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Two Moons

Dorfl

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Jun 19, 2005
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Lots of fantasy worlds, like those in Warhammer Fantasy or Elfquest depict planets with two moons, both appearing to be about the same size—or bigger—as our moon. Would that be possible in the real world, without either of the moons being flung into deep space or crashed into the nearby planet?

While many planets in the real world have several moons, I can't think of any which are not much smaller relative to their planet than our moon is to the Earth.
 
You realise they're not meant to be taken literally as...moons.
Think metaphorical heavenly bodies or parts thereof.

Actual answer- Well, Callisto, Europa, Ganymede and Io are all pretty big.
 
Soapy Sam said:
You realise they're not meant to be taken literally as...moons.
Think metaphorical heavenly bodies or parts thereof.

Actual answer- Well, Callisto, Europa, Ganymede and Io are all pretty big.
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But, those are in orbit around proportionately much bigger planets than the Earth-Moon ratio. The OP is asking whether an earth-sized planet could support more than one moon-sized moon, gravitationally speaking, or would the physics of it decree that it wouldn't work.

It's an interesting question, and I look forward to seeing the answer. Paging the Bad Astronomer (who is, after all, an astronomer...)
 
Rightoh. I'm unfamiliar with the worlds of Warhammer & Elfquest.
Guessing, I'd expect a three body problem involving an Earth size primary with two Luna sized satellites would have chaotic instabilities in the long term, unless one moon was a great deal further away than the other. But indeed there may be an astronomer along in a minute.
 
One must consider the unique circumstances that formed Earth's moon. Would it be possible for such a rare event to happen twice? Sure, but the odds of it happening are pretty low.
 
Travis said:
One must consider the unique circumstances that formed Earth's moon. Would it be possible for such a rare event to happen twice? Sure, but the odds of it happening are pretty low.
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Trouble with cosmology is there's always a counter to the "uniqueness " argument- namely that given we already know of one such pair and we really only know 8 planets and Pluto, such pairs may actually be extremely common.

But triplets...?
 
Sure it'd be possible, but you have too many variables for a solid answer. One of the moons cold be smaller, but closer and on a faster orbit, and appear to be the same size. Or they could share an orbit, but avoid collision like Epimetheus and Janus. I'd have to double ckeck the details, but these two moons of Saturn are nearly the same size, in nearly the same orbit, but not traveling at quite the same speed. When one moon starts catching up with the other, They exchange momentum due to the interplay of gravitational forces. The slightly slower moon becomes the slightly faster one, and then they slowly move apart until they catch up to each other again.

The real issue with two moons would be how it effects the tides. Depending on their mass and speed, the tide wouldn't be coming in twice a day any more. The moons could virtually cancel out each others effect on the tides. Or they could compliment their effects, and high tide might be rolling miles inland.

Or they could be like the windshield wipers on a school bus, gradually moving in and out of sync with each other, causing the tide to vary considerable over time, and making planning a trip to the beach a highly time sensitive issue.

It's not too tricky to do the math to sort all that out, but like I said, it's all about whatever numbers you pull out of the air to plug in to the equations. But basically the answer to your question is yes, it could work, but you wouldn't want to live too near the beach.
 
From what I've gathered from the simulations, if the original Moon forming impact generated two moons then they would either collide and combine or fall back to the planet. Two large moons close in would be unstable. You would need to postulate a different origins scenario.
 
Well, just for the sake of argument, and to give a starting point, let's stipulate that it's one Earth-sized planet, and two moon-sized moons, with the current size, shape, density, etc. of the current earth-sized planet and moon-sized moon. Would celestial mechanics allow those two moons to orbit around the planet?

Let's also stipulate for the sake of argument that the two moons arrived in orbit around this earth-sized planet via magic elves, who cast a spell to poof them into existence. Because, seriously, otherwise we'll get sidetracked into a discussion of what kind of creation event would cause two moons to arrive, and IMO that's kind of peripheral to the OP's question. It doesn't matter at this point how the moons arrived, just, "could you have an earth-sized planet with two moon-sized moons?"

