Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The moon?

  1. Apr 26, 2005 #1
    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. jcsd
  3. Apr 26, 2005 #2

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
     
  4. Apr 26, 2005 #3

    DaveC426913

    User Avatar
    Gold Member

    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.
     
  5. May 2, 2005 #4

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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?
     
  6. May 2, 2005 #5

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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?
     
  7. May 2, 2005 #6

    DaveC426913

    User Avatar
    Gold Member

    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.
     
  8. May 2, 2005 #7

    DaveC426913

    User Avatar
    Gold Member

    Would be very weird if it weren't, seeing as it is visually almost indistinguishable from Neptune.
     
  9. May 2, 2005 #8
    there is the oddity of mercury's perihelion in contrast.........something like 45 odd seconds per mecury year, if I remember right?
     
  10. May 3, 2005 #9

    SpaceTiger

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Are you talking about the perihelion advance? That's a GR effect.
     
  11. May 3, 2005 #10

    Phobos

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    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.
     
  12. May 3, 2005 #11
    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....
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: The moon?
  1. The moon. (Replies: 10)

  2. No moon (Replies: 13)

  3. The moon (Replies: 5)

  4. The Moon! (Replies: 16)

Loading...