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I'm writing a series that takes place on an earth-like planet, and I really want there to be two moons, specifically. I want there to be tides, but I don't want them to be so intense that they encompass entire coastal cities. I would prefer them to be in a similar/the same orbital pattern as each other, but as I have literally no understanding of how any of this works, I'm willing to compromise on that for the sake of realism, if possible.

If it helps at all, the visual I'm after is inspired by the ps2 game Dark Cloud, in which there are two moons.

game art featuring the two moons

How can I get this effect? Is it possible? What should I bear in mind with this?

Hypothetically, if the moons were the same size and density (or at least comparable) to our Luna, how would that affect the tides, and is there anything else in particular it could affect? How could they end up in orbit, theoretically, and how long would they have had to be there to become tidally locked (at least one of them)? Would they have to be made of something different to appear as a different color, or would that be more down to atmospheric gases?

I apologize ahead of time, I'm in no way an expert on matters of space, but I am trying! Thank you for your time and help!

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    $\begingroup$ Quick thing to point out: doesn't matter how big tides are, they're not going to encompass cities... for the simple reason that you wouldn't build a city where the tide would affect it. You might have a beach with more depth, due to a larger range of low and high tides... but that's about it. $\endgroup$
    – Kevin
    May 16, 2020 at 20:25
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    $\begingroup$ Here is a really complete write up on moon orbital dynamics, with pictures and cases, that is pretty easy reading. Likely useful to you: quora.com/… $\endgroup$
    – SRM
    May 17, 2020 at 14:36
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    $\begingroup$ Your "follow on questions" need to be asked as separate questions, with a link back to this one as "related to this question" This isn't a forum, so please don't add to the question with new questions. (But the great news is that you get to ask two more well scoped questions. ) The tour, How to Ask and How to Answer helps to explain how SE sites work. $\endgroup$ May 20, 2020 at 14:13

6 Answers 6

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It's probably not possible for two Luna-like moons to exist around an Earthlike planet. There are two major reasons why: formation and orbital dynamics.

There are two main ways that moons form. The first is similar to how planets themselves form. When the solar system was young, the material that would become the planets was essentially a huge disc of gas and dust. These small particles drew together over time, forming small rocks that gradually accreted into distinct bands. As those bands coalesced, they formed larger and larger bodies, eventually becoming planets. Smaller planets "sweep up" the lingering gas and dust that remains near their orbits, but for the largest planets like Jupiter, there's enough remaining material for it to undergo the same process and form new rocky bodies.

The second way moons occur is by capture. Asteroids and other small material that passes near a planet may be caught in its gravity well and drawn into a stable orbit. This may be how Mars's moons came to be; it's also a common origin for small moons of gas giants.

Neither of these explanations works for Earth's moon, though; among other things, it's far too large. The leading hypothesis of Luna's formation is the giant-impact hypothesis, which posits a collision between a smaller Earth and a Mars-like body. This would result in the smaller body and much of the Earth being liquefied, with the resulting magma either falling back onto Earth or remaining in orbit. Over time, the remnants in orbit then coalesced into the moon. (Although the hypothesis itself is well-supported, the exact details of the impact and later stages are the subject of a lot of speculation and modeling.) It's not clear to me whether it's possible for the cloud of debris to permanently become two distinct moons, but it seems unlikely; because of their orbital characteristics, they'd be too close together and would tend to merge together.

If an Earthlike planet did have two large moons, it would be hard-pressed to keep them. Large orbiting bodies will tend to interfere with one another's orbits, which usually results in one of them being pushed too far away from the planet (and escaping from orbit) or too close (and disintegrating). This would also prevent them from being tidally locked to Earth, which requires a relatively stable orbit over a fairly long period of time. A stable orbit involving three bodies similar in size, like Earth and two Luna-like moons, is extremely unlikely; combined with the formation problems, this means it would almost certainly be of artificial origin. Plenty of sci-fi stories have planets and moons shaped into artificial orbits by sufficiently advanced aliens for practical or aesthetic reasons, or just for the heck of it.