ETA: Let's just stipulate, IOW, that the moons are here, we don't care how they got here. Question: can they remain in orbit?
 
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Bearded One said:
From what I've gathered from the simulations, if the original Moon forming impact generated two moons then they would either collide and combine or fall back to the planet. Two large moons close in would be unstable. You would need to postulate a different origins scenario.
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Yeah that's true, and I feel stupid for not pointing out that I was talking from the assumption of "the clock is already in motion". The odds of an Earth sized body being able to form two proportionally sized moons from collision are absurdly slim. Managing to capture one or more bodies, and have them end up in a stable orbit, is a lot more unlikely.

So yeah, I should have said it could work, but you'd have to put the moons in place with a very large crane or something.
 
Goshawk said:
ETA: Let's just stipulate, IOW, that the moons are here, we don't care how they got here. Question: can they remain in orbit?
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I'm not sure and would guess that such a system might be unstable in the long run.

If we suppose that one is close in and the other farther away I would think that the enormous gravitational interplay would either knock the inner one into the Earth or the outer one out of the system altogether.
 
Travis said:
I'm not sure and would guess that such a system might be unstable in the long run.
Click to expand...

You know, I was thinking that would turn out to be the case myself. But I started playing with a gravity simulator I've got here. I dug up a preset module simulating of the orbits of Epimetheus and Janus, thinking it'd be a quick fix to just alter the mass of Saturn and the two moons (and their orbits), for a fast and dirty test run.

Just by accident, when I found the Saturn presets, I also stumbled across a series of simulations someone else had run. He was just introducing a cloud of a few hundred "moonlets" equal in total mass to our moon, into orbit around a moonless Earth. It turns out that occasionally the debris cloud would condense into two moons of equal mass, each half the size of our moon, and in stable orbits.

*Well, let's say close to a 50/50 split in mass. Some of the debris got thrown into space, and some hit Earth while the bulk of the cloud was jockeying for position. But the bulk of it ended up in the two remaining satellites.

When this did happen, it tended to consistently be one with a tighter, near circular orbit, and the other with a much wider highly elliptical orbit. He never got anything resembling the freaky relay race style orbits of Epimetheus and Janus, though.

If my insomnia is still in full effect tonight, I'll play around with it for a few hours and see what I come up with. So far though, it doesn't look nearly as implausible as I was expecting.
 
Dorfl said:
Lots of fantasy worlds, like those in Warhammer Fantasy or Elfquest depict planets with two moons, both appearing to be about the same size—or bigger—as our moon. Would that be possible in the real world, without either of the moons being flung into deep space or crashed into the nearby planet?

While many planets in the real world have several moons, I can't think of any which are not much smaller relative to their planet than our moon is to the Earth.
Click to expand...

Appearing to be about the same size, I believe, is going to be a factor. The sun and the moon appear to be about the same size from our vantage point (consider an eclipse).
 
it is my understanding that the world cannot "support" its moon.
isnt the moon drifting away from the earth, 4 centimeters every year?
 
Dictator Cheney said:
it is my understanding that the world cannot "support" its moon.
isnt the moon drifting away from the earth, 4 centimeters every year?
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Yes. This is because it is extracting energy & angular momentum from Earth's rotation via the tides. This also makes the day longer. It will stop when the day is the same length as the lunar month -- the Earth will show the same face to the moon -- but IIRC the sun will blow up before that.
 
shadron said:
Yeah, there aren't many with moons as big as Earth's is; we could almost be considered as a double world.
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An interesting fact about this is that the moon's orbit is always concave to the sun -- I think that also implies it is never retrograde either, but I've not done that calculation.
 
Sorry about the hit-and-run OP.

Thanks for your responses. If I understand the consensus correctly, an Earth–sized planet with two moon-sized moons is not necessarily impossible, but certainly very, very unlikely. And even if possible, the inhabitants might have to put up with very nasty tidal effects.

Kind of a pity, since it does look quite cool on paper.
 
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