I'm working on a story that takes place on a world that has 3 moons. At least 2 of the moons are the same size. They align extremely rarely. Otherwise, it's an earth-like planet.

There is one sun and possibly other planets nearby. Imagine if Earth had 3 moons that were all relatively the same size, perhaps in orbits that were so similar it would take them centuries to align. Assuming they would likely be tidally locked like Earth, would they effect each other, or gravity on the planet perhaps?

Similar, specifically about tides: How would having multiple moons affect tides?.

  • $\begingroup$ Welcome to Worldbuilding.SE. I did a heavy edit on your question because it was really about a half dozen questions. I changed things to focus it as much as possible. Otherwise people will vote to close it as too broad and unclear. If you're unhappy with my edit, please go ahead and change it. $\endgroup$
    – Cyn
    Jan 15, 2019 at 5:09
  • $\begingroup$ Fun thing to answer! Sorry if my answer isn't much help most of where I get my information has long since been forgotten. $\endgroup$ Jan 15, 2019 at 7:54
  • $\begingroup$ @dot_Sp0T Your possible duplicate only deals with tides & multiple moons. This question asks about effects on the moons themselves & gravity. Not exactly the same thing. $\endgroup$
    – a4android
    Jan 15, 2019 at 12:37
  • $\begingroup$ @a4android that's why it takes 4 more people to propose the same thing for it to stick; and then all we want is for the OP, in this case TiffanyMoon, to go through the proposed dupe and tell us why it doesn't help them. $\endgroup$
    – dot_Sp0T
    Jan 15, 2019 at 14:25
  • $\begingroup$ I have to agree with @a4android. The other question is a really good one but it focuses on tides. This question does not even mention tides. I would consider this a good followup question to the earlier one. After Tiffany weighs in, this may be a good candidate for reopening. Yes, it takes 5 people to close but it really only takes one to say "this is a dup" and 4 to say "sure, it sounds like it." $\endgroup$
    – Cyn
    Jan 15, 2019 at 15:17

3 Answers 3


Adding extra heavenly bodies to a planet will always have small effects on it. For a story however, it is up to you to choose if these are decorative pieces or if you want to plan out all the possible consequences of actually having 3 moons and the plausibility of such a scenario.

If your moons are the same size and are tidally locked, then they should be roughly the same distance from the Earth. This means that when your moons align, they will instead collide into each other shattering into billions of small fragments and leaving only one moon behind.

  • $\begingroup$ actually that gave me a good idea for it thanks ;) $\endgroup$ Jan 15, 2019 at 5:56
  • $\begingroup$ Well I mean what if one of the moons has a moon and one is recaptured? wouldn't that prevent a collision when they align from occuring? $\endgroup$ Jan 15, 2019 at 7:56
  • $\begingroup$ @SlothsAndMe For some reason I figured the planet cascade-juggliing with three moons, losing one and capturing one endlessly. $\endgroup$
    – kikirex
    Jan 15, 2019 at 12:50

in orbits that were so similar it would take them centuries to align

One of them would be ejected out of the system they made with the planet. This is close to a 1:1 resonance, and this is how protoplanets clear their orbits to become planets. It probably happened to our own Moon and the debris she came from.

would they effect each other

Just so you know: Pluto is smaller than the Moon and it affects Neptune. They never come even as proportionally close among themselves as the Moon and the Earth.

or gravity on the planet perhaps?

The barycenter of the Earth-Moons system would be moving in a more interesting way, but the most concrete effect from this would be adding one more topic which nerds could use to be boring at parties. Tides would be slightly different, a little more variable, but the stronger tides would not be much stronger than in the real world.


So basically I think it might affect tides, since tidal forces between the objects would affect the oceans. If you're concerned with creatures living there the moons would also probably alter how we see cycle functions in living things such as sexual period changes, blooming and such.

Edit: For story purposes I don't think that you would have to worry too much about the moons unless they are a central part of the plot. Also make sure to take anything I say with a grain of salt because I'm not always right about things.


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