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

129 viewsOtherPlanetary Science

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

9 Answers

Anonymous 0 Comments

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.

You are viewing 1 out of 9 answers, click here to view all answers.