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[Bulldog Psion]

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[The Grue]

While my knowledge of astronomy is "amateurish" at best, and most of my experience with orbital mechanics comes from Kerbal Space Program, nothing you've described sounds entirely unreasonable to me.

I know that at once point all of the inner planets did have rings, but they were unstable and eventually decayed. My understanding is that causes ring systems arise from a close orbiting satellite that gets tugged apart by tidal forces, which is why the gas giants have more stable ring systems that are visible today. Larger bodies are better able to capture objects into orbits close enough to shred them through tidal forces.

More on point, I don't think it's unreasonable to have a terrestrial Earthlike planet with a ring system that, while unstable and decaying, remains present and visible for a few millennia or so.

[Togath]

Also, as a note, in the modern era, Earth actually has a sphere of satellites, old scrap from ships and stations, and likely the occasional tiny meteor surrounding it. As for moons.. while they might be visible more often, there's nothing preventing a planet from having more than one from my knowledge tides might be a little funky, but not enough to prevent life, if I understand the mechanics of them.

[The Grue]

On the subject of moons, Earth's moon is abnormally large as moons go - so much so that the ESA has referred to the Earth-Moon system as a binary planet (http://en.wikipedia.org/wiki/Double_planet) - which means the tidal effects we see on Earth are larger and more visible than they might be on watery planet with a more average-size satellite.

For a planet with multiple satellites, tidal effects from each would fall off rapidly with orbital radius thanks to the inverse square law. They'd be measurable with sensitive enough instruments, but the third moon out wouldn't have very significant effects on the world's oceans - or at least, they wouldn't be noticeable in comparison with tidal effects from the closest one.

[Mando Knight]

Most of my experience is in two-body orbital mechanics, not astronomy, but the former was derived from the latter, so here's my thoughts:

An Earth-like planet could definitely have a planetary ring (we basically have a fairly thin ring of man-made satellites), though I can't say as to how stable or dense it'd be, other than that Saturn's rings are more-or-less unique in their thickness insofar as we know.

The problem with multiple moons and thick planetary rings is that the orbits become very complex when you include the masses of more than two bodies. The orbital resonances you brought up seem to be key to the stability of the orbits of multiple moons, though I doubt that any planet could have a stable system with multiple moons as relatively large as ours (which is actually the 5th largest moon in the solar system, 3 of the larger ones being Galilean moons of Jupiter), but fortunately round moons can be much smaller, such as Mimas (Saturn's "Death Star" moon).

As to phases, yes, the moons would definitely have phases, which is based on the angle between the planet, the moon, and the star, unless the moon orbited perpendicular to the planet's radial vector (which would be both weird and unstable). They might even have a "dark side" like our moon, since all large moons that we've found are tidally locked to their planet (which means that their orbital period and their rotational period match thanks to the planet's gravity, so the same face is always turned planet-side). Depending on the sizes and positions, there would be eclipses, but thanks to how unstable (and analytically unsolvable) 4-body problems are, I can't tell you the frequency or totality of any of them.

Tides would be fairly complex, with the alignment of the moons strengthening or weakening the tides. The one with the largest gravitational pull (directly proportional to mass, inversely proportional to the square of the orbital radius) would be the main driving force behind the main ebb and flow of the tides, with the others creating sub-tides that would either add to or subtract from the main tide. It's possible that if the two minor tides could together reverse the overall tide with some regularity, depending on specifics (I don't know the specifics, but thanks to the orbital resonances, it'd be regular).

[factotum]

One question I'm pretty sure has a simple answer: "would there ever be moonless nights?". Yes. If the moons are orbiting in 1:2:4 resonance then every 4th orbit of the inner moon would have all three moons on the same side of the planet, thus the other side would be moonless. The tidal effects would presumably be amplified in that situation, but how severe that would be depends on how big the moons are.

(Note that even without moons there would still be tides, because the planet's star would provide them--in the Earth-Moon-Sun system, the Sun's tidal influence is about 44% of what the Moon provides. This is what causes spring tides--they're when the Sun, Moon and Earth all line up and thus the Sun and Moon's tidal forces reinforce each other; when the Sun and Moon are at 90 degrees to each other the tides are at their lowest).

[The Grue]

*180 degrees :smallwink:

[ChristianSt]

I'm not really that far into astrophysics, but I personally see no problems.

The moons should be no problem at all. They should still have moon phases, though probably at certain times in the "month" they overlap each other, so it might not easy to figure out the exact phase at any given time. It just massively depends on their size and their orbit.

To tides: yes they get more complex, but I think basically can work like they work today. It depends on how masses are distributed. Wikipedia says that for Saturn most mass is from TItan: It has 90% of all mass in the orbit around Saturn. Basically from this I would think that the Tidal forces should work basically the same on Saturn as on Earth (maybe the other moons add a bit "noise", but probably not that much).


To the rings: On them I'm not sure. It depends on what you want your ring to be and how tilted it is compared to the sun. I couldn't find how tilted Saturn's ring is, so I can't say what should be "normal" (though most planets/things tend to orbit in the same plane in our solar system). Depending on how thick/solid/tilted your ring it it throws no shadow at all on the planet. Unless you want a really solid and wide ring I would assume no shadow.

