Why do we see the near side of the moon only?

  1. Why do we see the near side of the moon only??

    Why do we see the near side of the moon only???Why not the far side??
  2. jcsd
  3. russ_watters

    Staff: Mentor

    The moon is tidally locked to the earth. The same forces that cause our tides cause the moon to rotate once per revolution.
  4. unlike 24 hours and 365 days for earth, for moon periods of spin and orbit are roughly the same and are in opposite directions. [think of a ball rolling on the floor and at the same time rotating in opposite direction]
  5. Russ, could you expand on this? i also heard this on the Nova Origins series, but couldn't ask the TV (or at least expect a response).

    so, i s'pose some historical accident, the angular velocity of rotation of the moon is very close to the angular velocity of revolution about the Earth (ignoring the fact that both are revolving around the Sun together). now, i would not expect these two angular velocities to be sooo exactly equal that, over thousands of years, no relative turning motion would be detected. it's like with a continuous random variable, say a spinner, the probability of the spinner pointer landing precisely at 20o is zero (as opposed to the probability of the spinner landing between 20o and 21o which is not zero).

    so, somehow the angular velocity got close enough, what torque applied to the planet acts as a control system with negative feedback to keep the same side facing us? i don't understand that. it must be because of non-uniformity of the moon's surface. are there more mountains and highlands on the surface facing us than on the far side so that this extra mass is tugged toward the Earth? other than that, i don't get it.
    Last edited: Mar 14, 2008
  6. Wallace

    Wallace 1,253
    Science Advisor

    Tidal Locking is an expected phenomenon in systems with satellites, such as the Earth-Moon system. Eventually the Earth will become tidally locked to the Sun for instance (actually this assumes the Sun will last forever since tidal locking would take Billions of years). The Moon didn't start out being tidally locked, but this is the lowest energy configuration and hence what the system moves towards, it doesn't require any chance agreement between the initial orbital speed and rotation.

    There is some info on Wiki under 'Tidal Locking' if you are interested.
  7. russ_watters

    Staff: Mentor

    Yes, except in both cases, they rotate in the same direction.
  8. russ_watters

    Staff: Mentor

    No historical accident is necessary. Tidal friction is like applying the brakes on your car. It is a very real friction force that slows the rotation.
    Tidal friction is literally generated by the changing tides. It is perhaps easiest to understand with the flowing of water in and out of a bay. With any fluid flow, there is an associated viscous fluid friction that dissipates energy and generates heat. But the same force also kneads the earth and moon like a ball of dough, deforming the actual bodies themselves. This deformation is not perfectly elastic, and thus generates energy.

    This, by the way, is what causes the plate techtonics and volcanism on some of Jupiter's moons. There is just that much energy being generated by this friction.
  9. okay, so some of the rotational energy is converted to heat and eventually radiated. but, still, what torque is applied to change the angular momentum? because losing energy to heat that is radiated is a one-way street. what keeps the angular velocity from decreasing even further past the locked state?

    could it be that this deformation elongates the moon slightly toward the Earth and that hanging blob (being a little closer to the Earth) is the handle by which this torque that i seek is applied?

    i don't know, guys, i'm just guessing and trying to sew up the loose ends.
    Last edited: Mar 14, 2008
  10. yup, thanks for the pointer. i was sorta right, what i called a "blob" is actually called a "bulge" and that is the handle used to torque the rotation of the moon back in case it spins too slowly.
  11. I meant if orbital motion is to your left [as seen from earth lying face up looking at the moon], moon should rotate to right. To an earth observer aren't they in opposite directions?
  12. Chronos

    Chronos 9,872
    Science Advisor
    Gold Member

    Wallace gave the long and short of it. Read the references he gave again. You are missing the point.
  13. [I assume you are talking to me] I understand that. I was saying about the 'opposite direction'.
    BTW, I answered the question why moon always shows the same face, which is due to periods being roughly same and opposite from the view of the observer in earth.

    [which btw, is due to moon already having its axis which minimizes the torque ("tidal force") which is the preferred ("minimal energy") state oriented towards us] - first statement would be true, even if it is not due to it being minimal state of energy, but simply is the state currently as a matter of fact.
  14. russ_watters

    Staff: Mentor

    I'm actually not exactly sure what you mean by this, but the easiest way to see it is with a top-down view of the solar system. Most bodies both rotate and revolve counterclockwise.
  15. Chronos

    Chronos 9,872
    Science Advisor
    Gold Member

    You injected a new meaning into the term 'missing the point', fakr.
  16. Reply

    I also learnt that Earth's rotation is slowing down due to tidal friction, this means that the orbits would have to distance themselves further.
    Isn't moons attraction and the torque it provides enough to speed up the earth??
  17. Janus

    Janus 2,367
    Staff Emeritus
    Science Advisor
    Gold Member

    It is the tidal torque the Moon exerts on the Earth that is slowing the Earth's rotation down.
  18. No, while the moon distances itself from the earth, its energy is increased, so it receives energy from the earth. Part of the energy is lost due to tides on oceans, so the rotation of the earth must be reduced.
    In future, the earth will rotate at the same speed of the moon orbiting around the earth. They both are tidally locked exactly like Pluto and Chadron.
    Last edited: Mar 17, 2008
  19. They were probably refering to axial rotation, not orbital rotation.

    For the experts, is it safe to say that IF the sun lasted forever, all the material in the solar system would eventually be pulled into the Sun?
  20. Wallace

    Wallace 1,253
    Science Advisor

    Not at all, there is no reason that the orbits of the planets should decay over time. Eventually you might expect that say the Earth might become tidally locked to the Sun (so that one side was always day and the other always night) but the radius of its orbit wouldn't change. Note that this is only true under the Sun lasting forever hypothetical. In reality the Red Giant phase of the Sun in several billions years time will come long before this and cause significant disruption to the inner solar system, but that is a different issue.
  21. DaveC426913

    DaveC426913 16,081
    Gold Member

    Is this question satisfactorily answered for the OP?


    Cuz the real answer is:

    We only see the near side of the Moon for the same reason we only see the near side of the No. 3 Hook & Ladder Engine or the near side of the Pawtucket Light and Power building:

    in order to see the far side, it would have to be completely transparent.
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