**Question**

**Describe how Eratosthenes calculated Earth’s circumference.**

**Answer**

Eratosthenes, an ancient Greek mathematician and astronomer, calculated the circumference of the Earth in 240 BC. He noticed that at noon on the summer solstice, the sun shone directly down a deep well in the city of Syene (modern-day Aswan in Egypt), causing no shadow. At the same time, in Alexandria, Egypt, which was due north of Syene, the sun was at an angle, creating a shadow.

Eratosthenes measured the angle of the shadow using a gnomon (a vertical rod), and used his knowledge of geometry to calculate the angle of the sun’s rays. By using the distance between Alexandria and Syene, which he obtained from camel caravans, he was able to calculate the circumference of the Earth to be approximately 39,375 kilometres, which is very close to the modern measurement of 40,075 kilometres.