During the second century BCE, Greek scientist Eratosthenes figured out the circumference of the Earth by measuring the shadows caused by sunlight in two different cities at the same time. If the Earth were flat, the shadow’s angles would be equal, but they were 7.2 degrees different, roughly 1/50th of a circle. By multiplying the distance between the cities by 50, he came within 1 percent of the Earth’s circumference.