Have you ever noticed that statues of Atlas (pictured above), he of holding the earth upon his shoulders fame, always include the earth as a large orb? The earth is never pictured as a giant pizza-shaped object. One reason might be that the ancient Greeks figured out a long time ago the earth was not only round, but also it's circumference!

That Washington Irving! Little did people realize that he was writing fiction when he composed the Story of Christopher Columbus. It is Irving that is responsible for much of the myth surround Cristobal Colon and his epic journey, including the part where people in 1492 thought the earth was flat. That just wasn't true. Carl Sagan did an excellent job of explaining how one particular ancient Greek determined the girth of the earth over a thousand years before Columbus:

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 Eratosthenes 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.

The simple geometry behind his calculations.