Eratosthenes’ Measurement of the Earth’s Circumference

**Eratosthenes** was the first who calculated the **Earth's circumference**. He noticed that the sunlight in **Alexandria** fell at an angle of about **7 degree** from the vertical at noon on the summer solstice. It is possible to calculate it on the **base of the shape of a shadow**. Eighty kilometers to the southeast, the Sun's rays fell **vertically**.

Eratosthenes read that whole Sun can be seen as a reflection in a well. Eratosthenes assumed the Sun's distance to be very great and its rays therefore are practically **parallel** when they reach the Earth. He knew the distance between the two places. He was able to calculate the **circumference of the Earth**. The exact length of the units he used is doubtful, and the accuracy of his result is therefore uncertain.