kinmonth@dec-null.UUCP (06/12/84)
All this talk about the moon revolving or not has set me thinking. First of all, I should say that I have decided that the moon does indeed rotate on its own axis. My argument (to myself) was something like the following: If an object is rotating on its own axis, then it must have what we commonly call a North pole and a South pole. My definition of a "pole" was as follows: If you have a free-floating body, and there exists a point on that body from which the motion of all other bodies sufficiently distant appears circular, then that point is a pole. Thus if you are standing at a pole of the moon, then other visible bodies of sufficient distance (distance stars for instance) appear to have a circular motion. (of course, a body directly overhead would not appear to move, but that's just a degenerate circle). The "sufficiently distant" part is meant to minimize the effect of the actual motion of the distant bodies. i.e. if they are sufficiently distant and moving at a "reasonable" speed, they they can be considered stationary relative to you over a short time period. Since there are such points on the moon, the moon has poles, or equivalently, the moon is rotating on an axis. This brings me to another question. Can there be a free-floating body which is NOT rotating on its own axis (by the way, isn't "rotating on its own axis" synonymous with "rotating"? If something is rotating, it must be on SOME axis, and why would it be someone ELSE's? Of course it is its OWN axis....). Back to the question. It would seem that all free-floating object must rotate to some degree, no matter how slowly. The odds that something could be floating around prefectly aligned with the universe must be hovering near zero. This made me think of another thing. What are the chances that the period of rotation of the moon is EXACTLY the same as the time for one orbit of the Earth? I cannot imagine that this is a coincidence. The only explaination I can think of is that the side of the moon that always faces the Earth is more dense (perhaps much more?) than the other side. Thus the graviational attraction of this side eventually caused the moon's rotation to stabalize with the "heavy" side towards the Earth. If this were true, note that a rotating moon would appear to wobble. We can't observe this from earth however because we don't see the moon rotating; we always see the same side. There must be some hole in this argument though( i.e. the "heavy sided moon" argument). It seems that surely I would have heard of this unevenness in the moon's density if it were true. Does anyone out there have another explaination for why the lunar day and lunar month are equal? I cannot believe it is coincidence. Bruce Kinmonth ...decvax!decwrl!rhea!null!kinmonth
barmar@mit-eddie.UUCP (Barry Margolin) (06/12/84)
The fact that the moon always keeps the same face to the earth is no coincidence. It is the result of a couple of billions of years of tidal forces. No planet or satellite is perfectly circular, and after billions of years the forces on the uneven portions add up and the revolution speed slows down and the size of the orbit decays. I seem to recall that Asimov devoted one or two of his "Fantasy & Science Fiction" Science columns to this subject several years ago; check in the anthologies of his articles. -- Barry Margolin ARPA: barmar@MIT-Multics UUCP: ..!genrad!mit-eddie!barmar
wookie@alice.UUCP (06/13/84)
I don't know why, but I am amazed by all this moon stuff. This was something I learned in an elementary grade science class!! The moon does rotate on its own axis and due to gravitational affects etc it rotates at the same rate it travels around the Earth (about 28 days). Will someone please explain to me why this is a difficult concept!! Keith Bauer White Tiger Racing
stekas@hou2g.UUCP (J.STEKAS) (06/13/84)
This is a subtle problem. Technically, the Moon DOESN'T rotate on "ITS" axis. If you calculate the angular momentum of the Earth-Moon system you will find that the Moon contributes only to the orbital angular momentum - i.e. it is not spinning on its axis. As a consequence, a visitor to the Moon would not observe a Coreollus (sp?) force or other tell-tale sign that he was on a spinning body. Anyone willing to do the experiment? The rotation of the Moon with respect to the fixed stars is a consequence of the curvature of space-time in the Earth's neighborhood. Take away the Earth, Sun and planets suddenly, and the Moon would follow a straight trajectory with NO rotation with respect to the fixed stars. Do the same experiment with the Earth and it's rotation rate would change only 1 part in 365. Jim
mwm@ea.UUCP (06/14/84)
#R:decwrl:-136900:ea:4000003:000:1281 ea!mwm Jun 14 12:35:00 1984 /***** ea:net.games.triv / decwrl!kinmonth / 10:44 pm Jun 12, 1984 */ >It would seem that all free-floating object must rotate to some degree, no >matter how slowly. The odds that something could be floating around >prefectly aligned with the universe must be hovering near zero. "Perfectly aligned with the universe?" Huh? I'm not sure what you mean by "universe." How about "perfectly aligned with anything relatively at rest to you?" The probability of that is not "near zero", it *is* zero. You have exactly one chance in Aleph-null, which is none at all. >The only explaination I can think of is that the side of the moon that >always faces the Earth is more dense (perhaps much more?) than the other >side. Thus the graviational attraction of this side eventually caused the >moon's rotation to stabalize with the "heavy" side towards the Earth. If >this were true, note that a rotating moon would appear to wobble. We can't >observe this from earth however because we don't see the moon rotating; we >always see the same side. Your first hypotheses is correct, including the moons wobble. We *do* observe the moon wobbling from the earth, something like 10 degrees each way. We also pick up a little of the "dark" side of the moon do to the north-south wobble. <mike
aca@vaxine.UUCP (Alan Agostinelli) (06/23/84)
I don't think that's quite right. The reason that the moon doesn't rotate isn't a mass anomoly, it's tides. Picture a moon spinning on it's axis, which is roughly parallel to the Earth's. The gravitational field is some- what stronger on the side of the moon closest to the Earth, somewhat weaker on the opposite side (a small effect, but the *only* effect). As the moon rotates, mass must be "lifted" through this gradient on one side, and "lowered" on the other. This introduces stresses in the moons structure, and does work on it (moonquakes). This effect, over the eons, has turned the energy stored in the moon's former rotation into random heat. See Larry Niven's classic short story "Neutron Star" for a good discussion of this topic. Alan Agostinelli Automatix, Inc. Billerica, Ma. ...!allegra!linus!vaxine!aca