy used Eratosthenesí larger figure instead, Columbus might never have sailed west.