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Photo of the Day

August 21, 2001

Ancient Measuring Well, Aswan, Egypt, 1998

Photograph by Bob Sacha

"Several stories down but still sunlit, the waters of an ancient well near Aswan, Egypt, mark the arrival of the summer solstice. In the third century B.C. in Alexandria, the Greek scholar Eratosthenes relied on both geometry and reports of the sun's annual appearance almost directly over Aswan to calculate Earth's circumference, arriving at an estimate close to the actual distance. His successors put the figure at far less, giving Columbus the mistaken idea that India lay half as far from Portugal if traveling west across the Atlantic as east across land."

—From "Revolution in Mapping," February 1998, National Geographic magazine