In terms of Earth tides, the effects would be difficult to predict; it depends a lot on the specific orbits of the moons. If they have the same orbital period and are in the same part of the sky, their effects on tides will combine and you'll have much more powerful tides to deal with. (Also, they'll probably eventually run into each other.) In the more likely case that their orbital periods and/or inclinations vary, you'll have a partial interference pattern; similar to how spring and neap tides are driven by the Sun's tidal influence being out of sync with the moon's, your moon tides would sometimes line up (creating very high tides followed by very low tides) and sometimes interfere (creating relatively flat tides all day long).

A couple of other options spring to mind. Plenty of stories are set on the large moons of gas giants, and although we haven't found one yet (our existing methods of detecting planets around other stars aren't sensitive enough to pick up their moons), there's no reason why one couldn't exist at a habitable distance from its star. The sky on a moon like Europa would be pretty dramatic, featuring several other moons that would appear roughly the size of Earth's, plus the large disc of the gas giant itself. Tidal effects would be dominated by the gas giant, of course, but you would see a noticeable resonance from the other moons, more similar to spring tides than the huge swings of a smaller planet with multiple moons.

On the other extreme, you could have an Earthlike planet with multiple smaller moons. Mars's two moons are visible from its surface, and though Deimos is rather small and unimpressive Phobos at least is clearly visible and rather dramatic to look at (because of its fast orbit, it goes through a full set of moon phases about once a Martian day). There's no reason why a planet couldn't be host to even more small, captured moons, and though this also wouldn't be stable in the long run, it could provide pretty spectacular viewing for a few thousand years while it lasts. Tides from such small bodies would be minimal; the sun's influence would probably be the most significant.

A final note on color: the color of a stellar body is indeed mainly determined by its composition (or its atmosphere, if it has one). Mars is a famous example; its red coloration comes directly from the iron-oxide-heavy composition of its soil. Io is also well-known for having a yellow coloration based mainly on sulfur. Europa's surface of water ice is similar in color to Luna's gray but is over five times brighter. And so on.

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  • $\begingroup$ "If they have the same orbital period and are in the same part of the sky," -- wouldn't this put them in the same orbit? Their orbital period is determined by the distance from the planet. $\endgroup$
    – Mary
    May 16, 2020 at 14:17
  • $\begingroup$ Can't you have two large moons if they are in a resonance orbit? As to how it could form--what if Earth already had a couple of ordinary moons when Theia hit? Couldn't both moons sweep up debris and grow? And who cares if they tidally lock or not? $\endgroup$ May 16, 2020 at 20:30
  • $\begingroup$ @Mary By "same part of the sky", I mean from the perspective of an observer on the planet - in other words, they're "over" close points on the planet. One alternative would be for the orbits to be along different axes, for instance, so the moons would only be close together at two points each orbit. $\endgroup$
    – Cadence
    May 17, 2020 at 2:34
  • $\begingroup$ @LorenPechtel I was under the impression a resonance orbit would require the planet to be larger, relative to the moons, but I might be wrong about that. There's still the question of where it all comes from though. $\endgroup$
    – Cadence
    May 17, 2020 at 2:38
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Two moons would make the tides more intense, not less. Also more complex.

Here on earth we have "spring tides" and "neap tides" -- these are driven by the sun but there is enough of a difference that classical Greeks noticed them even though the Mediterranean is not strongly tidal. The reason the sun is of lesser effect despite its vastly greater size is that it is farther away.

To introduce another moon, even one much farther away than ours, would make the tides the sum of three celestial bodies. This would produce neap tides much lower, and spring tides much higher, than on our planet.

If you want to decrease tides, you could look at variation in tides on Earth, which is quite substantial, and use the geography, either for the region you want or all over the planet. If that's not feasible, increasing the distance between the planet and the moon would decrease them.

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This is a fascinating question!

As you mentioned, the effect on the tides would be most obvious characteristic, and the only way I can think of reducing them so as not to encompass cities would be to make the second moon smaller, and to move the second moon far away. Newton's law of gravitation says that the gravitational force exerted by a moon on a planet is given by Newton's law of gravitation equation

So, basically the force of gravity is proportional to the size of earth (m1), the size of the moon (m2), and square of the orbit radius (r). If you move the second moon 4 times as far away, you would have a gravitational force 16 times less. So in theory you can still have a beautiful moon of the same size and density in your landscape photography without having it destroy cities by just moving it farther away. You could also reduce its mass (But i believe you want to keep the same size/density).

