Original: Eratosthenes knew that on the summer solstice at local noon in the Ancient Egyptian city of Swenet ( known in Greek as Syene , and in the modern day as Aswan ) on the Tropic of Cancer , the sun would appear at the zenith , directly overhead .
Simplified: 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 .