Around 240 BCE, the Greek scholar Eratosthenes of Cyrene, who served as the head librarian at the Library of Alexandria, made a remarkably accurate estimate of the Earth’s circumference—without ever leaving Egypt.

Eratosthenes had heard that at Syene (modern Aswan), the Sun stood directly overhead at noon on the summer solstice, such that vertical objects cast no shadow. In contrast, in Alexandria, located north of Syene, vertical objects still cast a noticeable shadow at the same time. This discrepancy fascinated him.

Using a gnomon (a vertical stick or pole), he measured the angle of the shadow in Alexandria to be about 7.2 degrees, or 1/50th of a full circle. Knowing the approximate distance between Alexandria and Syene (roughly 5,000 stadia, an ancient Greek unit), he used simple geometry to calculate the Earth’s full circumference:

If 7.2° is 1/50th of a circle, then the total circumference = 50 × distance between the cities = 250,000 stadia.

Depending on the exact length of the stadion he used, this figure is astonishingly close to the modern measurement of Earth’s circumference (~40,075 km).

Eratosthenes’ method relied on empirical observation, deductive reasoning, and geometry—marking a major milestone in the development of science. His calculation not only confirmed that the Earth is spherical, but also demonstrated how logic and mathematics could reveal truths about the natural world on a grand scale.