How Did Eratosthenes Measure the Earth?
Could you measure our planet using only a stick, a shadow, and geometry?
The Answer
Eratosthenes compared the Sun’s angle in two Egyptian cities. In Syene, near modern Aswan, the noon Sun at the summer solstice was almost directly overhead. In Alexandria, a vertical object cast a shadow.
How Did It Work?
The Alexandria shadow showed an angle of about 7.2° — one-fiftieth of a full circle.
If the distance from Syene to Alexandria was about one-fiftieth of Earth’s circumference, multiplying that distance by 50 gave an estimate of the whole circumference.
Why Was This Brilliant?
He did not need to travel around the world. Observation + mathematics + reasoning turned a simple shadow into a tool for measuring a planet.
Try It Yourself
Place a straight stick vertically in sunlight and measure its noon shadow. Compare your measurement with one made at the same time far north or south of you. The difference in angles can be used to estimate Earth’s size.
From Voices of the Past: Origins
Great discoveries can begin with a tiny question: Why is there a shadow here… but not there?