[factotum]

*180 degrees :smallwink:

No, 90 degrees. If they're 180 degrees then they're in line and you get a spring tide--it doesn't matter that they're on opposite sides of the Earth because that's not how tides work.

[Kato]

There kind of is an important part you (OP) didn't mention: The desired dimensions. As Grue mentioned, our moon is huge, compared to Earth. If you want three of those, or three who look similar viewed from the planet things get difficult, I guess. (I'd assume you could get some projection of the behavior with not-too-complicated calculations but it's nothing I can do off the top of my head)
If it's just some arbitrary but visible spheres in the sky it's not really an issue.

I don't think there's a necessity for them to have a 1:2:4 symmetry.. but iirc all of Kepler's laws apply to satellites just the same as to planets (citation?) so the orbit times and distances are tied together. I'm not sure on it without doing some math but I guess you can pick your orbits pretty arbitrarily as long as there is some (in astronomical terms) distance between the moons. Also, while strongly inclined orbits are uncommon, you could include them if you want to. (Okay, it would be pretty unlikely in terms of "how did this happen" but in theory you should even be able to have all three perpendicular to each other... but having them in (roughly) a plane is much more likely)

Rings... depend on their size, again. A gaint, small ring won't influence the planet, much. Large, clearly visible rings would definitely cause the area where the shadow falls to be less habitable/colder. How the shadow moves, again, depends on inclination of the axis, size of rings, etc (reminder: Saturn's different rings are separated due to resonance with the moons. The size of the moons influences the size of the gaps, iirc)


Long story short: A lot of things you can imagine could work, few things might not, some things are unlikely but that's what a story is for, I guess, if you want to include them.

[Poldon]

RE: Moons.
As others have said, three moons should not pose a problem to the biosphere. The relative size of the moons would determine exactly how visible the moon phases would be (e.g., a small moon further away from the planet would have phases, but it might be very hard to see details other than a change in apparent brightness).
Tides would potentially be more complex. The main idea to remember with tides is that the moons alignment on either side of the planet will affect the magnitude of the tidal flows (e.g., two moons on opposite sides of the planet would still cause a higher tide tan just one moon in alignment). Rough diagram in spoiler box:
http://i98.photobucket.com/albums/l262/Poldon1/tides_zps567a2563.png (http://s98.photobucket.com/user/Poldon1/media/tides_zps567a2563.png.html)
Other notes: our large moon has probably helped to stabilize our axial tilt, helping to moderate seasons. And in addition to affecting the ocean, tidal forces also affect molten rock beneath the surface. Enough tidal force would create earthquakes/volcanoes.

RE: Rings.
Rings, assuming they are like Saturn's in density, will create a shadow. Where this shadow falls depends on the planet's axial tilt. If you want seasons, and therefore have a substantial axial tilt (Earth's is ~23 degrees), they would move based upon the time of year. Without an axial tilt, they would fall roughly on the equator.
Would this have a effect on the biosphere? Probably, but if you have shadows that move, the affect would be minimal. Maybe just a slightly slower growth of planets, and a few degrees of temperature change as the shadow passes. Think of it like a partial solar eclipse that lasts a while.
Other notes: rings are not as dense as moons! They would not block all of the sun's light, but rather dim it. Changing the density of your rings will change will magnitude of their effects. Also, unless you have a shepherding moon, your planet will likely be younger, as the rings will gradually condense into another moon. Finally, the material the rings is made of will also have an effect. Ice would create rainbow-like effects in the sunlight, both in the rings' shadow and possibly when looking at the rings from other locations. Rocks and dust, on the other hand, would probably be a bit duller.

[erikun]

You might want to consider how the moons appear from the planet. "Magic" might be the easiest explanation, as it means that all three moons could orbit at the same length and appear the same size without crashing into each other. In order for all three moons to orbit at 1:2:4 and appear the same size, they would need to be increasingly large in comparison to each other. The furthest moon would be four times the size of the closest moon, or the same size as the planet! (Using standard Earth-Luna comparisons)

By contrast, if the three moons were all the same size, or if only the closest one were as large as Earth's moon, then there would be a distinct difference in appearance between the three. The second moon would appear tiny in comparison to the closest, and the furthest away would probably look like an exceptionally bring lantern when compared to the closest moon. (You could view it clearly with a telescope, of course, but you can view a lantern clearly with a telescope as well.) Note that only the closest moon would have an eclipse as we see on Earth. The second moon would probably produce a visible disk, but not darken the daylight beyond some dimming similar to light clouds. The furthest moon might not even have a recognizable difference during its eclipse.

[factotum]

The furthest moon would be four times the size of the closest moon, or the same size as the planet! (Using standard Earth-Luna comparisons)


That's assuming the closest moon is the Luna equivalent, but why does that have to be the case? If the *furthest* moon is the size of our Moon then the other ones will be closer and smaller (although no idea if that would be a remotely stable system--may have to fire up Universe Sandbox and try it out).

