© Sierra College Astronomy Department
Ancient Astronomy Eratosthenes (276-195 B.C.)
Measuring the Size of Earth
Eratosthenes devised a clever way to measure the Earth’s size. He observed that when the Sun was overhead (at the zenith) at Syene, it was 7° from overhead at Alexandria.
Since 7° is about 1/50 of a full circle (360°), the circumference of the Earth should be 50 times the distance from Syene to Alexandria, or 50 x 5,000 stadia = 250,000 stadia.