The moon?

1. Apr 26, 2005

godzilla7

I've seen this mentioned on another post but some questions remain, the earth rotates about it's axis as I understand it and the moon rotates about the earth every 28 ish days, the moon has no axis rotation so we only see one side of the moon, is that right?

This I can grasp, however why doesn't the moon spin is this because according to one theory when it was spewed from the earth it had no spin and forces do not cause it to have a spin of it's own? or is there some balancing factor that stops it rotating about it's own axis.

Uranus doesn't spin about it's own axis( I think that's right) and is also a wandering planet, it is theorised, caught by our sun so never had any initial rotation, any similarities here. Any thoughts?

2. Apr 26, 2005

Integral

Staff Emeritus
If the moon did not spin on its axis we would see the other side. Think about it!

The rotation of the moon has become gravitationaly locked to its revolution about the earth. The moon rotates once every 28 days therefore we see only one side of it.

You can observe this easily. Simply observe some moveable object on your right. Pick it up and move it with out rotation to your left. You will now be looking at the other side of the object.

3. Apr 26, 2005

DaveC426913

The Moon rotates every 28 days. It happens that its rotation on its own axis and its revolution around Earth are exactly the same. This is not a coincidence.

Yes, it is called a tidal or gravitational lock. If you park a satellite (or anything) in orbit, and don't bother to correct its tilt, it will eventually point itself so that its long axis is pointing toward Earth - simple orbital mechanics. The Moon is not perfectly spherical. Over eons, this has slowed it rotation until it has one side facing toward Earth. If you measured the Moon it would be slightly bigger along a line parallel to the Earth-Moon axis, than if you measured it cross-wise.

Uranus rotates in 17h 24m, according to Wikipedia.

4. May 2, 2005

Phobos

Staff Emeritus
To spin off integral's model....(excuse the pun)...

Get 2 coins to represent the Earth and Moon. Have the moon's face pointing at the Earth. Now move the moon around (orbit) the earth but keep the face pointing to it all the way around. See how it has to rotate to keep one side facing the earth?

5. May 2, 2005

Phobos

Staff Emeritus
As noted above, Uranus does rotate on an axis (sounds crude, eh?).
The odd thing about it's axis is that it is dramatically tilted compared to the other planets.

As far as I know, the theory is that Uranus formed from the same stuff as the rest of this solar system. Where did you hear that it originated from somewhere other than this solar system?

6. May 2, 2005

DaveC426913

Pluto is often considered a captured body. Its orbit is eccentric compared to the other planets - for a brief time it is actually inside Neptune's orbit (as it was between 1979 and 1999. Its orbit is also and highly inclined (>10 degrees) wrt the solar plane.

7. May 2, 2005

DaveC426913

Would be very weird if it weren't, seeing as it is visually almost indistinguishable from Neptune.

8. May 2, 2005

x8jason8x

there is the oddity of mercury's perihelion in contrast.........something like 45 odd seconds per mecury year, if I remember right?

9. May 3, 2005

SpaceTiger

Staff Emeritus

10. May 3, 2005

Phobos

Staff Emeritus
Like SpaceTiger said, that has been explained by General Relativity (see "frame dragging"). When that oddity was first discovered (before GR), it was speculated that there was another hidden planet in the vicinity of Mercury that was affecting its orbit, but Einstein's work solved the mystery.

11. May 3, 2005

x8jason8x

I wasn't implying that it was still a mystery per se, just that it's an oddity comparable with the tilt of uranus' axis. I should have elaborated on what I meant, sorry about that....