Also, according to the wikipedia article for the moons of mars, there could be some other curious consequences:

  1. If the moon orbits quite closely, your second moon could look smaller near the planetary poles and bigger near the equator, like a bright star or planet.
  2. You may have more frequent lunar eclipses if the second moon is close enough. It may often block the sun.
  3. Depending on the direction of orbit, a smaller moon could rise, set, and rise again in less than a day (Eg., 11 hours for Phobos)., or a larger moon could take 3 days to set.
  4. The planet also exerts a gravitational pull on the moon, so at some point in the future, a smaller, closer, moon could be broken up by the tidal forces, and the fragments could crash into the planets, causing huge craters that can affect the surface of the planet. This could be a fun apocalyptic prophecy maybe?

Hope that is of some use! Such a great question!

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  • $\begingroup$ Yesss, I actually had something of a prophecy late in the series that involves something of that nature, a catastrophic series of impacts... but I didn't start considering it coming from the second moon until you said that! Hilariously enough, my inspiration from which I got the two moon theme actually does something similar in their second game, too. $\endgroup$ May 17, 2020 at 1:50
  • $\begingroup$ Tidal forces are more complicated than the force of gravity, and are proportional to the inverse cube of distance, not the inverse square. Which of course makes moving a moon further away even more effective at reducing the tidal force. $\endgroup$
    – Mike Scott
    Apr 28, 2023 at 4:59
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Cadence's answer is very good, but it might leave you with the the sense that you can't get away with anything interesting. I think there are definitely some good possibilities.

You want to imagine two moons with approximately circular orbits that have significantly different radii. If the radii are too similar, the system tends to be unstable. One way to have a stable system is for the moons to be in resonance. Do a search for "orbital resonance", and also read about the Galilean moons of Jupiter. A possible scenario is one where the inner moon goes around twice for every time the outer moon goes around once.

Edited to add that I also like M. A. Golding's answer, and the others aren't bad either. But I definitely still think you should consider having them be in resonant orbits.

Also I will add that you might have fun reading about theories of how Uranus and Neptune got their moons. Captures were quite common out there. One thing to realize about capturing a moon is that, without friction, it's hard for gravity to really "capture" something. Things speed up as they get closer, and slow down as they get farther away, so in the end they are moving just as fast but heading away from the planet. The way captures do happen is, either, (a) there is still a thick dust cloud that slows the body down, or (b) there is a third body in the system that maybe gets kicked into a higher orbit, slowing down the new body enough to capture it.

But this is all just background. I think you have some flexibility.

Cadence is right that it is hard to imagine how we could end up with two Luna-sized moons. But what you really want is for them to be big in the sky and affect the tides. A second moon that is significantly smaller and significantly closer could still look nice and big in the sky. In my judgment, not enough earthlike planets have been observed to rule out unexpected moon-acquisition scenarios. Really, the inner solar system planets are the only examples we have where we know what moons are present. You can be more confident that the orbits can't be too close together, or the moon much farther away than ours, because the orbital dynamics make it unstable.

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You can get rid of the tidal problem by making the two moon co-orbit, the second moon in a Trojan position.

Using Earth and Moon as examples of our planet and moon, the second moon needs to be around one hundredth of the Moon's mass. It can be less dense, in order to be safely larger and therefore more visible in spite of its smaller radius. To the same purpose, we can posit it having a higher albedo.

I think we can go with a S-type asteroid similar in size and density to 2 Pallas. Cross section is one tenth of the Moon, but albedo can be 3+ times higher (i.e. brighter), so in the end we get about one third as much reflected light.

Tides will actually be slightly less than Earth's. The second moon can barely get away with not having a perfectly spherical shape.

One drawback of this configuration is that there will never be a conjunction - the two moons will always keep the same relative positions in the sky, about 60° from each other.

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Have you see pictures or videos where Earth's Moon looks huge?

I believe that the angular diameter of the Moon, about half a degree of arc, is the same as a dime held at arm's length, which is pretty small. But the Moon looks vast in many pictures and videos because telephoto lenses are used to film it.

If you desire that both moons look like discs (when full) or partial discs (in other phases), both moons will have to be large enough to be (roughly) spherical.

