# Why does the Moon rotate around the Earth?

## Main Question or Discussion Point

I am doing a project about the earth, the sun, and the moon. My question is why does the moon rotate around the earth?

## Answers and Replies

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russ_watters
Mentor
Welcome to PF.

That's a little bit of a vague question....what would happen if it didn't?

Welcome to PF.

That's a little bit of a vague question....what would happen if it didn't?
who knows...

Borek
Mentor
The shortest answer (and probably not the one you expect) is: it doesn't

The shortest answer (and probably not the one you expect) is: it doesn't

The shortest answer (and probably not the one you expect) is: it doesn't
why?

Jonathan Scott
Gold Member
why?
I'm guessing that what Borek means is that from the point of view of someone at rest in the Solar System, the Earth and Moon orbit together around the sun, but their orbits wiggle slightly around each other because of their mutual gravitational pull. The orbit of the moon around the sun is always curved towards the sun, so from that point of view it would be difficult to see the moon as going around the earth.

However, from the point of view of the earth, the moon rotates about the earth. In fact, the earth and moon together orbit their common center of mass, but because the earth is so much heavier, that's approximately the same as the moon going round the earth.

The general answer is that because moon is passing fairly close to the earth, the earth's gravity pulls it round into a closed orbit, and the smoothing effect of tides over millions of years have made that orbit approximately circular and the moon's rotation to synchronize with the orbit so that it always presents approximately the same face.

I'm guessing that what Borek means is that from the point of view of someone at rest in the Solar System, the Earth and Moon orbit together around the sun, but their orbits wiggle slightly around each other because of their mutual gravitational pull. The orbit of the moon around the sun is always curved towards the sun, so from that point of view it would be difficult to see the moon as going around the earth.

However, from the point of view of the earth, the moon rotates about the earth. In fact, the earth and moon together orbit their common center of mass, but because the earth is so much heavier, that's approximately the same as the moon going round the earth.

The general answer is that because moon is passing fairly close to the earth, the earth's gravity pulls it round into a closed orbit, and the smoothing effect of tides over millions of years have made that orbit approximately circular and the moon's rotation to synchronize with the orbit so that it always presents approximately the same face.
But why does it go around the earth?

Borek
Mentor
I was aiming at the fact that both Earth and Moon rotate around barycenter. It happens that the barycenter is inside the Earth, but quite far from the Earth center.

Have you heard about momentum conservation?

I was aiming at the fact that both Earth and Moon rotate around barycenter. It happens that the barycenter is inside the Earth, but quite far from the Earth center.

Have you heard about momentum conservation?
yeah

If you are looking for a very basic answer, it is that the force of gravity between the Earth and the Moon is strong enough that the Moon can't "fly away" and leave the Earth, and the Moon just happens to be moving in the right direction so that it doesn't hit the Earth.

Gravity exists between any two bodies. It drops off rapidly with distance (1 / distance * distance). Earth is close to the Moon, so the Earth's gravity is much stronger than any other force on the Moon. The Moon doesn't orbit the Sun, for example (if we're keeping it simple).

Isaac Newton proved that the force of gravity between two objects results in elliptical orbits. Before him, Johannes Kepler figured out that planets and moons orbited in elliptical paths, but didn't know why. Wikipedia has lots of info on those guys.

The Moon keeps missing the Earth because it is what's left after everything else DID hit the Earth. It's been billions of years since the Moon's origin. There was a lot of stuff that could have been a moon to Earth, but it wasn't moving in the right way & eventually hit the planet.

Finally, the Earth is about 86x heavier than the Moon - that's why the Moon orbits the Earth instead of vice versa.

My apologies if this is too simplistic, but I wanted to cover the basics in case kyt_0903 was a little younger than most posters. Any question that asks "why" gets into extremely deep waters very quickly. Man has been trying to figure out "why" since He first took breath. The day we figure it out will be a sad day in many respects - it will mark the end of wonder.

Borek
Mentor
Have you heard about momentum conservation?
yeah
Try to imagine if it is possible for two objects to NOT rotate around their barycenter and still maintain constant momentum.

tony873004
Science Advisor
Gold Member
I was aiming at the fact that both Earth and Moon rotate around barycenter...
I thought you were aiming for the fact that the Moon revolves, with rotation being used for a single object about an axis.

Borek
Mentor
I thought you were aiming for the fact that the Moon revolves, with rotation being used for a single object about an axis.
Yep, sorry. Now and then my English plays tricks on me.

HallsofIvy
Science Advisor
Homework Helper
The reason the moon rotates around the earth, as opposed to rotating around mars, or the sun, or simply not existing at all, depends upon the origin of the moon which you can learn about here:
http://www.psi.edu/projects/moon/moon.html

D H
Staff Emeritus
Science Advisor
I thought you were aiming for the fact that the Moon revolves, with rotation being used for a single object about an axis.
That is a good point of distinction, and one not asked of the original poster.

So, kyt_0903, to which motion are you referring: That the Moon orbits the Earth, or that we on the Earth only see one side of the Moon?