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Origin of the Moon and it's Retreat

  1. Apr 14, 2013 #1
    In my reading about Astronomy, I have encountered two seemingly different scenarios for the origin of the Moon:

    (A) Planet Gaia (unsure of spelling) crashed into the Earth. It nearly destroyed the Earth, but from that collision, debris formed into the Moon.

    (B) Planet Orpheus crashed into the Earth at a very oblique angle, so oblique, that there were two collisions. Debris blew off and for reasons having to do with the Roche radius, debris formed into the Moon.

    How can there be two different planets that theoretically crashed into the Earth? That can't possibly be right.

    Why exactly does the Moon move 1/2 an inch further away every year from the Earth? Do other moons do this, in particular, my username? This seems really weird.
  2. jcsd
  3. Apr 14, 2013 #2


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    from wiki...

    another version of formation ... well another object name anyway ....

    who cares what the object was called ;) the general consensus is that the moon was formed by an impact when the earth was still relatively molten

    not really weird :) again from wiki ...

  4. Apr 14, 2013 #3

    Simon Bridge

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    Welcome to Science - there is always more than one theory for any situation. It is just a manifestation of the adage: there is more than one way to skin a cat.

    Notice that both theories involve a collision - don't worry about the names.
    Some big object collides somehow and different theorists give different details for the collision.
    The dominant theory is that detailed in wikipedia - thanks devann.

    You've seen slow-mo movies of a liquid drop into a surface?
    You've noticed that you always get a sphere popping up at the end?
    That's the basic picture of the Earth-Moon system formation... modify for being much bigger and involving molten rocks and two spheres instead of a sphere and a surface... things like the collision wouldn't have been exactly dead-on and so on. Option B would have been from investigating how undead-on the collision could have been and still get an Earth-Moon system something like what we see today.

    Life is weird - you get used to it.
    Yes - other moons do this [change their orbit radius over time].

    Not all moons were formed by this process though (i.e. Demos and Phobos).

    From the crude formation model (above) it should no longer be surprising that the Moon's orbit radius changes. To get a fixed orbit would involve a very exact random collision ... a contradiction in terms. As the Earth and Moon move apart, conservation of angular momentum means the orbit period also changes... think ice-skater throwing hands out to slow a spin (only the role of the arms is played by gravity).

    There are a lot of subtleties though and davenn is correct about the gravitational kick accelerating the Moon. There's also a tidal effect on the Moon - small perturbations from other planets etc.. I just wanted to show that these observations are not too surprising when you consider even a very simple model of the Moon's formation. When you realize what everything else is going on it becomes a slam dunk - there is no way any orbit stays the same in the long term.
    Last edited: Apr 14, 2013
  5. Apr 14, 2013 #4
    What exactly is the bulge nearest the moon? Does the moon have an equatorial bulge like the Earth does?
  6. Apr 15, 2013 #5

    Simon Bridge

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  7. Apr 16, 2013 #6
    Many thanks guys.
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