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Moon's Bulge Explained 204

Posted by Zonk
from the put-some-cream-on-that dept.
anthemaniac writes "The moon has an unexplained bulge that astronomers have been trying to find a source for since 1799. Finally, an apparent answer: The equatorial bulge developed back when the developing moon was like molasses (and you thought it was cheese!) and, rather than today's nearly circular orbit, it 'moved in an eccentric oval-shaped orbit 100 million years after its violent formation.'"
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Moon's Bulge Explained

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  • no (Score:5, Funny)

    by GigsVT (208848) on Thursday August 03, 2006 @07:39PM (#15843550) Journal
    It's just happy to see you.
    • Re:no (Score:2, Funny)

      by DurendalMac (736637)
      Well, seeing as this is a moon, it could just be prairie doggin'. When it finally squeezed that bulge out, we're all gonna be fucked.
    • Maybe there's something to those Father Sky, Mother Earth myths after all.
    • So that's why the moon's always holding a clipboard over its lap when I look skyward...
    • If viagra says to seek medical help if you have an erection more that 4 hours, surely the moon has something really potent!
  • Well (Score:2, Insightful)

    by styryx (952942)
    Hey, way to suck the fun out of this with 'the cheese' joke in the description.
  • Uh huh (Score:4, Funny)

    by Anonymous Coward on Thursday August 03, 2006 @07:40PM (#15843557)
    Its nothing more than a little baby fat.
  • Don't all circular/spherical objects bulge around the middle? take this o for example. The middle part of it is wide at the middle, and short at the top and bottom.
    • Re:Wait a minute... (Score:4, Informative)

      by Kelson (129150) * on Thursday August 03, 2006 @08:15PM (#15843711) Homepage Journal
      The radius at the equator is slightly longer than the radius at the poles. It's not quite a perfect sphere.* Sort of like if you took a rubber ball, set it on the floor, then pushed down slightly.

      The same is true of the Earth, though I believe it's generally attributed to the Earth's rotation.

      * Yes, I know that craters and such interfere with it being a perfect sphere too. No need to get pedantic, people of Slashdot. Well, no more than usual.
      • by Mr. Underbridge (666784) on Thursday August 03, 2006 @09:23PM (#15843962)
        Right. I'd assume these guys accounted for the tendency of a rotating body to form an oblate spheroid, and that the moon's current orbit can't account for the degree of its oblacity (if that's a word). Thus it would need to have exhibited some more violent orbit in the past.
      • IANACosmologist/Astronomer...but I think you can explain the Earth's bulge because it is still active;i.e. is not solid so it can expand here, contract there, sorta like a water balloon. The moon is probably completely solid and therefore should not bulge.
        • Astronomers think that the Moon still has a molten core, IIRC.
        • IANACosmologist/Astronomer...but I think you can explain the Earth's bulge because it is still active;i.e. is not solid so it can expand here, contract there, sorta like a water balloon. The moon is probably completely solid and therefore should not bulge.

          As you may or may not know, if you take liquid and keep it in a certain shape while it cools and solidifies, it stays in that shape. So, any bulges the Moon might have developed when it was still molten won't go away just because it solidifies.

          Beside

        • According to what I've heard (sorry no sources), the moon used to spin, but tidal forces from the Earth has slowed this down over time so that it's (almost) completely fixed. In fact, tidal forces from the moon on to the Earth is slowing it's rotation every so slightly. The result is that the day length is gradually getting longer!

          Could tidal forces from the Earth explain this bulge? I would have thought these forces would be big enough to make some impression on the moon and give it a bulge.
    • by Namarrgon (105036) on Thursday August 03, 2006 @08:18PM (#15843731) Homepage

      It's thin at one end, much much thicker in the middle, and thin again at the other end.

      I have another theory, you know...

    • by Billosaur (927319) * <wgrother@NosPAm.optonline.net> on Thursday August 03, 2006 @10:02PM (#15844107) Journal

      Don't all circular/spherical objects bulge around the middle?

      If they are planets and they are spinning, yes. Just look at pics of the Jovian worlds, especially Saturn. And the Sun has a definite bulge. Of course, for most of the planets, the bulge is pronounced because they are still elastic to some degree. The Earth bulges owing to the fact that the continents are riding around on their crustal plates, which ooze on molten material, and the Moon is tugging on them as it goes aroudn us. The Moon's is more fascinating because it is a geologically dead world, so the bulge happened some time in the past and then got frozen in place.

      • the continents are riding around on their crustal plates, which ooze on molten material

        Just nitpicking... The mantle isn't actually molten. The lava we see flowing from volcanoes is just a local effect. Though if you go way, way down to the outer core, you'll probably find liquid.

