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B Does the moon rotate on its axis?

  1. Jun 2, 2016 #1
    I googled this question and found sources which claimed the moon slowly, over about 27 days, rotates on its axis. Yet wouldn't rotating at a speed so that about 40% of the moon never faces the Earth suggest that it is rotating around an axis somewhere inside of the Earth, rather than inside of itself?

    For example if a person on a merry-go-round (near the edge) is rotating 360 degrees only after approximately a full revolution of the merry-go-round, I wouldn't consider that person to be rotating on their own axis, unless they are doing 360 degree rotations independently on the merry-go-round, which the moon doesn't appear to be doing?

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  2. jcsd
  3. Jun 2, 2016 #2

    Nugatory

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    The moon is rotating around its own axis, one revolution around the its own axis in the same time that it takes to make one full circle around the earth.
    One way to see this is to imagine that you are hovering motionless in space (relative to the earth) somewhere outside the moon's orbit, and looking in the direction of the earth. Because you're outside the moon's orbit, at some point the moon will pass between you and the earth; when it does the back side will be facing you and the earth-visible side will be facing the earth. But when you wait a half orbit the earth will be in between you and the moon, and the moon will be presenting its earth-visible face to both you and the earth. That is, the side that you could see switched as the moon went half-way around the earth, and the only way this can happen is if the moon turned on its axis one-half turn to switch the side facing you.
     
  4. Jun 2, 2016 #3

    A.T.

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    These are just two equivalent ways to describe the same kinematics:
    rotation around it's own center + synchronous circular translation around some point = rotation around that point
     
  5. Jun 2, 2016 #4

    russ_watters

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    You are trying to combine the rotational and revolutional motion into one motion. You can do that, but that just creates a rotating reference frame. Note that you can't always combined motions that way. For example if you're driving your car while a passenger is spinning a basketball on his finger, about what axis is the basketball spinning?

    That only works for the moon because it is tidally locked. You can't do it for earth and the sun; not only are the rotation rates different, but the axes aren't even aligned!
     
  6. Jun 3, 2016 #5

    A.T.

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    It has nothing to do with changing the reference frame. It is an equivalent way to describe the same motion, from the same inertial reference frame.
     
  7. Jun 3, 2016 #6

    russ_watters

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    I was responding to the OP; not you. Sorry if that wasn't clear.
     
  8. Jun 3, 2016 #7

    A.T.

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    I know. Different ways to decompose the same motion into rotation and translation still doesn't imply a rotating reference frame.
     
  9. Jun 3, 2016 #8
    If something is not rotating around its axis then any distant observer should see the same side of the object irrespective of the object's revolution

    That is not the case with moon as seen from the Sun or the person on merry go round as seen from someone outside.

    While walking if u take a right turn, u can do it by stopping and rotating 90degree or turn while walking. In both cases u have rotated around the axis of ur body
     
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