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Writing a werewolf story, and decided for a bit of flair, so I wanted two moons in orbit. Did a bit of digging, and I found out that to do that, I'd need to either have them in separate orbit (no thanks, I like calendar math, but that's the irritating kind, not the fun kind) or to have them in an L4/L5 position. Didn't like that either. So I dug a bit more.

Then I found binary star systems. Yes, I'm aware that this exact question has been asked. Except, it kind of wasn't. See, the answers seem to going towards where the moons were orbiting at each other's L3 point. That's not how a binary star system works. (Watch the video, it's cool.)

Instead, the stars share one of their foci, and kind of orbit around each other, given that. So, would it be possible to replicate this on a planet, having moons that share a foci on the Earth, and opposite orbit patterns. So, while it would appear from the planet that there are two moons in the same orbit, perfectly opposite each other, in actuality it'd be different. Something like this:

Yes, it's not to scale. Sue me.

(You'll probably need a guide to the picture, given that my talent is more scientific than artistic. The big yellow one is the sun, the green one is the planet, grey is moon, white is approximate orbit. And because I need the obligatory joke, 'Dammit Jim, I'm a scientist, not an artist'.)

The question is, would this work?

A note: For calculations, assume the planet is the size of Earth, or of a habitable nature, so 6x1024 kg. The moons must be visible, so assume around 1022 kg. The solar system is a single star, and the other planets are a single gas giant deep into the system and an asteroid belt beyond that.

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    $\begingroup$ I'm pretty sure you can't do that, but I'd need to do a much deeper analysis to be able to actually tell you exactly why not. (I actually might do that, if no one beats me to it.) But the short version is that the two moons would interact with each other gravitationally at every point in their respective orbits, which is unlikely to be stable. If you reduce the mass of the moons by another few orders of magnitude, you might be able to pull it off. In the meantime, I present thee Freefall 514--516. $\endgroup$ – a CVn Jun 5 at 19:35
  • $\begingroup$ A flaw here is your assumption that moons orbit "around" planets, while binary stars orbit a shared focus. In reality, everything orbits around a shared focus, it's just that when the masses are very different, the focus is inside the larger body. The New Horizons team made a nice video of Pluto & Charon orbiting their mutual center of gravity. $\endgroup$ – jamesqf Jun 6 at 17:42
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Your initial configuration, with the two moons orbiting perfectly out of phase, is not a stable one.

Over time their orbits will change very slightly as the result of various random effects, such as asteroid strikes, which will not affect both moons identically. The moons will tug on each other in surprising ways. Just read up on the three body problem and you'll see what you're up against.

This means that one moon will start orbiting ever so slightly faster than the other, at which point they will no longer orbit in perfect antiphase. As time progresses, the angle between the two moons decreases, which will only serve to amplify any instabilities in the system. Eventually they'll reach some minimum separation, which will be when the magic happens.

Edit: a CVn pointed out, with depressing plausibility, that what will probably happen is that one of the moons will simply get ejected from the Earth-Moon system and end up in a solar orbit.

Turns out you don't have to guess too hard at this sort of thing, but can instead just go look at someone else's simulator. I haven't actually researched how plausible the one I found was, but hey: it has actual greek letters and stuff in its configuration, so it seems pretty serious to me.

Behold, my two moon simulation, using "gravity simulator". One identical copy of the moon is placed next to earth, with an argument of periapsis in opposition to the regular moon. As anticipated, the moons slowly end up with their separating angle reducing, at which point one of them simply gets booted out into solar orbit. No earth shattering kaboom. Physics is so disappointing, but I console myself that it is just a simulation and in real life it might have been so much more dramatic.

You should fiddle with the simulator yourself, if you have sufficient patience; it is somewhat user-unfriendly, but workable. If you reduce the masses of the moons by at least a factor of 10 then you end up with something that merely looks a bit chaotic (and will probably fly apart given external gravitational influences, like your gas giant). Maybe if the moons were a hundred times lighter you'd succeed in getting them to work, but at the cost of your lovely big moons providing a scenic backdrop to your planet. Maybe your complex calendar system doesn't sound so bad anymore?


Here were my previous ideas, prior to running a simulation. The observation about the size differences between the moons and their parent is probably the important bit.

In our solar system, we have the moons Janus and Epimetheus which are co-orbital. They close within about 10000km of each other and then effectively swap orbits and slowly separate again, only to meet and swap again four years later. This is the best case scenario for your moons. It does mean that you do end up with moons in different orbits, and yes, that means you're going to have funny calendars. But conjunctions are cool, especially ones you can see so closely.

On the other hand, the parent planet of Janus and Epimetheus, Saturn, is about a billion times heavier than they are, which I suspect imposes a certain amount of stability on the whole system (or at least, stabilises it over a much longer timescale). In your example, the parent planet is only a hundred times heavier or so, and that does not fill me with warm fuzzy feelings.


These final ideas are clearly wrong. but I'll leave them here for future reference, if only to discourage later readers from making the same mistakes!

I'm not a skilled enough orbital mechanic to tell you exactly what happens next, but I suspect that it will be one of two things: your moons crash into each other, resulting in a Really Exciting period of meterorite bombardment on your planet... the sort best observed from a different planet. You'll get a funky ring system for a bit, which will be nice.

The final option looks a bit like this.

Theia colliding with Earth

Except, y'know, twice as much.

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    $\begingroup$ I doubt you'd get a Theia-style event. Certainly the possibility of a bad day, but if I'm not mistaken, Theia is hypothesized to have been on roughly the same order of magnitude mass as Earth at the time. While your point about relative mass remains valid, I don't think the outcome would be quite as bad as that. A more likely scenario seems to be one or both of the moons ejected out of the planet's sphere of influence, with the possibility of the remaining one ending up on a highly elliptical orbit, which itself would create all sorts of "fun". $\endgroup$ – a CVn Jun 5 at 19:38
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    $\begingroup$ @aCVn it was the nicest picture I could find of a major impact ;-) I suspect you may be right about one of the bodies being ejected. I'm going to do a little more investigation and see what turns up. $\endgroup$ – Starfish Prime Jun 5 at 19:42
  • $\begingroup$ @aCVn found a simulator, it agrees with you :-( There's no earth shattering kaboom. $\endgroup$ – Starfish Prime Jun 5 at 20:17

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