Two Moons? What would happen?

  1. Let's think about this for a second:
    Unlike earth, Mars has two moons. Let's switch the two. Instead of just one moon, what would happen if the Earth has two moons? I've been searching the Internet like crazy for answers on this, I haven't been able to find much. All I've found is stuff on moons orbiting gas giants, or what would a planet without a moon be like. Any help would be appreciated.
     
  2. jcsd
  3. mgb_phys

    mgb_phys 8,952
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    Depends on the moons.
    Mars's moons are very small and so have almost no effect mars.

    Two Earth moons sized moons (!) would have an effect, especially on tides.
     
  4. Okay, let's say that Earth's second moon is slightly (about a fourth) smaller than its first moon (by first, I mean the one we already have). What would be the effect on temperature, tidal forces, etc.? We have to remember that the moon's gravity has an impact on the tides. If both moons are orbiting the Earth in the same direction, then I can assume that there will no change. However, if the smaller moon orbited in the opposite direction, it's possible that this would cause a major change in Earth's environment.
     
  5. mgb_phys

    mgb_phys 8,952
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    Temperature - not much effect, the moon doesn't block a significant amount of the sun.
    Tides - tricky, tides are tricky enough as it is with two moons in different orbits it would be 'fun'.
    It's not clear what the interaction of tides and weather is but odds are it's complicated but significant.
     
  6. Nabeshin

    Nabeshin 2,200
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    My gut reaction is tides would become borderline chaotic with two substantially sized moons. Also, it would be interesting to see what (if any) tidal locking and/or resonance occurs with such a configuration.
     
  7. Okay, how could it be chaotic? If the two moons are moving in the same direction, wouldn't the tides be about the same? I can understand the tides being chaotic if the moons moved in opposite directions.
     
  8. Nabeshin

    Nabeshin 2,200
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    I'm obviously just speculating, but even if the orbital motion of the moons is fairly simple, the fact is tides do not occur exactly in synch with orbital motion. It takes a while for the tides to "catch up" to the moon's position. It is this catching up period that makes me think that with two different orbital periods, a complicated or chaotic motion could be produced.
     
  9. Alright, we all know that a moon also affect a planet's rotation and seasonal change, so what would happen if a planet had two moons?
     
  10. sophiecentaur

    sophiecentaur 13,796
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    How big and how far away? If their orbits had similar radii then they would interact a lot - 'tumbling around' each other and exchanging inner and outer positions periodically. If they each had similar mass to our present Moon then the tidal range would increase (by more than twice) and the cycles would become more complex. Would they both be in the same plane?
    The scarey thing would be that you could get wild variations in their orbits. If that were to bring one very close to Earth every so often it could be a nightmare in low lying areas of coast and it could be much harder to predict, I imagine. (Hence the 'chaotic' idea, introduced earlier)
     
  11. if we had two moons, we would more than likely lose most of the earth's tilt, greatly affecting seasons and life on earth. depending on how big the other moon is and which way it revolves, it may or may not affect tides that much.
     
  12. Chronos

    Chronos 10,060
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    Having two moons in stable, separate orbits would be interesting. Another lunar mass moon say at half again the distance from earth would have interesting tidal effects. Tides might occasionally resemble small tsunamis, and almost unnoticeable at other times. It is hard to say how this might affect things like ocean currents. So long as both moons were in the same general orbital plane, it would little effect on the tilt of earth's axis.
     
  13. imagine having the two moons orbiting each other in a binary configuration with both orbiting around the earth. I'd imagine you would probably have super long high tides at the equator with very low low tides, but seemingly normal tides as you move towards and away from the poles.
     
  14. Wait, tides, "[resembling] small tsunamis," is "interesting"?! I'd hate to hear about what you find intriguing or terrifying. Seriously though, if it effects ocean currents that effects weather patterns... I wonder if you'd see slightly different land-masses above sea level (or below)?
     
  15. Janus

    Janus 2,426
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    Assuming that at least one of the Moon's was that mass of our own, the maximum separation between the two while maintaining a stable orbit around each other would be ~20500 km (1/3 the Hill sphere to be conservative).

    If we assume that the moon's are of equal mass, Each will swing a little closer to Earth and a little further from Earth as it orbits the other, This means that when one is at its closest to the Earth, the other is at its furthest. This will tend to smooth out differences in tidal force due to the individual moons changing distance from the Earth. Any remaining difference would be very small compared to the difference due to the fact that our Moon already changes its distance by more than the separation of the two, due to the eccentricity of its orbit.

    So, having two moons orbiting each other in binary fashion would not be much different than having one moon of the same combined mass.
     

  16. Sorry, I should have elaborated on what I meant rather than the two moons moving far and close to earth in their orbit, I meant that they would rotate from the north pole to east to the south to the west. so both moons would generally be the same distance from the earth at all times. though as you explained, with the maximum distance between the two moons being so small I'd imagine that moons in a binary configuration have much more effect than a single moon.
     
  17. Chronos

    Chronos 10,060
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    Two equally massive moons orbiting each other while orbiting earth would not have stable orbits.
     
  18. Meaning... it would decay and they'd fall to earth? Well, that's definitely something happening! :0
     
  19. Janus

    Janus 2,426
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    Such an arrangement is not possible. Such a polar orbit would hold its orientation to the fixed stars (minus precession) so at different points of the month it would have a different orientation as seen from Earth.
     
  20. Janus

    Janus 2,426
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    Meaning that the moons would separate into their own independent orbits around the Earth.
     
  21. sophiecentaur

    sophiecentaur 13,796
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    At certain times, the two moons would be at the same 'position' in the sky. Their total effect, however, would be greater than the effect of a single Moon at their centre of mass - the nearer moon would dominate as their separation could be comparable with the distances from the Earth. This would cause higher tides, occasionally, surely than for a single Moon.
     
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