I'm building an alien world for a game mod, and want to avoid any blatant mistakes from a world-building perspective. The world has three moons visible in the night sky - two on one side near each other, while the third is on the opposite horizon. When viewed from the equator (my main landmass is tropical), the pair is located in the northwestern sky and the single is to the southeast. The pair are roughly both a little larger and a little smaller than Earth's moon, while the single is medium - say the same size as Earth's moon.

What affect, if any, would this arrangement have on the formation of landmasses and/or oceans on the planet? I'm currently using Earth-type erosion to form both topographies, but is this the correct approach?

Also, as a result I'm keeping the nights fairly well-lit instead of dark, similar to dusk but with a slight purplish tint. Would this affect the growth of flora or fauna to any significant degree?

I hadn't really put much thought into the details, but let's assume for this question that the paired moons are locked together and all three bodies are roughly the same distance as Earth > Luna with around the same rotation period. Under these assumptions, we can also assume that the pair would have a larger accumulative mass than the single on the other side, and that they would always be more-or-less opposite of each other throughout the year.

  • $\begingroup$ what is the distance between your satellites and the planet ? Also, you said that they near on the same side but this is only temporary since they rotate around the planet at different speed. $\endgroup$
    – Vincent
    Oct 15, 2014 at 19:17
  • $\begingroup$ Is your planet tide locked with one of the satellite? $\endgroup$
    – Vincent
    Oct 15, 2014 at 19:18
  • $\begingroup$ Good points - hadn't put that much thought into it yet. I'll add both to the question. $\endgroup$
    – Omegacron
    Oct 15, 2014 at 19:19
  • $\begingroup$ so, you have a pair of MOONS that are locked together. Their center of rotation is at Earth-Moon distance. The other Moon is alone and at the same distance. So, they rotate at the same speed if the have the same mass. A small difference will make them crash in the long run. Not if they have a different orbit. Time for some Universal sandbox ! $\endgroup$
    – Vincent
    Oct 15, 2014 at 19:36
  • 1
    $\begingroup$ This question will get you started on the tides at least: worldbuilding.stackexchange.com/questions/71/… $\endgroup$
    – Liath
    Oct 16, 2014 at 7:04

2 Answers 2


I'm going to ignore orbital stability issues (I suspect your arrangement is unstable, resulting in one or more of the moons being ejected from the system, but I can't prove it).

"Well-lit" nights is a relative term. The Moon provides only 1/500,000th the light of the Sun; even if your moons were mirror-reflective, you could only boost that light to 1/60,000th or so. That's enough light for nocturnal predators to have an easy time of it, but not enough for photosynthesis.

I wouldn't expect any major changes in landscapes. You'd see a slight increase in tidal flexing of the crust, but it won't be a major geological factor. The main effect I'd expect is that earthquakes are slightly more common and slightly weaker; you might also see a slight increase in vulcanism.

The big change is the tides. The moons you describe are not in an equatorial orbit, so the strength of their tidal contribution varies based on how high above the horizon they are at peak: a day when a moon is low on the southern horizon will have a weak tide from that moon; a day when the moon passes overhead will have a strong tide.

If the singleton moon and the pair are in different orbits with different inclinations, predicting the tides would be a nightmare -- set it up right, and you've got effectively chaotic tides.

  • 1
    $\begingroup$ That may actually work to my advantage - this world is a plane of Oblivion (it's a mod for Skyrim) and is generally a chaotic place anyway. I don't know if it's actually possible to simulate tide differences with the world engine, though. Might be fun to try. $\endgroup$
    – Omegacron
    Oct 16, 2014 at 15:46


three moons visible in the night sky - two on one side near each other, while the third is on the opposite horizon.

Three moons, two together and one off at an angle.

And MAINTAINING this relative separation at all times? So they MUST be in the same orbital plane, and at the same orbital distance from the planet.

It would require suspension of the laws of gravity and/or motion, as the two moons near each other would gravitationally attract each other, and collide.

The closest you could arrange, and this still would require a bit of celestial handwaving:

The two moons orbit each other closely, and the pair orbits the planet. The third one is 60 degrees ahead or behind the pair, in a stable Lagrange point. The handwaving required is because Lagrangian points are not stable if the relative masses of the moons are similar, and they absolutely require that the moons be singular and stable, not a binary. Still, this will only outrage astrophysics nerds, so be happy.

the pair is located in the northwestern sky and the single is to the southeast

This requires all the moons to be in an inclined orbit, relative to the planet's equator. No problem with that, in the short(astronomical-scale) timespan.

Finally, to answer your question:

What affect, if any, would this arrangement have on the formation of landmasses and/or oceans on the planet?

As the relative positions of the Moons are fixed, AND their orbit is circular, their gravitational effect on the planet will be the same as a single moon positioned between the three, somewhat closer to the pair than the singleton, massing a bit less than the sum of the moons. (because the are not perfectly aligned)

You have given them very Earth-Moon type distances and masses, so the end result will be a tide that is 2.5-3 times as great as Earth tides. The tide height would also vary immensely by Latitude, as the inclined orbit will sometimes cause tides to be greater when "under" the moons, and less when in the opposite hemisphere, even when the moons are at their highest.

On the astronomical medium to long term your planet has problems. The inclined orbit will cause the precession of your planet's rotation to be hugely amplified. Expect the rotational north pole to drift around a lot, causing seasons to be ...interesting.

Also, expect that third moon to eventually drift out of its Lagrange position, start orbiting the planet by itself, and disturbing the twins. After the first close encounter, you now have three rogue moons rampaging around the planet until they run into something, most likely themselves. Big fireworks, meteor swarms and eventual planetery ring will result.


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