        Devon

  • by Bamafan77 (565893) on Thursday August 03, 2006 @07:43PM (#15843568)
    Everyone knows your metabolism slows down after a certain age. Still though, a half hour a day on the treadmill probably wouldn't hurt either.
    • by Anonymous Coward on Thursday August 03, 2006 @07:47PM (#15843583)
      30 minutes a day on the treadmill? The fucker goes 1km/sec ALL DAY LONG!

      YOU do that, fat-ass!
  • by BoBathan (166436) <bobathan @ s o f t h o m e .net> on Thursday August 03, 2006 @07:49PM (#15843591) Homepage Journal
    Bulge at the equator, violent formation, clearly the Moon is American.
  • Oblig... (Score:2, Insightful)

    by Anonymous Coward
    That's no moon...
  • by CarnivoreMan (827905) on Thursday August 03, 2006 @07:52PM (#15843610)
    Well... perhaps the man in the moon was thinkin about some of them women from Venus....
  • by Quaoar (614366) on Thursday August 03, 2006 @07:54PM (#15843616)
    Now when I have to explain the bulge in my pants, I'll say it's because I'm eccentric! /Badump bump
  • by peektwice (726616) on Thursday August 03, 2006 @07:56PM (#15843628)
    It's a codpiece.
  • Monolith? (Score:4, Funny)

    by wingfoot (769619) on Thursday August 03, 2006 @08:01PM (#15843650) Homepage
    There wouldn't happen to be a strong magnetic field at the bulge would there....? How about a black monolith buried beneath the surface causing the bulge....?
  • Missing energy (Score:5, Interesting)

    by Harmonious Botch (921977) on Thursday August 03, 2006 @08:01PM (#15843653) Homepage Journal
    So how did the eccentric orbit become so nearly circular? That takes a lot of energy ( and a little coincidence )
    • Re:Missing energy (Score:5, Informative)

      by Jazzer_Techie (800432) on Thursday August 03, 2006 @08:16PM (#15843719)
      Actually, circular orbits are the lowest energy state. Thus, tidal forces cause the system to gradually lose energy until it settles into a circular orbit. When you add up the 1/r potential of gravity and the repulsive 1/r^2 centrifugal potential, you get a function with a nice minimum which is the radius of a circular orbit. The reason that elliptical orbits occur is because the period of the orbit exactly matches the period of oscillations around the minimum potential. Thus when you go around once, you end up right back where you started and get a closed, elliptical orbit. (Note that this is true only for Newtonian mechanics. Once you take General Relativity into account, the periods aren't the same and orbits precess. We can directly observe this in the orbit of Mercury as a perhelion shift of 43 arcseconds/century.)
      • Re:Missing energy (Score:4, Interesting)

        by gardyloo (512791) on Thursday August 03, 2006 @09:54PM (#15844075)
        Whoa. What's the repulsive 1/r^2 "centrifugal" potential? At first I thought you were including G.R. with the talk of tidal forces, but then I realized (via the 1/r potential) that you're talking about regular old Newtonian gravity. That's fine. But where's the repulsive potential?
              The only tidal forces I can see in this problem are evinced in the deformation of the earth or the moon, their atmospheres, and the ocean of the earth. Wikipedia has this to say about tidal locking:

        There is a tendency for a moon to orient itself in the lowest energy configuration, with the heavy side facing the planet. Irregular shaped bodies will align their long axis to point towards the planet. Both cases are analogous to how a rounded floating object will orient itself with its heavy end downwards. In many cases this planet-facing hemisphere is visibly different from the rest of the moon's surface.

        The orientation of the Earth's moon might be related to this process. The lunar maria are composed of basalt, which is heavier than the surrounding highland crust, and were formed on the side of the moon on which the crust is markedly thinner. The Earth-facing hemisphere contains all the large maria. The simple picture of the moon stabilising with its heavy side towards the Earth is incorrect, however, because the tidal locking occurred over a very short timescale of a thousand years or less, while the Maria formed much later.


            I'll have to try to work out how tidal forces within one astronomical body might lead to a circular orbit. It might be a well-known effect, but it's not obvious to me.
        • Ah. If you're talking about the pseudo-repulsive potential mentioned in the article (away from the barycenter), fine. BUT the sum of conservative potentials (as the 1/r and 1/r^2 ones are) is also conservative, and will STILL lead to elliptical orbits in general. So you've got to propose some non-conservative forces acting, and then you can't even really use "potential" in its usual sense of leading to a force via the gradient.
        • A previous moon-related story got me to do some calcs I have been thinking about for a while, to find how large the forward dragging (thrust) force on the moon would have to be to cause the orbit to increase by 3.8 cm a year.

