Working on a D&D campaign. As a physics nerd, I'd like the orbital mechanics of the planet, its sun, and its moons, to follow standard Newtonian/Keplerian mechanics. I'm trying to come up with an interesting set of parameters so that there are neat and tidy alignments at certain times. The system was semi-intelligently designed, and so everything here can be in nice round numbers. That's all mostly just background.

There are two moons, one with a circular orbit and one with a very elliptical orbit. The circular moon has a very short period and the elliptical one a very long period. Here's the thing. I would like the elliptical orbit to have a smaller perigee than the other's altitude, but an apogee several times larger. This means that, if they aren't inclined, their orbits will have to intersect at two points, 90 degrees from the apogee.

I'm not sure if this matters, but I plan for them to have harmonic orbits. Right now the numbers I'm thinking are that the planet has a year of 243 days, the circular moon has a period of 15 days and the elliptical moon has a period of 61 days (seasons, basically). Every 15 orbits/915 days, they align at the apogee and really cool stuff happens.

My question is, with all this, is it possible to say somehow that if these two moons are in the same orbital plane, their orbits intersect at two points, and they both align at the apogee periodically, can it be shown that they either will or will not eventually collide? My rationale for hoping has something to do with the fact that they are harmonic, and at the point 90 degrees around the orbit, where they'd collide, is going to have something to do with pi, so rational and irrational numbers mean they'll never be the same value at the same time. ¯\_(ツ)_/¯

If this isn't the case, either if it can be shown that they definitely will collide, or that it can't be shown one way or another, I can work with that. I know that I can incline one or both orbits as an easy fix, and I know I can ALSO say "yep, magically they never collide" because it's D&D, but it would be super cool if there was a way they could both be in the same plane.

EDIT: This is an aside, in response to Morris' answer, it was getting too long for a comment. Since you mentioned the Dark Crystally-type stuff, there are a few other things going on here, if I may elaborate. :) First, I didn't mention but the planet's year equals its day, as if it were tidally locked. So one half is always baking, the other half is always frozen, and the ring in the middle is roughly habitable. So since the sun never moves and they don't have seasons, they use a lunar calendar. The solar year is 243 days long, the elliptical period is 61 days and the circular period is 15 days. So the elliptical's apogee happens exactly four times a year (1 cycle = 1 "season"), and the circular moon orbits 4 times plus one day for each of the elliptical moon's orbits. So the alignments happen once every 15 of those "seasons", or every 915 days/3.75 years. The alignment happens along the orbital equator at four different points, 90 degrees apart. Each of those four points has an alignment every 60 seasons or 15 years. Very different good/bad things happen depending on which point they overlap. But it works so that every 15 years the planet, sun and moons all align, which is a pretty ominous time.

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    $\begingroup$ I mean, I think we have an example of that in our own solar system, just with planets instead of moons. The orbits of Neptune and Pluto intersect, they just have an orbital resonance that prevents collision, which is true of your proposal as well. The only thing about your proposal that might make things tricky is the alignment at apogee.... not sure if that changes anything. en.wikipedia.org/wiki/Pluto#Relationship_with_Neptune $\endgroup$ Commented Dec 4, 2019 at 18:49
  • $\begingroup$ Hey @MorrisTheCat, I forgot all about Neptune and Pluto! Excellent point. I would have thought that Pluto's inclination was the reason for not colliding, but that link does a good job of explaining how their resonance creates a stable equilibrium. However, that might be harder to make happen in the much wackier case of 15:61, and in fact that rationale makes me think they're MAYBE actually more likely to find an equilibrium at 1:4. $\endgroup$ Commented Dec 4, 2019 at 19:17
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    $\begingroup$ @MorrisTheCat It's not a resonance. Pluto's orbit does not intersect Neptune's. When Pluto is at the same radius from the sun as part of Neptune's orbit, Pluto is not in the same plane as Neptune. Imagine two "linked" rings tilted to each other but they don't touch. It's like that. Now, precession could move the orbits around so they did intersect. But right now, they don't. $\endgroup$
    – puppetsock
    Commented Dec 4, 2019 at 19:31
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    $\begingroup$ What has magic to do with this? $\endgroup$
    – puppetsock
    Commented Dec 4, 2019 at 19:35
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    $\begingroup$ en.wikipedia.org/wiki/Horseshoe_orbit might be of interest $\endgroup$
    – Slarty
    Commented Dec 4, 2019 at 19:47

5 Answers 5


Ok, so you say 'Harmonic Orbits', but actual Space-Talking-Dudes call that 'orbital resonance', and it's the solution to your problem.

