# How come the moon spins at the same speed it orbits around earth?

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It takes 27 days for the moon to spin completely and the exact same time for it to orbit around earth, thus creating a situation where we can always only see the same side of the moon.

Surely, this synchronization is not a coincidence but how exactly did we reach that point? I read some articles but found them vague.

In: Planetary Science

It’s because of tidal locking. The moon is close enough to the Earth that the Earth’s gravity makes the moon slightly oblong. The “bulge” of the moon that faces the Earth is pulled more strongly to the Earth than the rear, and more strongly than the sides would be if it was turned 90 degrees. This means if the “bulge” of the moon ever began to turn away from the Earth, the Earth would try to pull it back. Therefore, this bulge will try to always face the Earth, and it’s this bulge that we see from the Earth’s surface.

You’re right, it isn’t a coincidence, and it’s due to something called Tidal Locking.

Before we go further, take a tennis ball, hold it at arm’s length with the logo facing you, and have it orbit you such that the logo is always facing you.

You will realise that if the ball did not rotate at all, when it got behind your head, the logo would be facing away. This would mean that the people on the world on the opposite side to your would be seeing the other side of the moon, which, as you established, doesn’t happen.

This means that the moon must be rotating constantly so that the “logo” side is facing Earth at all times.

Now to explain Tidal Locking.

We have known since the time of Newton in the 1600s that gravity is stronger on objects that are closer, and it increases by the square of the distance.

For an object like Earth, which is very very far away from the sun, the difference between the near and far sides is negligible, so the force on either end is roughly the same.

Not so for the moon and the earth. It’s about a 100 times closer, which means that the force on the near side is a fair bit stronger than the force on the far side. Natural consequence of this is that the near side never faces away from us. This isn’t exclusive to the moon and earth system either btw – Mercury is tidally locked to the Sun too, so it has one face absolutely scorched, and the other half is actually freezing cold because it is always facing away from the sun.

Imagine the Earth and the moon are perfect spheres, now put one big mountain on each. Those two mountains are going to attract each other, so eventually they will line up to each other. Same thing on the real Earth and moon, just with a more complicated surface.

For something to be tidally locked doesn’t it require some mass asymmetry?

It’s called [Tidal Locking](https://en.m.wikipedia.org/wiki/Tidal_locking). It occurs because when the moon is rotating on its axis at a different speed than its orbit, it causes the sphere to deform just a little bit, causing bulges in its shape as it rotates about its axis. Movement in these bulges require energy and are released as heat in the moon’s interior. This energy comes from the moon’s angular momentum (rotational energy) and slows its spin down over time as it is dissipated as heat. Eventually, after millions of years, the moon’s spin slows to a point where it no longer spins faster than its orbit with the earth. Since it now spins at the same rate as its orbit, the bulges caused by the gravity of the earth acting on it are locked in place and no longer move across the moon’s body and no longer generate heat, and therefore, no longer slow the spin of the moon.

Hopefully this makes sense. If not, the Wikipedia article I linked above should help explain further.

The Moon isn’t perfectly symmetrical, so the Earth “pulls” on one side more. Overtime, this pull locked one side of the Moon towards the Earth.

The Moon became tidally locked with the Earth. The clue to how that happens is the the name Tidally.

At the start the Moon was spinning. And as now was making tides on Earth. But, the Earth was also making tides on the Moon. Not of water, but of the rock itself. So the Earth pulls the near side of the Moon slight more because it is closer. It also pulls slightly less on the far side, and because the far side is further out it is moving at above orbital velocity, so the backside bulges also. (same thing happens on Earth now. It’s just more noticable with water as it moves easier.)

But the Moon is spinning, and it takes time for these bulges to form and fade. So instead of pointing directly at the Earth the bulge pulled slightly further along the rotation. But now the force of gravity is slightly forward of the direct line between centers of mass of Earth and the Moon. So this slight slows the Moon, and over time stops the rotaton.

You may ask were all that angular momentum went? It went into the Earth Moon system and slightly speed up how quickly they orbit each other.

Fun fact: this is actually currently happening to the Earth. The tides are slooooowly slowing the Earths rotation. And at the same time slowly raising the moons orbit. So it is moving slowly away from Earth.

I think it’s easier to ignore the motion around the earth and just look at the moon rotating relative to earth. The earth will deform the moon slightly just like a ball rolling on the ground is deformed. With the rolling ball the deformation will be in front of the ball but the moon is deformed facing the earth.

Moving the deformation around the ball or the moon takes energy. This will cause the ball / moon to slow down to a standstill. (We ignore the friction).

Now add back the rotation around earth. The bulge doesn’t need to move so the moon doesn’t slow down (or accelerate) it’s motion.