The Earth has one moon at about 1/80 of Earth's mass. Is it possible to have two moons large enough each to subtend a >30 minute disk as viewed from the surface?

I have tried with various simulators and systems fail spectacularly within a few dozen orbits, either with one colliding with the planet, or being thrown out of the system.

This question was prompted my many SF novels with covers showing several large moons in the sky.

Preliminary thoughts:

One possibility is to have the two moons co-orbiting each other and the pair orbiting the planet. As long as they are outside Roche's limit this is fairly reasonable. But it is boring.

If we use Luna as the first moon (our present moon) then if we locate Selene (the new one) at half the distance it has to be only half the diameter and 1/8 the mass. This would make it 1/640th the mass of earth. (Because tides go up as inverse cube of the distance, but direct as mass, little Selene would cause about the same size tides as Luna)

Is it easier to make a long term stable system with large differences in mass?

Are there resonances that increase stability (negative feedback loop) rather than scramble the eggs?

I've asked this question on Physics Forums, and gotten no good reply.


1 Answer 1


If you make the moons the same size they can be placed simultaineously in each other's $L_4$ or $L_5$ Lagrange points, which should be stable. https://en.wikipedia.org/wiki/Lagrangian_point

  • $\begingroup$ You get a point. I should have thought of that. But the solution is even more boring than the co-orbiting one. $\endgroup$ Oct 15, 2018 at 2:39
  • 1
    $\begingroup$ Lol, if it is excitement you want, let the moons collide. $\endgroup$
    – JMLCarter
    Oct 15, 2018 at 2:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.