Gulf of understanding: The way we see the Earth has never been set in stone

Katherine Dunn is a journalist based in London. Her book on the history of GPS will be published in 2026. Some time around 250 BC, Eratosthenes - a Greek poet, mathematician and chief librarian of the Library of Alexandria - looked into a well in Syene, which is now the city of Aswan in Egypt, and saw the shape of the Earth. It was the summer solstice, and the light of the sun shining directly overhead disappeared neatly into the darkness of the well. At exactly the same time, Eratosthenes later realized, the sun was casting a shadow in Alexandria….