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How come the moon is circling the earth?

  1. Jul 3, 2004 #1
    Can some one explain to me how did the moon started rotating around the earth, especially, assuming that the moon's material has earth's origin?
    In other words, if the moon was initially a peace of the earth how come it has the eliptic trajectory as it does?
     
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  3. Jul 3, 2004 #2

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    Look for the current (July) issue of Astronomy magazine. It has an article called "How Earth got its Moon." There is a sequence of computer-generated pictures showing "a Mars-size impactor [that] hits the proto-Earth a glancing blow, shooting debris into orbit. This later coalesces into the Moon."
     
  4. Jul 3, 2004 #3

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    If the moon was Earth material sent out by an impact, the impact itself would provide angular momentum, assuming it did not perform the unlikely act of hitting the Earth dead centre.
     
  5. Jul 4, 2004 #4
    I'm not really sure how to ask or phrase this question but...I'm assuming the Earth had it's molten core back then. The collision occurs, the Moon-sized chunk of matter leaves, now what is happening to the Remainder of the Earth? I know its not an "ice cream scoop" shaped hole lifted out but more likely a "chip off the ol' block" that was sheared away. I'm guessing it wouldn't have exposed the core but would it have been enough to allow for all the pressure from everywhere else in the world to be redirected to/concentrated in that particular spot? Maybe like digging your thumb into a water baloon?
     
  6. Jul 4, 2004 #5

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    In the computer simulation sequence in the magazine article, a color code is used to show temperature. In the second frame of the sequence, when the striking body is still partially attached to the Earth, you can kind of see a high-temperature gouge in the Earth, though it looks like maybe it does not extend more than 10% of the way toward the center of Earth. (Hard to say for sure from the diagram.) By the next frame the Earth is essentially spherical, and remains so for the remaining eight frames of the simulation. The red color indicating a temperature of 13,000 F eventually spreads over the entire surface of Earth. That tells me, by the way, that if there was any biology going on before the collision, it all got destroyed.
     
  7. Jul 5, 2004 #6

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    Haven't seem the sim; how long of a time does it represent? I'm partcularly interested in knowing how long the ensuing "nuclear winter" would take to drop temperatures from that 13,000o F to around 150-200, when water would begin to precipitate.
     
  8. Jul 5, 2004 #7

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    As far as how long the simulation represents, all I can say is that in the body of the article it says the accretion of the disk into a moon was "from a few to several thousand years, very rapid in geological terms. The energy of accretion appears as heat that cannot easily dissipate--and the result is a body that melted almost totally." It does not explicitly say how long the part that became the new Earth stayed above the boiling point of water.

    EDIT: spelling error
     
    Last edited: Jul 6, 2004
  9. Jul 6, 2004 #8

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    Seems much simpler to assume the moon formed in the same manner as most of the rest of the solar system. It coalesced out of the primordial accretion disc. And since earth was the gravitational bully on the block, they became became orbital buddies. The evidence suggests most planetary moons evolved in this manner. Most moons in our system orbit consistent with the spin axis of the planet they are gravitationally bound to. A few little strays were captured along the way by the big dudes.
     
  10. Jul 6, 2004 #9
    It is my understanding that it wasn’t a direct impact, in that the centres of mass of the colliding object and earth were not aligned (to the motion of the object). In a sense, the object ‘grazed’ the Earth, though still impacted. This gave Earth its tilt (from what I’ve read, if I’m mistaken please correct me). Anyway, my question is: what happened to the ‘planet’ that impacted with the proto-Earth? Does the model in the magazine suggest that it just remained as debris, fell back to earth, condensed as part of the moon and/or flew off into space? Or did it condense as a single mass again? If so, what might have happened to it?

    I wonder if it’s possible that the “Mars-sized object” was indeed Mars or perhaps even Mercury, although I think Mercury is the better candidate. Here’s why: Mercury’s rather large orbital eccentricity (0.206) compared to most other planets (except Pluto) to me suggests that it did not form as a condensing ball of dust in its present orbit around the sun. As well, Mercury has a somewhat large orbital inclination (7 degrees).

    ..Just a thought.
     
  11. Jul 6, 2004 #10
    here's an idea

    There are many superficial assumptions made by many posting about the moon's creation here. Who cares anyway, when we can finally find an accurate (very accurate) path the moon is taking, Which< I believe is presently good but could be better. We could simulate, in reverse, the exact path and the stellar bodies around it. We should observe the fusion, if you will, of the moon and earth if this theory and model are correct. Theoretically, we should be able to identify this configuration if we could find a referance time.So, we might be able to search for debris and possibly locate super stellar objects that would have strayed to their present point. Also, what if it was an ENERGY Impulse that did it and not the result of a celestial bodies influence.
    my point is anything could have been. And biology wouldn't necessarily have ceased during this time. Sure if we assume that the lower forms of life were not in some naturally sealed chamber or all the atmosphere left at this time. But, there are some amazing things about this planet that could have been happening.
    IDK but, try not to assume anything.
     
  12. Jul 6, 2004 #11

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    No doubt that holds true for some of the moons. The problem in the case of Earth’s moon is pointed out by the article I referenced: “The Apollo missions proved that Earth and the Moon are closely related… the Moon has a low content of volatile elements, suggesting its birth involved high temperatures… While the Moon’s chemistry most closely resembles Earth’s mantle, the mantle is a secondary feature of our planet’s evolution. It emerged when Earth, soon after formation, differentiated into mantle and core. How could any kind of moon, born as an independent body, have a composition matching the mantle.?”

    As best I can explain the sequence in the article, it looks like about a third of the body that struck Earth just kind of slides on past, warping out of shape as it goes. It stretches into a tendril with an arcuate shape. Other bits and pieces continue being vomited out of the gouge in Earth for a while. In frame 6 it looks like a pretty fair chunk of stuff lands back on Earth, but the long tendril continues to revolve around Earth, eventually smoothing out into more of a disk shape. The simulation doesn’t proceed far enough to show the disk coagulating into the Moon proper.

    Years ago I remember reading a brief item about someone putting a bunch of digital processors together in a way so as to make a special-purpose device to do one thing: calculate the positions of planetary bodies in the solar system, forward or backward in time. The device was called a “digital orrery.” Whether it handled Earth’s moon as well, or just the planets, I don’t quite remember. At any rate, what you are talking about could surely be done, but the results would get less trustworthy the farther back in time you look with the calculations. For instance, what if we only know the mass of the (modern) moon to 1 part in 10,000? I imagine that kind of uncertainty causes all sorts of havoc in the calculations by the time you get back millions of years in the past, let alone billions. And of course we don’t know the history of long-term comets that pass through the solar system and ever-so-slightly perturb the planets, since we only know of ones that have passed by in historic times, and that would be a tiny fraction of the overall number of those critters.
     
  13. Jul 6, 2004 #12

    Chronos

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    To each his own. It seems much more probable the moon evolved independent, but, in a similiar manner to the earth: as opposed to a chip off the old 'block'. The moon is compositionally different because the heavy elements were more attracted to the large body [earth], than the small body. Given that most moons of any consequential mass in the solar system are aligned along the spin axis of the planets they orbit is pretty good evidence.
     
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