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 '14 at 19:17
  • $\begingroup$ Is your planet tide locked with one of the satellite? $\endgroup$ – Vincent Oct 15 '14 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 '14 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 '14 at 19:36
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    $\begingroup$ This question will get you started on the tides at least: worldbuilding.stackexchange.com/questions/71/… $\endgroup$ – Liath Oct 16 '14 at 7:04

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.

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  • $\begingroup$ 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 '14 at 15:46

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