# Dark side of the Moon

#### richbass

Here's another observation of nature

"I don't think the moon's spin cancels it's orbit (causing the same side to face us) by tidal forces or by accident. That would be one big coincidence. I think that the moon's center of gravity is offset from it's center point along it's diameter. So "The heavy side faces us"

Is this mentioned somewhere? I couldn't find it. If not does anyone agree that this could be possible?" or am I again "out of my mind"?

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#### HallsofIvy

No, it's not by "accident" nor does it follow that one side must be heavier than the other- the moon is "tidally locked" to the earth. Why do you say you don't think it could be caused by tidal forces? That's certainly not a coincidence!

Do you understand what is meant by "tidally locked"? It has nothing to do with the tides on earth- more to do with "tides" on the moon!

Here is a good explanation:
http://en.wikipedia.org/wiki/Tidal_locking

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#### Meir Achuz

Homework Helper
Gold Member
It is not just the heavier side facing us.
It's more complicated than that. It has to do with the quadrupole moment of the moon due to non-spherically symmetric distribution of it's mass.

#### richbass

It is not just the heavier side facing us.
It's more complicated than that. It has to do with the quadrupole moment of the moon due to non-spherically symmetric distribution of it's mass.
non-spherically symmetric distribution of it's mass? Doesn't that mean it's center of gravity is offset from it's center of mass?

#### russ_watters

Mentor
No, the moon is basically football-shaped (an ellipsoid) and since the force of earth's gravity on the near-side is greater than that on the far-side (because of the distance), it stays in that orientation. The earth technically has an ellipsoid component as well (same reason - the tides), but it is very small compared to earth's rotation-induced oblateness.

The link you provided before that mentioned the crust being thicker on the other side doesn't mean that side is heavier - the crust is thin and light.

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