探索頻道 Cosmos 宇宙的邊疆09
It is about 50 kilometres west of what was once one of the great cities of the world, Alexandria. In Alexandria at that time, there lived a man named Eratosthenes. One of his envious contemporaries called him "Beta", the second letter of Greek alphabet, because he said Eratosthenes was second best in the world in everything. But it seems clear that in many fields, Eratosthenes was "Alpha". He was an astronomer, historian, geographer, philosopher, poet, theater critic and mathematician. He was also the chief librarian at the Great Library of Alexandria. And one day while reading a papyrus book in the library, he came upon a curious account.
"Far to the south", he read, "at the frontier outpost of Syene, something notable could be seen on the longest day of the year." On June 21st, the shadows of a temple column or a vertical stick would grow shorter as noon approached. And as the hours crept to its midday, the sun's rays would slither down the sides of a deep well, which on other days would remain in shadow. And then precisely at noon, columns would cast no shadows and the sun would shine directly down into the water of the well. At that moment, the sun was exactly overhead.
It was an observation that someone else might easily have ignored--sticks, shadows, reflections in wells, the position of the sun—simple, everyday matters. Of what possible importance might they be? But Eratosthenes was a scientist and his contemplation of these homely matters changed the world, in a way, made the world. Because Eratosthenes had the presence of mind to experiment to actually ask whether back here, near Alexandria, a stick cast a shadow near noon on June the 21st. And it turns out, sticks do.
An overly skeptical person might have said that the report from Syene was an error, but it’s an absolutely straightforward observation. Why would anyone lie on such a trivial matter? Eratosthenes asked himself how it could be that at the same moment, a stick in Syene would cast no shadow and a stick in Alexandria 800 kilometers to the north would cast a very definite shadow.
Here's a map of ancient Egypt. I have inserted two sticks, or obelisks, one up here in Alexandria and one down here in Syene. Now if at a certain moment, each stick casts no shadow, no shadow at all. That's perfectly easy to understand, provided the earth is flat. If the shadow at Syene is at a certain length and the shadow at Alexandria is the same length, they also make sense……
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