Eratosthenes was an astronomer, historian, mathematician, poet, philosopher, and a director of the great Library of Alexandria (in Egypt, circa 3rd century B.C.E.)... Eratosthenes had learned that at noon on June 21^st  an obelisk in the city of Syene (near present-day Aswan) cast no shadow, however at that very same day and time, he knew from personal experience that similar objects at Alexandria still cast an observable shadow...