   Does the moon rotate about its own axis?
  A friend of mine asked me this yesterday. As a trained computer graphics programmer, my gut reaction was no, because from earth you always see the same face. If you take the local coordinate system of the earth and the moon, and put earth at the origin and the moon some distance away, to simulate the moon-earth relationship all you have do to transform the moon is rotate it about the origin -- you never rotate the moon about its own center.Then my friend says I am wrong (neener, neener), the moon does rotate about its axis. Its rotational speed just happens to be the same speed that it takes to orbit the earth. Later on I looked it up on the internet, which confirmed that this is the correct answer.Perhaps I am just too stubborn, but I can't let this go. I tried to think about it in a different way, like instead of rotating the moon about the earth's axis, do a translate + self-rotate to achieve the same thing. But that's extra work.Then I decided to stop thinking like a programmer and like a physicist, but I still can't get the problem out of my head. The moon is in 'tidal lock' with the earth, which according to internet sources makes the moon rotate at the same rate that it orbits. But that explanation is not good enough for me. If I tie a string to a ball and whirl it around my head, I will always see the same side of the ball. But do we say the ball rotates about its own axis? No, that would be silly. It rotates about me. How is tidal lock different from the string? Heck, if I were to literally lasso the moon from here on earth, would I ever get flung to the other side, such that the moon blocks the earth from my view? I wouldn't stay in one place because the moon is not in geo-synchronous orbit, but I don't think I'd get flung into space (assuming the rope stays straight).So yeah, I'm going to stick with my answer until someone comes up with a better explanation.   
  When you rotate something around some point, it rotates around its own axis.   ;Quote:If I tie a string to a ball and whirl it around my head, I will always see the same side of the ball. But do we say the ball rotates about its own axis? No, that would be silly.Well, it DOES rotate about its own internal axis. If it didn't you wouldn't see the same face all the time. The side with the string connected would always be facing the exact same direction.But instead, you pull that side towards the center constantly. This causes the ball to rotate so as to face the center. It is constantly rotating about its own axis.For a more simplistic proof of concept, have your ball string experiment with a light and a heavy ball.  As you spin up to speed, watch as the heavier ball (with more rotational inertia) takes more force to get going AND more force to make it actually face you.The ball wants to go in a straight line, and wants to stay facing the original direction.  The heavier ball will visibly lag further behind from the ideal position and facing (represented by the lighter ball, or a ball-on-a-stick). If you let it go, it goes off in a straight line, while spinning.(even happens if you have a ball on a stick, the "baton" will spin as it flys off).Your idealized "Graphics" situation though approximates reality for a visual sense, but lacks the physical nature of the situation.   ;Quote:Original post by ScintBut do we say the ball rotates about its own axis?Yes, yes we do. Try cutting the string and watching the ball as it flies off; it continues to rotate.Quote:So yeah, I'm going to stick with my answer until someone comes up with a better explanation.Do you find comfort in obstinacy?[grin]   ;  The key difference is that the earth's gravitational pull on the moon isn't like a string attached to it.  Think of it more like a slope in the "fabric" of space.  The moon sits within this slope, and so tends to move inward toward earth.  However, its spin generates a force that pushes it outward (back up the slope).  This is the primary mechanism through which orbiting bodies maintain their orbital altitudes.  Anyway, there are certain rotational speeds at which the outward force generated by rotation completely cancels out the inward force due to gravity.  In some conditions, this happens to be such a speed that the same face of a body always faces its orbital parent.  This is the basic observational characteristic of tidal locking.  The moon happens to be in an orbit with these characteristics.   ;  The thing which gives away the fact the moon IS rotating about its own axis is that we can see upto 59% of its surface due to libration.;Quote:Original post by ScintBut do we say the ball rotates about its own axis? No, that would be silly. It rotates about me.The problem here is that you're assuming that rotating about its own axis and rotating about you are mutually exclusive.  It can do both.  For example, do you think the Earth "spins" about its own axis?  Do you think the Earth "orbits" the sun?Or, if you want to think in terms of computer graphics, consider a vector drawn from the center of the moon through some point on the moon's surface.  Now, transform it and consider a vector drawn from the center to the same point on the surface of the transformed moon.  Are these two vectors parallel?  Or, if you don't render the whole sphere and just render a picture of the moon on a flat surface, will the normal of this surface always point the same direction as you rotate it?Of course, there are frames of reference where the moon isn't rotating.  Similarly, the Earth isn't rotating in the frame of reference that I usually use.  "The Earth revolves around the Sun" is perhaps more enlightening than "The Sun revolves around the Earth", but they're both equally incorrect, and the latter's frame of reference is usually more convenient.;  There is something both awesome and frightening about that animation   ;  Yeah, anyone else reminded of the Indiana Jones ride at Disneyland?   ;Quote:Original post by ScintThen my friend says I am wrong (neener, neener), the moon does rotate about its axis. Its rotational speed just happens to be the same speed that it takes to orbit the earth. Later on I looked it up on the internet, which confirmed that this is the correct answer.1.Moon and  Earth are in "orbital resonance",as  it was happened with many other satellites of planets in Solar system.2. From phisical point of view,the Moon rotate.All rotating bodyes are non-inertial systems,they are "selected" in comparison with inertial which are equal in  Newton mechanics.I.e you can determine the fact of rotation your coordinate system without observing  another bodyes (measuring Coriolis force etc.) Observing another bodyes shows that all universe rotate around you,it's also a property of such systems.I think your friend simply share  Ptolemeus point of view [smile][Edited by - Krokhin on May 19, 2009 3:37:19 PM]