Proving the Earth Turns

Foucault’s pendulum and the bulging Earth

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• A Foucault pendulum's swing rotates once every $24\text{ h}/\sin\varphi$.
• Earth's spin reduces effective gravity at the equator by $\omega^2 R \approx 0.034$ m/s².
• The spin also flattens the Earth: it bulges by about 21 km at the equator.

Steps

1. The Panthéon, 1851
2. At 30°
3. At the equator
4. Lighter at the equator
5. Ten degrees an hour
6. In Paris