Eratosthenes knew that at local noon on the summer solstice in the Ancient Egyptian city of Swenet ( known in ancient Greek as Syene , and now as Aswan ) on the Tropic of Cancer , the Sun would appear at the zenith , directly overhead .
He knew that on the summer solstice at local noon at the city now called Aswan on the Tropic of Cancer , the sun would appear at the zenith , directly overhead .