The fact that the earth is a sphere is a fact that few people dispute today. The Greek philosopher and mathematician Aristotle realised this about 2,300 years ago, based on an amazingly simple observation: he was fascinated by lunar eclipses. And the shadow, in the form of the earth, which pushed itself in front of the moon, was… round. Aristotle concluded quite correctly that the earth must be a sphere. However, the definitive proof of this theory was only found some 150 years later by a library administrator.
All the knowledge of the whole world, one well and one summer solstice
The Greek Eratosthenes administered the largest library in the world: the Library of Alexandria in what is now Egypt, which housed all the knowledge of the time. He knows about Aristotle's assumption regarding the shape of our planet but wants to know exactly. The earth is round – but is it a sphere or a disk? The solution begins with a well in Syene (today Aswan). Eratosthenes observes that the water level of the fountain is in shadow all year round, except on one single day: at noon on 21 June, the summer solstice, the water reflects the sun. The library administrator concludes that the sun must be exactly perpendicular above him. On the same day in Alexandria, 800 kilometers away, however, an upright rod casts a shadow. Accordingly, the sun is not perpendicular above the viewer. From these two observations, Eratosthenes is the first person ever to calculate the circumference of the Earth and proves that the Earth is a sphere.