Tidal Locking

moon · gravity · space · rotation

The Moon's lock. The Moon is tidally locked to Earth, which is why we always see the same face of the Moon from our planet. The Moon takes about 27 days to spin around once, and that's exactly how long it takes to orbit Earth.

Gravity pulls it still. Tidal locking happens because gravity from the larger body gradually slows down the smaller one's spin over millions of years. Eventually, the smaller object rotates at just the right speed so one side always points toward its partner.

Two-way locking. Usually only the moon gets locked to the planet, but sometimes both objects lock to each other. Pluto and its moon Charon are locked together this way, always showing each other the same face.

Takes time to happen. Tidal locking doesn't happen overnight, it takes millions or billions of years depending on how big the objects are and how close they orbit. Moons that are closer to their planets get locked faster.

Common in space. Many moons throughout our solar system are tidally locked to their planets, which means alien astronomers visiting other worlds would see the same landscape every time they look up.