‘The discovery that the Earth is a little world was made, as so many important human discoveries were, in the ancient Near East, in a time some humans call the third century BC, in the greatest metropolis of the age, the Egyptian city of Alexandria. Here there lived a man named Eratosthenes.

… He was an astronomer, historian, geographer, philosopher, poet, theater critic and mathematician…. He was also the director of the great library of Alexandria, where one day he read in a papyrus book that in the southern frontier outpost of Syene … at noon on June 21 vertical sticks cast no shadows. On the summer solstice, the longest day of the year, the shadows of temple columns grew shorter. At noon, they were gone. The sun was directly overhead.

It was an observation that someone else might easily have ignored. Sticks, shadows, reflections in wells, the position of the Sun—