Earth's circumference was once measured using stick's shadow

Greek astronomer Eratosthenes made the first recorded measurements of Earth's circumference around 240 BC, using only a stick. Eratosthenes observed during the summer solstice, the year's longest day, Sun wasn't casting a shadow of the stick in one city but was casting a shadow in another city. He used this difference and distance between the cities to calculate Earth's circumference.