Why we always see the same side of the Moon
Physics · Key Stage 2 · The School
A common but wrong assumption
From Earth, the Moon always shows the same face. This is true night after night, year after year. It is tempting to conclude the Moon simply does not spin at all. But that conclusion is wrong. The Moon DOES spin on its own axis. It just spins at exactly the right speed to make it look like it never does.
One spin, one orbit, perfectly matched
The Moon takes about 27 days to spin once on its axis. It also takes about 27 days to orbit the Earth once. The two times match almost exactly. Because the spin and the orbit are locked together like this, the Moon always shows Earth the same face. Picture someone walking in a slow circle around a chair. If they keep turning to always face the chair, they show it the same side of themselves the whole way round.
Why this matching is called tidal locking
This matching is called TIDAL LOCKING. Earth's gravity caused it, over billions of years. It gradually slowed the Moon's original, faster spin. That slowing continued until the spin matched the orbit exactly. This is not a coincidence, and it is not a special property of the Moon alone. Many moons around other planets are tidally locked to their planet in exactly the same way.
One pupil stands still in the middle of a room as "Earth". Another pupil walks slowly in a full circle around them. They keep turning, to always face the person in the middle. This walker is the "Moon", orbiting once. By the time they have gone all the way round, they have also turned exactly once on the spot. It did not feel like spinning at all, from the inside. A pupil watching from outside the circle could see the walker's back turn to face every direction in turn. The person in the middle, though, never saw anything but the walker's face. That is exactly the situation Earth is in with the Moon.