Eratosthenes’ Shadow

Measuring a planet with a stick

An interactive lesson with 8 steps, about 7 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Parallel sunlight makes different shadow angles at different places on a round Earth.
• The angle difference is the angle at the Earth’s centre between the two places.
• Eratosthenes found a circumference close to the true 40,000 km.

Steps

1. A round Earth
2. Noon in two cities
3. Your turn
4. The key assumption
5. A flat Earth with a nearby Sun
6. Shadow to angle
7. The radius
8. An error in the distance