I am looking for help on the following:

  1. Will that have any affect on the length of days and nights?

  2. Would having two moons create a higher body of water on the planet due to increased tides? (i.e. the Earth is 71% and now with two moons it's 82% water or something like that)

  3. Would there be any affect on the stable axial tilt?

  4. Would eclipses happen more frequently?

  5. Are there any gravity considerations I should be aware of (I know, probably stupid question among many)?

  6. Is there anything else I should take into consideration with having two moons?

Thanks in advance for any help provided.

Update: Here's a bit more detailed information which I hope will help. Using a planet exactly like Earth, with two moons both the size size as our moon, and in the same orbit so that both are present at night. I'm not sure the exact separation distance, but they would be far enough apart not to be in each others gravitational pull if possible.

  • 1
    $\begingroup$ It would help if you gave a diagram showing what you meant by "adjacent", also, trying playing with <a href="phet.colorado.edu/en/simulation/…> orbital simulator to see if such a gravitational system is stable, if you haven't already. $\endgroup$
    – Luna
    Commented Dec 29, 2016 at 20:56
  • $\begingroup$ @Luna Link is 404. Oh, actually just malformed. Use the [link text](link-url) format for comment links. Like this. $\endgroup$
    – Samuel
    Commented Dec 29, 2016 at 22:16
  • 2
    $\begingroup$ +1 I second more description of these "co-orbital" moons. If it's Lagrange points, yes Earth-Moon L3 exists but wikipedia says only L4 and L5 are stable. And if you put a second massive body there, I don't know if that would still be stable either, see the Theia. And the only other config I could imagine, the two moons orbiting each other, and their barycenter orbiting parent planet, would require a real-life example. Even massive bodies of Jupiter and Saturn's moons don't have that I think. $\endgroup$
    – IT Bear
    Commented Dec 29, 2016 at 22:20
  • $\begingroup$ I don't know what you mean by adject+co-orbital, but this question about tides might help with some of this. $\endgroup$ Commented Dec 30, 2016 at 4:06

2 Answers 2


Assuming that when you're talking about 'two moons adjacent to each other in a co-orbital configuration', you mean that a planet is being orbited by two moons, which are one gravitational unit in a binary configuration, something like a hierarchical trinary star system.

I think it is very unlikely that something like this would ever form inside a solar system — if the moons were formed from Earth like how the real moon was formed via a giant impact – the resulting ring of debris around the primordial Earth would most likely coalesce into one object.

One also ought note that the Moon and the Earth, in our world, are very far away from each other compared to their size (see picture). I will assume that the total mass of the orbiting bodies is identical to the mass of the real moon.

Earth-Moon system

  1. Yes, the effect on days and night times would be pronounced. The reason why Earth has a roughly 24 hour day is because of the pull of the Moon. When the Earth was formed, it had a much faster rotation, on the order of 6 hours. It slowed down over time due to tidal locking with the Moon. Because the Earth's tidal bulge would be dragged by the two orbiting bodies, the Earth's rotation would slow.

  2. The sea level itself is based on the amount of water on the planet. Due to the large distance and the same mass, the tides would have the same strength. If there were two Earth sized moons orbiting, they would be twice as strong, but that doesn't mean that there is more or less water on the planet, as the amount of water is not related to lunar gravity.

  3. The stable axial tilt is caused by the effect of there being a moon which stabilises the axis of rotation. Its stability doesn't change by their being two moons, so long as this binary orbits far way from the planet. Thus, it would still be stable.

  4. Eclipses (total eclipses, at least) only occur because the Moon is at such a distance that its angular size is basically the same as that of the Sun. These moons, if they had the same mass as the real Moon, would be smaller, and therefore, there would not be any eclipses at all, though there would still be transits if it orbited around the planet on the same plane as that of the planet around the Sun.

