Eratosthenes Measures Earth's Circumference

Around 240 BCE, Eratosthenes of Cyrene, head librarian at the Library of Alexandria, calculated Earth's circumference. He observed that sunlight reached the bottom of a well at noon during the summer solstice in Syene but cast shadows in Alexandria. Eratosthenes calculated the angle difference between locations as approximately 7.2 degrees (1/50th of a circle). By multiplying the known distance between the cities by 50, he determined Earth's circumference to be approximately 250,000 stadia, close to modern measurements. This calculation demonstrated Earth's spherical shape and size using geometric principles and observations.