Tidal locking
A Socratic walk-through of tidal locking — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does the Moon keep nearly the same face turned toward Earth?
The Moon shows the same markings year after year, so the first thought is that it must not be spinning. But it circles us every month, and a body that did not turn on its own axis would, over one lap, present every side to us in turn. So the Moon is turning — at precisely the rate that keeps one face aimed our way. What could arrange that?
Reasoning it through
REASONING #Gravity weakens with distance, so Earth pulls harder on the Moon's near side than its far side. What should that do to a body of rock that can flex? Stretch it — a bulge toward Earth, another away. The Moon has tides too, in stone rather than water.
Now suppose the young Moon spun faster than it orbited. The bulge belongs where Earth is, but rotation keeps carrying material out of that spot, and rock does not flex instantly, so the high point is dragged slightly ahead of the Earth-Moon line. Being nearer, it is pulled harder, and off-centre — which twists the Moon backwards against its own spin.
When does that braking stop? Only when the bulge no longer has to travel. Once the Moon turns once per orbit, the pull runs straight down the line, there is no lever, and the torque falls to zero. The spin was not switched off; it was drained until nothing further could be taken.
The analogy
THE ANALOGY #A compass needle dropped into a magnetic field swings, overshoots, swings back, and settles pointing one way as friction bleeds the wobble off. The Moon's long axis is that needle, Earth's uneven pull the field, and friction inside the Moon's rock is what bleeds the motion away.
a needle is a rigid magnet placed in someone else's field, whereas the Moon's "needle" — its bulge — is raised by the very body it then points at.
Clarifying the model
THE MODEL #"The same face" is only nearly true. The Moon spins steadily but its orbit is elliptical, so it runs ahead of schedule near perigee and behind near apogee; that mismatch rocks the visible face side to side, and its tilted axis nods it up and down. Over time we see about 59 percent of the surface — an effect called libration.
Nor is 1:1 the only resting place: Mercury settled at 3:2, so tides drive a body into some stable resonance, which one depending on eccentricity and shape.
A picture of it
THE PICTURE #How to readRead left to right as real elapsed time. The first four entries are a one-way process that ran to completion; the last two are where the system has stayed since. The turning point is Locked — nothing new happens there, the braking simply runs out of anything to act on.
What became clearer
WHAT CLEARED #The Moon is not frozen. It turns once per orbit, and that rate is no coincidence: it is the one condition in which Earth's uneven pull stops taking spin away.
Where to go next
ONWARD #- How the same tides are lengthening Earth's day and pushing the Moon away.
- Why Mercury settled at 3:2 instead of 1:1.
Key terms
TERMS #| Term | What it means |
|---|---|
| Tidal bulge | the stretching caused by gravity pulling harder on a body's near side than its far side. |
| Torque | an off-centre force that twists something rather than moving it. |
| Libration | the Moon's small rocking, which lets us see slightly more than half its surface. |
Every term the collection defines is gathered in the glossary.