| Eratosthenes'
calculations were based on two assumptions. The
first was that Syene lay on the Tropic of Cancer.
The second assumption was that Alexandria lay due
north of Syene on exactly the same line of longitude
(the meridian line). At noon during the summer
solstice, the rays of the sun always shine directly
perpendicular to the Earth's surface, but only
on the Tropic of Cancer. If Alexandria was exactly
due north of Syene, then Eratosthenes could argue
that the key measurements he used -- the length
of the column's shadow in Alexandria and the distance
between Alexandria and Syene -- were geographically
sound. |
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