Eratosthenes’ Measurement of the Earth’s Circumference

**Eratosthenes** was the first to calculate Earth's circumference. He observed that in Alexandria, sunlight struck the ground at an angle of approximately 7 degrees from the vertical at noon on the summer solstice, which could be determined by the shape of the shadow. Approximately 80 kilometers to the southeast, the Sun's rays were observed to fall vertically. Eratosthenes knew that the entire Sun could be seen as a reflection in a well at that location, indicating the Sun was directly overhead. He assumed that the Sun was very far away, its rays being practically parallel when they reached Earth. Knowing the distance between the two locations, he was able to calculate Earth's circumference. However, the exact length of the units he used is uncertain, and as a result, the precision of his calculation remains unknown.