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No moon

  1. Sep 13, 2005 #1
    What would be the effects of Earth not having a moon? I know that our moon is slowly drifting away. But what would happen then?
     
  2. jcsd
  3. Sep 13, 2005 #2
    The moon provides a stabilizing influence on the Earth's axial tilt (obliquity). With the moon in place where it is, our tilt, which is currently at 23.5°, moves between only 21.5 and 24.5° about every 41,000 years. This cycle used to be in synch with ice ages, but it hasn't been for the past million years or so.

    Without the moon in place, our tilt would vary much more, maybe to extremes of 60° or more. Mars does this since its moons are too small to stabilize the planet's tilt. This could cause extreme climate swings and big-time ice ages.

    There have been arguments that, without our moon, life on Earth would not be possible, but I think that's a little over the top myself.
     
  4. Sep 13, 2005 #3
    Well, I think complex life would have a hard time getting a foot hold.
     
  5. Sep 13, 2005 #4
    I guess your name fits!
     
  6. Sep 14, 2005 #5
    Tides would be significantly reduced. This could have had important implications for the emergence of terrestrial life had there been no moon.
    The 'day' would be much less than twenty four hours long. A substantial part of the reduction in day length is a consequence of the tidal interaction of the Earth and the moon.
    There would be an increased risk of being struck by a large bolide: a proportion of these are intercepted by the moon.
    The absence of eclipses might have delayed the early development of astronomy.
    Romantic songwriters would have been lyrically challenged.
     
  7. Sep 14, 2005 #6

    marcus

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    could say a subtantial part of the reduction of rotation rate is due to tidal drag
    (reducing the reciprocal of day-length same as lengthening day)
    this was probably intended, consistent with what else you said

    good summary, also like what tropo said about stabilizing the axis, so seasons and iceages not so extreme, and what entropy said about that helping complex multicell critters evolve

    this is a neat thread, thanks vincent for posing an interesting question.


    I don't think the moon is drifting away fast enough to worry about,
    does anyone remember an estimate about that?
    intelligent life, once evolved, could probably cope with more radical tilt of the axis, and even slightly more bolides. and so could get along with much more distant moon (or none at all)

    but the moon sure seems to have helped life succeed EARLY ON
     
  8. Sep 14, 2005 #7

    EnumaElish

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    Are planets also drifting away from the sun, or are they getting nearer to it? (I am under the impression that the latter is true.)

    I forgot what made the moon fall to Earth in H.G. Wells' Time Machine.
     
  9. Sep 14, 2005 #8

    JesseM

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    It didn't in the original H.G. Wells book, it was only the 2002 Time Machine movie where the moon blew up...the explanation they gave there was that people were using nuclear bombs as part of mining operations on the moon.
     
  10. Sep 14, 2005 #9

    JesseM

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    According to this, it's drifting away at about 4 cm per year, and billions of years in the future it will stop drifting away when the length of the earth's day is the same as the length of the moon's orbit (around 47 modern days), at which point the moon will be about 1.35 times as far away as it is now.
     
  11. Sep 14, 2005 #10

    EnumaElish

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    Oh, I hope life will not imitate art in this case!
     
  12. Sep 14, 2005 #11

    EnumaElish

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    And what is the reason for the day stretching to 47 times its current length?
     
  13. Sep 14, 2005 #12

    JesseM

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    According to the page, the drifting away of the moon is related to the slowing of the rotation of the earth because of "tidal friction", which transfers the earth's rotational momentum to the moon's orbital momentum. This page gives a slightly more detailed explanation of how tidal friction works:
     
  14. Sep 15, 2005 #13

    Chronos

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    Tidal locking is the name of the game. The moon is already tidally locked to earth [i.e., not rotating with respect to the earth's axis]. The earth will become tidally locked with the moon in the distant future, as JesseM noted. It will take earth much longer to become tidally locked to the moon due to its much greater mass, oceans and denser atmosphere
     
  15. Sep 15, 2005 #14

    marcus

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    Thanks Jesse!
    this incidentally confirms the point Chronos made

    I will put down an attempted explanation and then check the link to see if it agrees
    basic principles are conservation of angular momentum, and friction

    the total angular momentum represented by moon orbit PLUS rotation of earth cannot change (system approximately isolated, nothing to take it up)

    so if earth rotation slows down from friction with the tidal bulge
    then it loses spin, and the moon orbit has to gain it----by having orbit radius extend

    ===================
    yes, I checked Jesse's link and at least my explanation is consistent with it.

    the way I picture it is the tidal bulge tries to keep pointing at the moon (two bulges aligned with the earthmoon line)

    and the solid earth is rotating UNDER that bulge, and by friction of the water having to flow around obstacles etc, the solid earth pulls the bulge around so that instead of being exactly aligned it is just a little bit IN ADVANCE of the moon

    now that bulge is dragging the moon ahead, like when you whirl something tied to the end of a string, and to put energy into the whirling you make your hand just slightly LEAD the object, so your hand is slightly in advance of the object and the string is not only holding the object inwards but also dragging it ahead.

    so that puts some energy into the moon's orbit (there is also energy lost by the turbulence of the flowing water which just heats the water slightly, we dont have a simple energy conservation here because some leaks off)

    and the way a roughly circular satellite orbit absorbs energy is gets a longer radius, goes a little bit slower, with a longer period, and has an increase in angular momentum.

    so Jesse's link says this has to keep happening, with the moon getting farther and farther away, UNTIL the earth is no longer rotating faster than the moon is orbiting (until Day = Month) so that friction is no longer dragging the bulge ahead-----and then the tidal bulge will be perfectly aligned with the earthmoon line and will no longer be in advance, so no further transfer of energy.
    =========================

    OOPS AND SHUCKS
    I just looked at jesse's OTHER link
    http://curious.astro.cornell.edu/question.php?number=124
    and see that it says essentially the same as I just wrote so I didnt need to add the explanation.
     
    Last edited: Sep 15, 2005
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