Excerpt from
Cosmos by Carl Sagan
The discovery that the Earth is a little world
was made, as to many important human discoveries were, in the ancient
Near East, in
a time some humans call the third century BC, in the greatest
metropolis of
the age, the Egyptian city of Alexandria. Here there lived a man named
Eratosthenes. 
One of his envious contemporaries called him "Beta," the
second
letter of the Greek alphabet, because, he said, Eratosthenes was second
best
in the world in everything. But it seems clear that in almost
everything
Eratoshtenes was "Alpha."
He was an astronomer, historian, geographer, philosopher, poet, theater critic and mathematician. He was also the director of the great library of Alexandria, where one day he read in a papyrus book that in the southern frontier outpost of Syene, at noon on June 21 vertical sticks cast no shadows. On this the summer solstice, the longest day of the year, shadows of temple columns grew shorter. At noon, they were gone. The sun was directly overhead.
It was an observation that someone else might easily have ignored. Sticks, shadows, reflections in well, the of the Sun - of what possible importance could simple everyday matters be? But Eratosthenes was a scientist, and his musings on these commonplaces changed the world; in a way, they made the world. Eratosthenes had the presence of mind to do an experiment, actually to observe whether in Alexandria vertical sticks cast shadows near noon on June 21. And, he discovered, sticks do.
Eratosthenes asked himself how, at the same moment, a stick in Syene could cast no shadow and a stick in Alexandria, far to the north, could cast a pronounced shadow. Consider a map of ancient egypt with two vertical sticks of equal length one stuck in Alexandria, the other in Syene. Suppose that, at a certain moment, each stick casts no shadow at all. This is perfectly easy to understand - provided the Earth is flat. The Sun would then be directly overhead. If the two sticks cast shadows of equal length, that also would make sense of a flat Earth: the Sun's rays would then be inclined at the same angle to the two sticks. But how could it be that at the same instant there was no shadow at Syene and a substantial shadow at Alexandria?
The only possible answer, he saw was, that the surface of the Earth is curved. Not only that: the greater the curvature, the greater the difference in the shadow lengths. The Sun is so far away that its rays are parallel when they reach the Earth. Sticks placed at different angles to the Sun's rays cast shadows of different lengths. For the observed difference in the shadow lengths, the distance between Alexandria and Syene had to be about seven degrees along the surface of the Earth; that is, if you imagine the sticks extending down to the center of the Earth, they would there intersect at an angle of seven degrees. Seven degrees is something like one-fiftieth of three hundred and sixty degrees, the full circumference of the Earth. Eratosthenes knew that the distance between Alexandria and Syene was approximately 800 kilometers, because he hired a man to pace it out. Eight hundred kilometers times 50 is 40000 kilometers: so that must be the circumference of the Earth. This is the right answer. Eratosthenes' only tools were sticks, eyes, feet and brains, plus a taste for experiment.Next