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Can two moons rotate around a world in opposite directions of each other

Discussion in 'Writing Discussions' started by Vicki27, Jul 7, 2020.

  1. Vicki27

    Vicki27 Minstrel

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    Hi, this may be a completely stupid question, but in my world there are two moons, one silver, one red. I would like them to rotate in opposite directions if possible, but I am not sure if this is physically possible. I know it shouldn't matter seeing as it's my world but I do like to be correct if I can.

    So I guess I have 2 questions, is this possible and if so, can someone give me a quick, SIMPLE explanation of how it would work.
     
  2. Orc Knight

    Orc Knight Auror

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    Not being a physics person of any sort, but I kind of doubt it. Can't rule out that it can't happen though. Space is weird and doesn't like to listen sometimes. If you have a decent enough explanation, it'll probably just be shrugged off. Maybe a deity did it, or a mage of some sorts, or it is a deity. Or some big magical thing happened out their and put the moons in opposite orbits.

    Or magnets.
     
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  3. A. E. Lowan

    A. E. Lowan Forum Mom Leadership

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    Yes, it is absolutely physically possible. Gonna do some weird things to your tides, though, assuming your world has oceans.
     
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  4. Steerpike

    Steerpike Felis amatus Moderator

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    Sure. I believe Neptune and Uranus both have at least one moon that orbits in the opposite direction of the rest of them. If they're orbiting in the direction the planet rotates, it's prograde. If they're going the opposite direction it's retrograde.
     
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  5. Chuck

    Chuck Scribe

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    Triton has a retrograde orbit. It is thought to be a Kuiper Belt object (similar to Pluto) that was captured, and not a natural satellite. It is extremely unlikely that a natural satellite would develop a retrograde orbit.
     
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  6. Penpilot

    Penpilot Staff Article Team

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  7. Vicki27

    Vicki27 Minstrel

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    Thanks for your comments, info and links. When I was inventing my world I wanted 2 moons as a way of distinguishing two different lifestyles good v evil, magic v non-magic you know the usual stuff..... but it wasn't until I was developing a particular subplot that I thought perhaps I had made the impossible happen. Even better, I've learnt something as well :)
     
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  8. S J Lee

    S J Lee Inkling

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    Why do planets orbit in the same direction?

    basically, if the planets coalesced at the same time, they would ORBIT the same way.... but if a moon was a passing space rock, caught later on, by gravity, why not opposite? I am not a physicist. Just wondering? Do you mean rotate on the moons' OWN axes in opposite directions? or orbit the PLANET in opposite directions? I think most astronomers refer to orbit + the sun and rotate + own axis. Venus and Uranus apparently spin in opposite directions ON THEIR OWN AXES

    and of course, if your world is simple "a magical creation", then don't even worry about it
     
  9. Vicki27

    Vicki27 Minstrel

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    Orbit the planet in opposite directions - my world is magical but it is a very limited magic otherwise, I think you can be in danger of making everything magical and to me that is just a little bit too easy! Also I don't really want to go against the nature of things if they are impossible - I don't mind stretching poetic license a little but not too far!
     
  10. Azaraiha

    Azaraiha Scribe

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    A moon is locked in its orbit by the gravity of the planet it is slave to. It is absolutely possible for two moons to have opposite orbits from each other. The moons of Jupiter and Saturn do this.
     
  11. J.W. Golan

    J.W. Golan Scribe

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    It should be pointed out that all of the examples provided for moons in retrograde orbits are bodies that are much, much smaller compared to the planet they orbit, than Earth’s moon is compared to the Earth. This is not an accident. A larger body in retrograde orbit would cause tidal drag on both the planet, and any other moon or moons - which would result in orbital decay. It’s not a stable arrangement.

    Earth’s moon, in contrast, experiences tidal acceleration. Because its orbit is in the same direction as Earth’s rotation, the same tidal forces which gradually slow the Earth’s rotation also accelerate the moon to a higher orbit (further from Earth) across hundreds of millions of years.

    For a retrograde moon to be a lasting feature (on geological time scales), it will need to be much smaller than the planet it orbits about, and probably at a greater distance than any large, naturally occurring moon.
     
  12. scink

    scink Acolyte

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    Given that you are working in the realm of fantasy can you not simply create your own rules. Otherwise you open yourself up to all kinds of interrogation from readers. If it were me I would call one the light and the other the mirror or maybe the dark and the shadow. One exists because of the other and they are opposites. Maybe a gas based planet that pulsates with energy that feeds off the essence of the other.. Maybe drawn to each other for reasons that give meaning and purpose to those that populate the main planet....' for this is the world of Enethia where anything is possible '. Maybe the occupants have shadows that can split off from their physical form and reflect the way the orbiting bodies behave. And their essence is drawn to one or the other. Light and dark and good and evil. Me I wouldn't worry, and the more fragmented and unfathomable the concept the more fantasy you can envelop in your story. Have fun with it.... Reminds me of Universe on the Amiga. Planet jumping.

    But what do I know. I have no idea. They orbit cus of the power of imagination.
     
  13. Vaporo

    Vaporo Inkling

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    Actually, I don't think the issue is so much that it's unstable, it's that large objects orbiting retrograde don't tend to form in the first place. The solar system was formed from a rotating cloud of dust, therefore everything in it tends to rotate in the direction that the cloud of dust did. Retrograde objects are usually only formed when a stray object like an asteroid is captured in orbit around a planet and therefore the direction of rotation rule doesn't apply.
     
  14. Sunny dewbae

    Sunny dewbae Dreamer

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    when I read the question I kept thinking about this video
     
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  15. J.W. Golan

    J.W. Golan Scribe

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    You are correct that they don’t commonly form - but they also experience orbital decay due to tidal forces that a natural satellite will not experience. Neptune’s moon Triton, for example, is expected to eventually pass inside the planet’s Roche limit, where tidal forces will tear it apart.
     
  16. Vaporo

    Vaporo Inkling

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    Not necessarily. If orbiting bodies become tidally locked, or are otherwise distant enough that tidal locking occurs very slowly, this is a non-issue.
    The opposite can happen too. A body orbiting prograde can gradually gains momentum due to tidal forces until it is flung out into empty space.
     
  17. Carl Brothers

    Carl Brothers Scribe

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    The way i see it, as long as it is not a new phenomenon, then life on your world has had plenty of time to adjust and evolve to whatever magnetics and gravity govern your world. May be worth a small mention of how those make your world unique ...
     
  18. J.W. Golan

    J.W. Golan Scribe

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    That’s if the orbit is prograde. As I mentioned before. Earth’s moon experiences tidal acceleration, its orbit gradually raised over time. A retrograde moon will experience tidal drag, its orbit lowering until it reaches the Roche limit.
     
  19. Vaporo

    Vaporo Inkling

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    Yes. But the drag is usually very, very gradual and stops once both bodies become tidally locked. The moon is tidally locked to the earth. As the earth transfers its rotational momentum to the moon, eventually it too will be tidally locked to the moon. You get the exact same phenomenon in a retrograde orbit. Once the bodies are tidally locked, there is no more tidal drag.
     
  20. J.W. Golan

    J.W. Golan Scribe

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    To be tidally locked with a retrograde moon, the parent body would have to reverse its direction of rotation. Stop for a moment an consider just how much momentum you’re talking about transferring. The moon will lose orbital momentum and pass the Roche limit long before then.

    Also, recall that the question was posed for a planet with two moons: one retrograde, the other not.

    This is why I don’t write science fiction. I deal enough with the physics during my day job.
     

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