          I'm notoriously dicey with complicated algebra, but if I've done it right (assuming a circular orbit) the total (kinetic plus potential) energy of the moon is -.5*G*me*mm*delta(1/r); me, mm masses of earth & moon, r radius of the orbit. The delta can be approximated as -delta(r)/r^

      • Actually, circular orbits are the lowest energy state. Thus, tidal forces cause the system to gradually lose energy until it settles into a circular orbit.

        Except that the tidal forces in the Earth-Moon system give energy to the Moon, not take it (to be more exact, they transfer Earth's rotational energy to the Moon, causing Moon's orbital speed to grow and Earth's day to lengthen). And those tidal forces are the strongest when the Moon is closest to the Earth. Now, all you astronavigators out there, wha

  • All the time I was thinking it was due to eating too much Double Quarter Pounders at McDonalds
  • by Anonymous Coward
    It's where they covered up the "CHA" that Chairface Chippendale put there. They used a little too much moondust.
  • shapes (Score:4, Funny)

    by ah247msg (993074) on Thursday August 03, 2006 @08:33PM (#15843782)
    I wish the bulge around my middle had as good an explanation... adam0@247msg.com
  • Enough already (Score:2, Informative)

    by midkay (984862)
    Isn't that enough of the "Because it's fat!" and "Because it's horny!" comments? If I knew how to mod all of you "Redundant", make no mistake, I would!
  • mmmm.... (Score:5, Funny)

    by denttford (579202) on Thursday August 03, 2006 @09:13PM (#15843925) Homepage
    Space Beer.
  • I don't think so (Score:4, Informative)

    by npietraniec (519210) <.ten.evitsiser. .ta. .narteipn.> on Thursday August 03, 2006 @09:13PM (#15843926) Homepage
    The moon is actually a spaceship. http://www.theforbiddenknowledge.com/hardtruth/moo n_spaceship.htm [theforbidd...wledge.com]
  • by helioquake (841463) * on Thursday August 03, 2006 @11:03PM (#15844301) Journal
    How did Laplace determine the existence of the bulge?

    Was it a "simple" measurement of the shape of the Moon or something more sophisticated via his favorite mathematic tricks? Considering it is Laplace, he must have measured its eccentricity fairly accurately. I wonder what he used to do that in 1799.
  • Fondue (Score:3, Funny)

    by fattybob (196045) on Thursday August 03, 2006 @11:13PM (#15844334)
    Mollases is more of a textural camparsion, but it must be remembered that this occured as the moon cooled, cooling from molten to solid cheese, so perhaps a better explanation would be that the bulge occured during cooling while the moon was like Fondue.
  • Iapetus (Score:3, Interesting)

    by j1m+5n0w (749199) on Friday August 04, 2006 @02:00AM (#15844878) Homepage Journal
    Maybe next they'll be able to explain this [wikipedia.org]...
  • Explorers from the Japanese Moon base, excavating the "bulge", have uncovered a large, black, granite slab of some sort. Asked about the possibility of un-earthing other pieces (so to speak), a researcher was quoted as saying, "no, it looks farily monothlic". We hope to have film at 11 pm.
  • it's attracted to the earth.

    ba-dum chhhhh......

    thanks folks, I'll be here all week... try the veal...

  • The right term is elliptical.

    Slashdot - news for ... people who are not expected to know math terminology?

  • The article doesn't really explain how we "know" what the moon's orbit was 100 million years ago. Also, we don't really know exactly how the moon formed. The theory that it was formed out of an immense object striking earth and tearing a chunk out of it is perhaps the currently prevailing theory, but it's not something that we know for absolutely certain.

    Also, 100 million years ago wasn't really that long ago. The earth's age is estimated at about 4 billion years. I'd expect that the moon formed way ear

    • The article doesn't really explain how we "know" what the moon's orbit was 100 million years ago.

      Celestial mechanics is very deterministic. Just add in extra energy the moon gets from tides, and work backwards. It's really not all that complicated.

      Also, we don't really know exactly how the moon formed. The theory that it was formed out of an immense object striking earth and tearing a chunk out of it is perhaps the currently prevailing theory, but it's not something that we know for absolutely certain.

      Wel
  • What if the moon, on a planetary scale, had a very very low viscosity?
    http://www.kirchersociety.org/blog/?p=541? [kirchersociety.org]

    Just and idea. But sometimes you have to think big.
  • ...and caught a glimpse of Uranus!

    sorry - I couldn't resist ;-)

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