Tidal Locking

Why the Moon shows one face

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

• Tidal friction slows Earth’s spin and pushes the Moon outward (3.8 cm per year).
• The Moon is tidally locked: it spins once per orbit.
• Locking is common among moons; Pluto and Charon are locked to each other.

Steps

1. Tidal friction
2. The Moon’s spin
3. A million years
4. Slowing spin
5. Counting ancient days
6. The far future