We've got an example of something ALMOST exactly like what you're talking about right here in our own solar system with Pluto and Neptune. As puppetsock rightly points out, their orbits don't actually intersect because of Pluto's high inclination, but if they DID intersect, the planets still wouldn't collide because of their 2:3 orbital resonance.

So far so good. At first I was concerned about the alignment possibly creating a problem, but then I realized that ANY two bodies orbiting the same primary are going to align at the convergence of their orbital periods, resonance or not, so now I don't think that's really a problem either.

Now, if you REALLY want to be clever, you'll make the resonant periods of your two satellites harmonic with the planet's orbit around the sun TOO, which means that your alignment will come at the same time of year every time it happens, which is all mystical and stuff and VERY Dark Crystal.

Just don't try to wipe out the Gelflings. It never works.

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    $\begingroup$ The ratio of their orbits is 1.50449. It's not 3:2. This only works for a while. If they were co-planar, eventually they'd have an encounter close enough to drastically alter their orbits. That might take hundreds of orbits, but eventually it would happen. A collision is less likely, but possible. $\endgroup$
    – puppetsock
    Commented Dec 4, 2019 at 20:07
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    $\begingroup$ @puppetsock that's not what the wikipedia article says, unless I'm missing something. "The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.[82] This prevents their orbits from changing relative to one another, and so the two bodies can never pass near each other. Even if Pluto's orbit were not inclined, the two bodies could never collide" $\endgroup$ Commented Dec 4, 2019 at 20:15
  • $\begingroup$ Response to that last bit as an edit to the question. Thanks :) $\endgroup$ Commented Dec 4, 2019 at 20:26
  • $\begingroup$ @FrankHarris you didn't ask about this, but having your planet rotate that slowly creates a different problem for you in that it's not rotating anywhere NEAR quickly enough to generate a magnetic field, which means no protection from solar wind, which means your atmosphere gets ripped off like Mars' did. You know, unless magic. $\endgroup$ Commented Dec 4, 2019 at 20:29
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    $\begingroup$ @FrankHarris I mean, it's D&D, so hand-wave away =) $\endgroup$ Commented Dec 4, 2019 at 20:42

You should have a look at Janus and Epimetheus. They are two moons of Saturn that exchange orbits approximately every four Earth years. This setup is probably not stable for more than a few billion years but it might do for what you want.

Janus and Epimetheus dance explained

Epimetheus orbits closer to Saturn, so has a shorter orbital period and eventually approached Janus from behind.

As they get closer, they tug on each other gravitationally.

The tugging by Epimetheus slows down Janus, which makes it fall toward Saturn in its orbit; Janus speeds up Epimetheus, which makes it rise. Janus has four times the mass of Epimetheus, so it moves inward by less than Epimetheus moves forward.

Closer to Saturn, Janus speeds up in its orbit; farther from Saturn, Epimetheus slows down. Janus will slowly creep ahead of Epimetheus; for years later, they'll do the same dance in reverse.

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    $\begingroup$ Probably worth adding a source for the diagram (or stating it is your own, of course) $\endgroup$ Commented Dec 4, 2019 at 20:28
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    $\begingroup$ I asked a question about horseshoe orbits here some time ago, for anyone who's interested in a little more detail on the topic. $\endgroup$
    – Palarran
    Commented Dec 5, 2019 at 11:39
  • $\begingroup$ @Jasper aproximately four years. $\endgroup$ Commented Dec 6, 2019 at 14:35
  • $\begingroup$ @Jasper indeed, I'm editing it now. $\endgroup$ Commented Dec 12, 2019 at 16:33

NASA recently discovered a very interesting resonant pair in two moons of Neptune, Naiad and Thalassa. Their orbits (nearly <2000km) intersect and have periods of 7 and 7.5hrs respectively. Even though they are quite close at nearest pass (<4000km) they never actually collide because of this ”unprecedented” 69:73 resonance.