Solar System objects more massive than 10 to the 21st power kilograms (one yottagram [Yg]) are known or expected to be approximately spherical. Astronomical bodies relax into rounded shapes (ellipsoids), achieving hydrostatic equilibrium, when their own gravity is sufficient to overcome the structural strength of their material. It was believed that the cutoff for round objects is somewhere between 100 km and 200 km in radius if they have a large amount of ice in their makeup;1 however, later studies revealed that icy satellites as large as Iapetus (1,470 kilometers in diameter) are not in hydrostatic equilibrium at this time,2 and a 2019 assessment suggests that many TNOs in the size range of 400-1000 kilometers may not even be fully solid bodies, much less gravitationally rounded.3 Objects that are ellipsoids due to their own gravity are here generally referred to as being "round", whether or not they are actually in equilibrium today, while objects that are clearly not ellipsoidal are referred to as being "irregular".

https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size1

It was once expected that any icy body larger than approximately 200 km in radius was likely to be in hydrostatic equilibrium (HE).4 However, Ceres (r = 470 km) is the smallest body for which detailed measurements are consistent with hydrostatic equilibrium,8 whereas Iapetus (r = 735 km) is the largest icy body that has been found to not be in hydrostatic equilibrium.[9] Earth's moon (r = 1,737 km) is also not in hydrostatic equilibrium, but—unlike icy Ceres and Iapetus—it is composed primarily of silicate rock, which has a much higher tensile strength than ice.

https://en.wikipedia.org/wiki/List_of_Solar_System_objects_by_size#Larger_than_400_km3

Vesta, which has a radius of 262.7 kilometers (166.3 miles), is the largest solar system object that looks irregular in a good photograph, although some of the ones larger than Vesta appear just as dots of light in the photograph.

More or less arbitrarily assuming that a radius of 400 kilometers is necessary for a rocky body to appear round, one can calculate how close such a small moon would have to be to appear as a disc and not a dot of light, in the sky of your world.

The maximum angular resolution of the human eye is 28 arc seconds or 0.47 arc minutes,[18] this gives an angular resolution of 0.008 degrees, and at a distance of 1 km corresponds to 136 mm. This is equal to 0.94 arc minutes per line pair (one white and one black line), or 0.016 degrees. For a pixel pair (one white and one black pixel) this gives a pixel density of 128 pixels per degree (PPD).

https://en.wikipedia.org/wiki/Visual_acuity#Physiology5

So an object with a radius of 400 kilometers and a diameter of 800 kilometers would appear to be a tiny disc instead of a point of light if it had an angular diameter of at least 0.008 degrees of arc.

According to my rough calculations, that means that a minimum size round moon, with a radius or 400 kilometers (248.5 miles) and a diameter of 800 kilometers (497 miles), would have to be less than roughly 5,729,582.7 kilometers, or 3,560,197.6 miles, distant to be seen as a tiny disc and not as a mere point of light.

The Roche limit of an an astronomical body is the distance at which it will cause a smaller astronomical body to break up. For Earth the Roche limit is 9,492 kilometers (5,898 miles).

https://en.wikipedia.org/wiki/Roche_limit#Selected_examples6

The Hill sphere of a planet, calculated from the masses of the planet and its star, and the distance between them, is the minimum distance that moon of the planet would have to be closer than in order stay in orbit around that planet.

The Hill sphere is only an approximation, and other forces (such as radiation pressure or the Yarkovsky effect) can eventually perturb an object out of the sphere. This third object should also be of small enough mass that it introduces no additional complications through its own gravity. Detailed numerical calculations show that orbits at or just within the Hill sphere are not stable in the long term; it appears that stable satellite orbits exist only inside 1/2 to 1/3 of the Hill radius. The region of stability for retrograde orbits at a large distance from the primary is larger than the region for prograde orbits at a large distance from the primary. This was thought to explain the preponderance of retrograde moons around Jupiter; however, Saturn has a more even mix of retrograde/prograde moons so the reasons are more complicated.3

https://en.wikipedia.org/wiki/Hill_sphere#True_region_of_stability7

In the Earth-Sun example, the Earth (5.97×1024 kg) orbits the Sun (1.99×1030 kg) at a distance of 149.6 million km, or one astronomical unit (AU). The Hill sphere for Earth thus extends out to about 1.5 million km (0.01 AU). The Moon's orbit, at a distance of 0.384 million km from Earth, is comfortably within the gravitational sphere of influence of Earth and it is therefore not at risk of being pulled into an independent orbit around the Sun. All stable satellites of the Earth (those within the Earth's Hill sphere) must have an orbital period shorter than seven months.

