Measuring the Earth

The Times (June 21st, 2013) also had a piece on the Greek scholar Eratosthenes, going back to the BC period. In the Egyptian city of Syene (near the modern Aswan Dam) he noted that in a deep well, at summer solstice (June 21st) a shaft of sunlight shone straight down into the well without casting a shadow – implying the Sun was directly overhead. At Alexandria, in contrast, he found objects cast a short shadow in the midday Sun on the summer solstice. The Sun was therefore not directly overhead. He surmised the difference was due to the curvature of the Earth. Using geometry he calculated the circumference of the Earth at 250,000 stades (= 39,300km). This turned out to be surprisingly accurate, within 98 per cent of modern calculations. In addition, the angle of the shadow allowed Eratosthenes to work out the tilt of the Earth's axis.