This resonance was mathematically shown to be extremely stable, to where it could persist on the order of billions of years.

Unfortunately for these two, as Triton’s retrograde orbit slowly saps their orbital energy they’ll approach their Roche limit in a few million years and shatter into beautiful rings rivaling Saturn’s.

enter image description here


Oh, maybe that is what the magic is about.

Under normal circumstances, you can't have perfect synchronous orbits. Suppose the orbits are, to pick numbers, 1000 hours and 10,000 hours. That's about 40 days and about 400 days.

So whatever is supposed to happen at the first coincidence might be off by some tiny amount. This tiny discrepancy grows for 400 days until the next encounter. So the second encounter is off by a much larger amount.

Say the first encounter has an error of only 1 centimeter per second. After an hour that's 36 meters. After 10,000 hours that's 360 km. So the second encounter winds up being 360 km off target. Which gives a much larger rate of drift for the next encounter. If the moon is only, say, 1000 km across, then it probably misses entirely on the third encounter.

The most probably occurrence is a near miss that massively alters the orbits.

So you would need some way for the encounters to be tuned. That means you would need to be able to detect the motion of the moons to an accuracy of better than 1 cm/s. Much better. Since 1 cm/s produces 360 km after only one orbit. Probably you need something like no more than a few km per orbit. So call it .01 cm/s, or 3.6 km in 400 days. And you'd need some way to give one or both of the moons just the barest little push, presumably by using one moon against the other. Accurately and in the right direction. And you'd need to do that at the correct time, every time the moons encounter.

That sure looks like magic.

By the way, if I did the math right, to move our moon by .01 cm/s would require the energy equivalent of 100,000 tonnes of TNT. It really starts to look like magic.

  • $\begingroup$ Yeah...I had given some thought to perturbations/n-body stuff, but concluded I'd pretty much have to sweep it under the rug. So yes, that's def where the magic comes in. My in-universe rationale is that these orbits were desired and put into being by some very smart and powerful mages or demigods. Long ago, they might have been able to magically redirect an asteroid with some implausible precision, so that it would collide with the system and cause this. Alternately, within the planet, they might have magic things that affect the planet's magnetosphere to produce the desired perturbations. $\endgroup$ Commented Dec 4, 2019 at 20:20
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    $\begingroup$ "100,000 tonnes of TNT. It really starts to look like magic." Russia built a bomb that, even at only half power, released 500 times that much energy. $\endgroup$ Commented Dec 5, 2019 at 4:10
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    $\begingroup$ "Under normal circumstances, you can't have perfect synchronous orbits." - um, that's what orbital resonances do. $\endgroup$ Commented Dec 5, 2019 at 16:34
  • $\begingroup$ @JosephSible-ReinstateMonica ...and Tsar Bomba doesn't look like magic? $\endgroup$
    – jeffB
    Commented Dec 5, 2019 at 17:53
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    $\begingroup$ Also, you seem to have some false beliefs about the difficulty and likelihood of "tuning" orbits. Lots of orbits are self-tuning; it is not a coincidence that there are so many resonant orbits in our solar system. Resonances are stable; if orbits are perturbed from a stable resonance, gravity tends to gradually pull the bodies back towards a resonant orbit. $\endgroup$ Commented Dec 5, 2019 at 19:16

Coincidentally I just watched a Scott Manley video on this topic published in May 2018.

A small asteroid called 2015 BZ509, and a large gas giant named Jupiter have a resonance in their orbits which is self-correcting. Every time they approach, if the smaller body is too fast or too slow (that is, early or late) the influence of the larger body applies a correction.

The video goes into detail of how simulations have been done in reverse to find whether the asteroid was a captured interplanetary body. Upshot is the model is stable over billions of years for multiple possible inputs; that is while it is unusual but not vanishing off toward impossibly-improbable.

An interesting gotcha is that the asteroid is in a retrograde orbit with respect to the planet.

I'm not doing it justice, for great inspiration I suggest you spent 8 minutes watching it.


  • $\begingroup$ Hopefully this is enough explanation to avoid "link only answer" - if it needs more please comment. Are we permitted to post stills from video ? $\endgroup$
    – Criggie
    Commented Dec 7, 2019 at 11:19

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