I'm writing a story set in a world with 3 moons and I have a couple of questions about how that would work. I understand that the tidal patterns would be significantly different than Earths and I'm writing that in as part of the story (wild oceans, unpredictable shores/ports).

My question is what would be a logical setup for 3 moons? What I mean is, would one moon be the size of ours (which I understand is unique relative to moons on other planets) the second 1/5th, the third 1/10th ect? What would be a logical size differences between them to be functional (by functional, I mean functional to a "normal" sort of life on the planet)? I'm asking because I'd like to describe how they look at night in certain parts of my story.

Also, how much time would pass between when the 3 moons established an orbit and to when one crashed into another? Millions of years? Billions? I'm curious as to this because I'd like to know if there is enough time since they all 3 arrived to have developed life and if there is enough time left over till the crash and cause mass extinction on the habitable planet.

  • 1
    $\begingroup$ 1. I suggest you search existing questions here first. IMHO, this recent question covered a fair bit of what you'll want to know: worldbuilding.stackexchange.com/questions/66290/… $\endgroup$
    – Catalyst
    Jan 5, 2017 at 21:08
  • $\begingroup$ What's the size of base planet? Why don't you take, say, Jupiter, and just remove one of galilean moons (and ignore small ones)? $\endgroup$
    – Mołot
    Jan 5, 2017 at 21:08
  • $\begingroup$ Roughly the size of Earth but with much more water, less land space. I'll take a look at the Jupiter moons however, I just don't know enough about the moons around other planets to know exactly where to research so thank you for that direction! $\endgroup$
    – Benjamin
    Jan 5, 2017 at 21:14
  • $\begingroup$ Actually, from the limited sample we have, multiple moons is the norm. Earth is the only planet with just one. Venus & Mercury have none, all the rest have more. Note that if they're of any great size, they're likely to be locked in orbital resonance, like Jupiter's inner 3 Galilean moons: en.wikipedia.org/wiki/Orbital_resonance $\endgroup$
    – jamesqf
    Jan 5, 2017 at 22:22
  • $\begingroup$ On tides, see this question. You can model that. $\endgroup$ Jan 6, 2017 at 4:30

1 Answer 1


Planets and moons typically form at roughly the same time when a solar system is forming. In the early stages of formation, life is impossible. Every time bodies collide, they turn molten again, destroying any life on them. It takes millions of years for the worlds to stop being molten, etc. By that time, all of the moons around a planet are in stable orbits. They are not going to crash into each other. Or anything else.

With three moons, each is at a different distance from the planet. They are all in the same plane. Their sizes can be invented. Each formed from whatever material was around it at the time. Our moon is very large, so don't invent one much bigger, not to mention three big moons.

Sometimes a planet "captures" a celestial body that has formed but not been captured yet. This captured moon could strike an existing one before settling into an orbit that prevents this. Note that once a moon is orbiting, it's unlikely to hit another moon except in the earliest part of lifetime as a new moon before its orbit has stabilized.

Distance from the planet is the only thing that controls orbital speed. Nearer = faster, so the three moons are orbiting at different speeds. The nearest moon has the most impact on tides due to proximity. The effect of other moons will be less. When all three moons are roughly in the same area of the sky (called a conjunction), this will produce stronger tides.

  • $\begingroup$ I had read a few other multiple moon posts and articles and they almost always suggested that at some point one of the moons would crash into another. Perhaps they meant early on as they established their orbits. What you said makes a lot of sense , thank you for that clarification. That helps me a great deal. $\endgroup$
    – Benjamin
    Jan 5, 2017 at 21:59
  • $\begingroup$ @Benjamin Jupiter has 67 moons, 3 of which are larger than Earth's moon and one that is only slightly smaller. On the other hand, there's a lot more space around Jupiter for the moons to orbit without colliding with one another. $\endgroup$
    – user45623
    Jan 5, 2017 at 23:16
  • $\begingroup$ The nearest moon has the most effect on tides if it is also the largest. If it's smaller and a larger one is farther out, it becomes more complicated. $\endgroup$ Jan 6, 2017 at 4:29
  • $\begingroup$ By largest, we must be specific and say largest mass. The diameter has no bearing tides. $\endgroup$
    – Geminirand
    Jan 6, 2017 at 12:19

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