  5. I'm very doubtful that this would be gravitationally stable. I cannot imagine something forcing a return to equilibrium after leaving it. Actually, looking at hierarchical trinary star systems, this is certainly possible given that the moons were formed far away and orbit close enough such that it can be treated as if it were one gravitational body. Solving a three-body problem like for the exact parameters and required separations is something that I leave for your consideration, as it is certainly beyond my mathematics. A practicable solution would probably start with determinations on whether the planetary system's centre of mass is stable. If there is an unstable centre of mass, then the gravitational system is likely going to be unstable as it tries to find a new equilibrium by ejecting one of the bodies.

  6. Gravity in a ring, outside the Roche limit, will generally coalesce into one object. I believe it is quite unlikely that a binary moon would naturally coalesce from such a ring if the source of that ring is a giant impact.

If you're talking about multiple moons, that certainly is possible. Jupiter and Saturn are shining examples of multiple moons. If you mean that these moons were in the same orbit, but separated at Lagrangian points L4 and L5, such a configuration is not likely to be stable over the extremely long time-scales necessary for the evolution of intelligent life.

  • $\begingroup$ Re "the Moon and the Earth, in our world, are very far away from each other", Pluto and Charon are much, much closer relative to their size, about 4 times Pluto's diameter. $\endgroup$
    – jamesqf
    Commented Dec 30, 2016 at 5:03
  • $\begingroup$ Thank you and yes the moons would be in the same orbit. The planet is exactly like our Earth and the moons would both be the same size as our moon and in the same orbit so both would be present at night but not be in each others gravitational pull if that's possible. $\endgroup$
    – user31604
    Commented Dec 31, 2016 at 3:31
  • $\begingroup$ I don't know what that means. What do you mean 'same orbit'. How would two objects be in the same place at the same time? Consider the moon system. Is it a binary system? Or, are the two moons not gravitationally bound? $\endgroup$
    – ifly6
    Commented Dec 31, 2016 at 3:34

I'm pretty sure that two moons in the same orbit around a shared planet is not at all stable (won't last long.)

If your two moons were exactly opposite each other, in a perfectly circular orbit (around a perfectly spherical planet), you'd have a "stable" system like balancing on top of an igloo -- not truly a long-term stable situation, but just waiting for any bit of cosmic dust to destabilize it. And then moons collide eventually; wham! There's a reason that we haven't spotted such a planet/moon system; it wouldn't last.

On the other hand, you ought to look at Trojan Points: https://en.wikipedia.org/wiki/Trojan_(astronomy)

In a Trojan Points' situation, you would have three moons sharing the same orbit, separated by 60 degrees of angle. I'm pretty sure the middle one has to be the most massive, to maintain stability. Can you live with three moons?

There may be constraints on the masses of the 'side' moons relative to the middle one, but such systems exist and have been studied:

from https://en.wikipedia.org/wiki/Co-orbital_configuration

"The Saturnian system contains two sets of trojan moons. Both Tethys and Dione have two trojan moons, Telesto and Calypso in Tethys's L4 and L5 respectively, and Helene and Polydeuces in Dione's L4 and L5 respectively."

  • $\begingroup$ The issue with Lagrangian points L4 and L5 is that we generally assume that the body occupying that point is effectively massless because it is negligible. Similarly, the idea of a large mass being at point L3 has the same issue. $\endgroup$
    – ifly6
    Commented Dec 31, 2016 at 4:06
  • $\begingroup$ @ifly6: So long as the center moon is the most massive, and the moons (in toto) are low mass compared to the planet, I'm pretty sure it works, to some degree. There may be a maximum mass for what's parked in the Trojan points (relative to the center/largest moon), but I don't know what it is. The main point is that a Lagrange-aware orbital situation might work, whereas two moons seems very unlikely, from a stability perspective. $\endgroup$
    – Catalyst
    Commented Dec 31, 2016 at 4:13
  • $\begingroup$ If you want something orbiting in a planet's (or moon's) L4 or L5, your mass limit is about 1/25 of whatever the mass of the main body is. The Moon is about 1/81 the mass of the Earth, so you could be on to something here. $\endgroup$
    – Palarran
    Commented Dec 31, 2016 at 16:19

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