[erikun]

That's assuming the closest moon is the Luna equivalent, but why does that have to be the case? If the *furthest* moon is the size of our Moon then the other ones will be closer and smaller (although no idea if that would be a remotely stable system--may have to fire up Universe Sandbox and try it out).
It's a good point because that's what most people will imagine and assume in that situation. When designing a RPG or a fantasy world for a story, people will probably assume "the moon" and "an eclipse" will behave in the same manner as on Earth. Your suggestion of the furthest moon being the same size as Luna, and presumably that the closest would be 1/4th the size, means that an eclipse would hardly be noticeable.

Also, I doubt that having the largest moon the furthest would be very stable. I have no calculations to back it up, but given that with other planets the larger moons are the closer ones, I think that it would potentially cause instability.

[Mando Knight]

Also, I doubt that having the largest moon the furthest would be very stable. I have no calculations to back it up, but given that with other planets the larger moons are the closer ones, I think that it would potentially cause instability.

Actually, of the Galilean moons, Callisto and Ganymede have the furthest orbits, and the five "inner" moons sit inside Io's (the closest Galilean moon) orbit. (Not that the non-Galilean moons contribute any relatively significant force to the system... the sum of the masses of all Jupiter's smaller natural satellites is much less than 1% of Europa's mass)

Of Saturn's moons, Titan is much larger and further out than Rhea, and Rhea is larger and further out than Mimas. Titan is 96% of the mass orbiting Saturn.

[Douglas]

It's a good point because that's what most people will imagine and assume in that situation. When designing a RPG or a fantasy world for a story, people will probably assume "the moon" and "an eclipse" will behave in the same manner as on Earth. Your suggestion of the furthest moon being the same size as Luna, and presumably that the closest would be 1/4th the size, means that an eclipse would hardly be noticeable.
I think he meant the furthest moon being the same size and distance as Luna, and the other two being closer.

[factotum]

I think he meant the furthest moon being the same size and distance as Luna, and the other two being closer.

Yes, that's exactly what I meant. Had a quick play around last night and was able to get the asteroid Ceres into a reasonably stable orbit inside the Moon (yes, its orbit wobbled slightly, but within fairly restrictive limits)--didn't get as far as trying it with a third moon as well, though.

[Radar]

Most important points about stability and all other details were already said, so I'll just drop in a nice and simple way of estimating the tidal forces from moons: assuming the same density for all of them, the tidal effect from a moon or any other astronomical object relies mostly on the angular size of said object as seen from the planet. How does it work?
Tidal forces are proportional to the mass of the moon and inversly proportional to the third power of the distance between moon and the planet. The mass is proportional to the density and the third power of the radius of the moon in question. Thus the tidal forces are proportional to the density of the moon and the third power of the radius/distance proportion. Said proportion is simply the tangent of half the angular size of the moon as seen from the planet (give or take the planet's radius).

If you have three moons, that all look the same from the planet, then they all equally affect tides.

[factotum]

If you have three moons, that all look the same from the planet, then they all equally affect tides.

Assuming they're made of material with the same density, of course?

[Radar]

Assuming they're made of material with the same density, of course?
Yes, that's exactly, what I wrote earlier in the very same post. :smallsmile:

[factotum]

Yes, that's exactly, what I wrote earlier in the very same post. :smallsmile:

Well, doesn't hurt to reinforce the point for people who might be as blind as I am... :smalleek:

[The Grue]

Also, that assumes all three moons are composed of material with roughly equivalent densities.

[The Extinguisher]

Yes, that's exactly what I meant. Had a quick play around last night and was able to get the asteroid Ceres into a reasonably stable orbit inside the Moon (yes, its orbit wobbled slightly, but within fairly restrictive limits)--didn't get as far as trying it with a third moon as well, though.

I was playing around with Universe Sandbox and I was able to get three moons in there, one half the mass and half the distance away, one quarter the mass and quarter the distance.

It was actually pretty stable. So yeah, I guess this would work pretty well.

[The Grue]

Doesn't Universe Sandbox start to break when you get it doing n-body gravitation for long enough?

[Mando Knight]

Doesn't Universe Sandbox start to break when you get it doing n-body gravitation for long enough?

Maybe?

The problem with n-body mechanics isn't that we have no idea how things will react in an instant. The problem is getting an analytical solution for the shape of the objects' paths. If Universe Sandbox is running a numerical solution for orbital mechanics, then how it will behave at long time periods depends on the fidelity of its model.

[factotum]

Doesn't Universe Sandbox start to break when you get it doing n-body gravitation for long enough?

I think it runs into problems when you set the time acceleration too high--orbits go all to heck at 1 day per "tick". At 1 hour per tick it's usually reasonably stable, at least for a few years of simulated time.

[Bulldog Psion]

Thanks guys! My computer died so I was unable to reply for quite some time, but I'm back in the saddle again, so to speak. More to come soon. :smallwink:

[Mando Knight]

I think it runs into problems when you set the time acceleration too high--orbits go all to heck at 1 day per "tick". At 1 hour per tick it's usually reasonably stable, at least for a few years of simulated time.

Ooh, yeah. That would break things. Numerical solutions for the n-body problem are only really any good if you use relatively small time steps.