Eratosthenes accurately calculated Earth's circumference in ancient times using observations of shadows.

Around 240 BCE, the Greek polymath Eratosthenes conducted a groundbreaking experiment. He noted that on the summer solstice, the sun cast no shadows at noon in Syene (modern-day Aswan, Egypt), indicating the sun was directly overhead. Simultaneously, in Alexandria, located approximately 800 kilometers north, shadows were cast at an angle of about 7.2 degrees. By assuming the Sun's rays were parallel and that Earth was a sphere, he deduced that the angular difference represented the curvature between the two cities. His calculation for Earth's circumference was remarkably close to modern measurements, proving the planet's spherical shape thousands of years ago, using nothing more than geometry and observation.