http://en.wikipedia.org/wiki/Spherical_Earth

Aristotle provided physical and observational arguments supporting the idea of a spherical Earth:

Every portion of the Earth tends toward the center until by compression and convergence they form a sphere. (De caelo, 297a9–21)
Travelers going south see southern constellations rise higher above the horizon; and
The shadow of Earth on the Moon during a lunar eclipse is round. (De caelo, 297b31–298a10).

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Eratosthenes, a Greek astronomer from Hellenistic Libya (276–194 BC), estimated Earth's circumference around 240 BC. He had heard that in Syene the Sun was directly overhead at the summer solstice whereas in Alexandria it still cast a shadow. Using the differing angles the shadows made as the basis of his trigonometric calculations he estimated a circumference of around 250,000 stades. The length of a 'stade' is not precisely known, but Eratosthenes' figure only has an error of around five to fifteen percent.[19][20][21] Eratosthenes used rough estimates and round numbers, but depending on the length of the stadion, his result is within a margin of between 2% and 20% of the actual meridional circumference, 40,008 kilometres (24,860 mi). Note that Eratosthenes could only measure the circumference of the Earth by assuming that the distance to the Sun is so great that the rays of sunlight are essentially parallel.

陶哲轩的演讲讲了eratosthenes的故事。

罗马帝国覆灭后，愚昧的天主教是西方文明进入黑暗世纪。是阿拉伯人摆脱了基督的咀咒revive了希腊并引进了波斯和印度的科学。