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I want two moons in my novel's new world, but hope to have minimal changes in tides, etc. Would it be possible to have a very small moon that doesn't wreak havoc on nature? Also, is it possible for a small moon to have an orbit that only brings it around a planet intermittently, e.g. once every few months?

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  • $\begingroup$ Do you think that the Moon "wreak(s) havoc on (Earth's) nature"? $\endgroup$ – RonJohn Apr 19 '18 at 22:11
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    $\begingroup$ "intermittently" means "at irregular intervals; not continuously or steadily", so... no. But, just like the outer planets in the solar system, when you push out the orbital radius, the time it takes to revolve around the planet increases. So, have the second moon orbit farther away from the planet. $\endgroup$ – RonJohn Apr 19 '18 at 22:15
  • $\begingroup$ @RonJohn, I think the moon wreaks havoc... nothing's worse than the tide coming in when I'm asleep on a beach :-p. $\endgroup$ – JBH Apr 19 '18 at 23:10
  • $\begingroup$ Welcome to worldbuilding.SE, RL! We appreciate the question you've brought us and, as you already see, there's people on this site who know a fair bit about orbital mechanics. If you haven't already, please take our tour and review our help center pages to learn more about us. Also, you may want to read about Mars' two moons. They'll give you some practical insight into double-moon systems. Cheers! $\endgroup$ – JBH Apr 19 '18 at 23:12
  • $\begingroup$ It looks like I didn't ask part of my question quite right, I meant "Would it be possible to have a very small second moon that doesn't wreak havoc on nature." I've heard that a second moon would cause many changes -- but I love the responses! $\endgroup$ – RL George Apr 20 '18 at 0:28
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Yes. You could absolutely have two moons and not wreak havoc on tides. The force of gravity that moon exerts on the surface of the planet, pulling the oceans, is the same force of gravity that holds the moon in orbit. If the moons were further out, or if one moon was much further out, the effect on the tides could be negligible. I would recommend a much less dense outer moon, that has a large volume of lighter material, say frozen water instead of rock, that would still reflect a lot of moonlight, and even a comet trail. That's it! Make it a large comet that caught in the orbit of a planet. That's highly improbable, but possible, and the visual effect would be stunning.

The Force of Gravity equation is as follows:

     F = (G•m1•m2)/D²

where:

F is the force of gravity
G is the Universal Gravitational Constant is 6.67384E-11 m³/kg•sec².  
m1 is the mass of the first object
m2 is the mass of the second object
D is the distance between them.
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    $\begingroup$ Tides are caused by tidal forces (go figure). You need to consider a lot more than the equation for the force of gravity to determine the effects. $\endgroup$ – Samuel Apr 19 '18 at 23:07
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    $\begingroup$ The comet idea is brilliant! $\endgroup$ – RL George Apr 20 '18 at 0:38
  • $\begingroup$ I am not sure an icy body would last too much along a life hosting planet $\endgroup$ – L.Dutch - Reinstate Monica Apr 20 '18 at 2:53
  • $\begingroup$ In fact the Earth quite often has multiple moons, and the moon we generally refer to is moving further away, albeit at a slight rate. When a lot closer, the tides would have been quite different. $\endgroup$ – mckenzm Dec 12 '18 at 3:51
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Yes, both of these scenarios are quite plausible.

Our moon influences the tides so much because it is very large compared to other natural satellites in our solar system and is fairly close to Earth.

Check out the table on this wikipedia page : https://en.wikipedia.org/wiki/List_of_natural_satellites

Sort them by 'mean radius' and you can see that our moon is the 5th largest in the solar system. It is also closer to Earth (smaller Semi-major axis) than any of the larger ones are to their planet.

There are moons that are much closer to their host planet than our moon is to us, but most of them are significantly smaller.

The total gravitational effect they have on the planet is therefore much less than our moon's, which means way less tidal forces.

In order for the new moon to only come around every few months, it would need a highly eccentric orbit. https://en.wikipedia.org/wiki/Orbital_eccentricity .

Check out the list of the objects on that page, note that Halley's Comet has an eccentricity of ~0.96 compared to the moon's ~0.05.

Check out this page to see that visualized: https://en.wikipedia.org/wiki/Halley%27s_Comet

So your new moon would have to be quite small, probably only a few 100 kms across at max, and would spend most of its orbit far away from the planet. Then it would come screaming in, pass close by to the planet quickly, and get flung back out again.

As a result, your new moon probably won't be super visible, and it probably won't have a stable long term orbit. It would probably more closely resemble a captured asteroid than an actual moon.

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    $\begingroup$ Truly excellent feedback! $\endgroup$ – RL George Apr 20 '18 at 0:39

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