https://en.wikipedia.org/wiki/Hill_sphere#Formula_and_examples4

So Earth's Hill sphere extends to about 1,500,000 kilometers (932,056.7 miles), and its true region of stability extends to about 500,000 to 750,000 kilometers, (310,685.5 to 466,028.3 miles).

The semi-major axis of the orbit of the Moon is 384,399 kilometers, or 238,854.4 miles.

So this means that if a habitable planet has similar mass and distance from its star (a star that should have a mass similar to that of the Sun) any moons which it has which are large enough to be round will be close enough to the planet to always appear round (except or their phases), and never appear as mere dots in the sky.

The minimum size of a rocky moon large enough to be round, with a radius or 400 kilometers (248.5 miles) and a diameter of 800 kilometers (497 miles), would be about 0.230229 of the diameter of the Moon, and thus about 0.0124228 of the volume of the Moon. If that moon had the same average density as the Moon, it would have about 0.0124228 of the mass of the Moon.

The gravitational pull of astronomical bodies one each other are proportional to their masses and distances. So if a smallest possible round moon was at the distance of Earth's moon, it would have only 0.0124228 as much gravitational attraction on Earth as the Moon does. According to my rough calculations, if a moon with a mass of 0.0124228 that of the Moon was at a distance of 0.1114576 of the Moon's distance, it would have a gravitational attraction on Earth equal to that of the Moon. That distance would be about 42,844.189 kilometers, or about 26,623.144 miles.

At that distance the minimum size round moon should appear to be roughly one arc degree wide, about twice the angular diameter of the Moon.

Thus my rough calculations indicate that it should be possible for a moon smaller than Earth's moon to be close enough to an Earth like planet to appear as large or larger than the Moon does from Earth without raising any higher tides.

Of course an astronomical situation which could possibly exist, is not the same thing as an astronomical situation which could form naturally, and an astronomical situation which could form naturally is not necessarily the same thing as an astronomical situation which could exist for the billions of years necessary until a planet developed an atmosphere with a lot of free oxygen and became habitable for beings with requirements similar to those of humans.

The Moon is believed to have formed out of debris much closer to Earth than it is now, and to have raised large tides on ancient Earth, and gradually slowed down the rotation of Earth and moved farther and farther from Earth.

The interactions between the two moons you desire, the planet, and its star could possibly eject one of the moons out of orbit around the planet long before the planet develops advanced multi celled life forms.

One possibility you might want to consider is making your "planet" a giant habitable moon of a giant planet the size of Saturn or Jupiter. The larger "moon" in the sky could be the giant planet, and the smaller "moon" could be another moon of the giant planet.

Rene Heller and jorge I. Zuluaga in "Magnetic Shielding of exomoons Beyond the Circumplanetary Edge" The Astrophysical Journal Letters, Volume 776, Issue 2, article id. L33, 6 pp. (2013) calculated the distances from a giant planet that a giant moon could potentially be habitable in. According to their calculations, the exomoon would have to be between 5 and 20 planetary radii to be shielded from radiation. https://iopscience.iop.org/article/10.1088/2041-8205/776/2/L338

At a distance of 5 to 20 planetary radii, the planet would appear to be about 5.7295 to 22.9183 degrees wide, about 11 to 45 times the angular diameter of the Moon.

Any other moons of the giant planet that were large enough to be round would appear as discs and not dots whenever they were closer than roughly 5,729,582.7 kilometers, or 3,560,197.6 miles. Larger rounded moons would appear as discs at even larger distances.

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  • $\begingroup$ Oh wow this is a lot of awesome stuff to consider! Particularly about the planet the story takes place on being a moon of larger planet. I intensely enjoy this idea, and it would actually make other things make sense later in the series! I'll have to think more on all of this and maybe try to build it in universe sandbox. Thank you! $\endgroup$ May 17, 2020 at 1:58

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