Multiple Moons

Is the planet habitable without life support systems?

One of the recent theories is that advanced life on Earth wouldn't be possible without the one, single moon.

Depending on the size and distance from the planet the moons are, the gravitational pulls will definitely make things interesting. If there is large bodies of water on the planet, the tides might be huge depending on the moon orientation.

Yes, you could have complete different cycles for the moons depending on where they are at with relation to the star and the planet.

Read Asimov's "Tragedy of the Moon". http://www.powells.com/biblio/2-9780440189992-1

The moon reduces the perturbation of our rotation and keeps he planet from flipping. It gives us tides and cycles. The moon is our planets stabilizer.

You might be able to find a multi-moon simulation on the Sky & Telescope site. A three moon cycle sounds fun. You can have lots of weird social events when the moons are in same cycles, or alignment.

Look at Jupiter's four major moons cycle.

Look at Jupiter's four major moons cycle.

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Tallent read my mind! Jupiter has 3 moons, or maybe 4, that are always in conjunction. That means, for every time moon A orbits around Jupiter, Moon B orbits twice, and Moon C orbits 4 times. Sorry if I'm kinda vague on the deatils . . . been a while since I took Astronomy classes!

Anyway, cool idea!

You might try posing this question in the Story Research Forum.

knight_tour said:

Religion never evolved on this world, though the people do believe (to a degree) that the red moon means bad luck. I need to find an astronomy expert!

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Fun stuff.

Is this an advanced society? Maybe they don't have much technology because they have magic, but they've studied and understand science?

I'm asking because just about any race of people who live under three moons are going to have more than a single idea that a red moon means bad luck. They are going to see portents in just about all of the moon phases, especially if it is complicated. Entire industries of reading the moon phases and what that means for an individual would exist. Maybe they don't have religion, but they would most certainly see the moons as indicators of something.

Anyway, that's just a thought. It's your story.

The Earth has more than one moon - depending on how you classify 'moon' (actually there are at least five thingys that orbit the earth, so the answer to how many moons does the earth have is one or five depending on how you class them)


Just because you have a moon doesn't mean you actually can see it with the naked eye.

knight_tour said:

Different peoples do have different ideas about the moons, but the major civilized race's major characteristic is their pragmatism. They didn't evolve as independently as we did. When they were still just past caveman days this particular tribe encountered a group of scientists from earth, so they ended up evolving far differently than we would normally expect. Note that many things we take for granted simply don't work on this planet, i.e. the scientists' attempts to create gunpowder produce only fizzles; electricity won't work, etc. (This explains why their shuttle crashlanded when they first arrived.) Even the time continuum affects the scientists differently there than the inhabitants, as over the next 6,000 years the surviving scientists only appear to age about 25 years.

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Can't you just be as freewheeling with the science as this and do whatever you want with your moons?

Well what I can collect from common sense is this:

Do your 3 moons collide? What kind of effects would this provoke if they did?
Moons usually reflect the light of the start of the system so for different colors you might need to work something out.
How close are these to the planet?, the closer they are, the more extreme the waves and weather will be and the larger they will appear.
I kinda watched a national geographic "Mysteries of the moon" recently.

Maybe they can each cause different seasons as they individually pass over the landscape, one could cause deadly winter etc etc

You don't really have to answer any of these, I say just make your moons fun and interesting the way you want them to be.

Wait, electricity is not existent within the limit of this particular planet?, what is causing this or is it just the outside, I think humans actually function with electricity somewhat and no electricity at all could probably be a problem for the scientist survivability.

knight_tour said:

Nah, I believe that in order to get away with some crazy ideas one must ground things in as much reality as possible.

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Yes, but I also believe that you have to have some kind of justification for your crazy ideas. There has to be a reason why gunpowder just fizzles on this world. Even if you never fully explain it to the readers, there still should be a solid idea in your head as to why. Otherwise, you run the risk of Deus ex machina coming to the rescue. That's just something to think about, not a recrimination.

Moon periods of multiple moons could be different, but phases would be identical. Those are created by the sunlight hitting the moons at an angle, and since the moons are all hit by sunlight from the same direction, their phases will be the same.

sneaky_squirrel said:

Do your 3 moons collide? What kind of effects would this provoke if they did?

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Effect. A collision would happen only once. At stellar size, all materials are liquids, colliding moons would form a single bigger moon and stay that way. Unless they collide at extremely high speed, in which case the result would be a ring, pretty much.

Moon phases are indeed the same assuming ordinary orbital mechanics. However there is a case where an observer on the planet might see two moons in apparently different phases. It is a situation that is nearly impossible, but not unheard of: One of the moons orbits in the opposite direction from the others.

If one moon orbits like our moon, Luna, does, and another orbits in the opposite direction, one can be waxing as the other is waning. It is, in orbital mechanics a highly rare occurance, and unstable. But it certainly could be stable "enough" to allow a population of sentients on the planet to observe the phenomenon.

Three moons of some size would stabilize the planet as well as the Earth's single Moon, and maybe better. It would be best if the moons were of different sizes, because that would put them in different orbits with different orbital speeds. Tides would be difficult to predict, but that wouldn't make much difference, except at the highest and lowest tides.

knight_tour said:

Assuming a single star system, how would two or more moons appear in a night sky? Far apart or near, would they show similar states or could they be quite different, i.e. would they both have to close to full/half/sliver at the same time? This is complex, as they can be at very different distances from the planet. I have a planet with three moons...

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Orbital mechanics aside (because that can get really complicated) ... heck if this is a more fantasy-oriented story, just make it all up. Just remember one basic rule, the farther away the moon is, the longer its orbit and cycle will be, and we hardcore SF folk won't scream TOO loud.

But the good news is the phase has no relation to how far away from the planet its orbit is. It's determined solely by where it is in the sky in relation to the Sun.

Each moon is a ball (or sphere) in the sky. The bright side is where the Sun shines on it. If you know where the Sun is relative to each Moon, it's easy to figure out what part is lit by the Sun and what phase it's in. If the Sun has just set in the East and there's a moon in the Eastern sky, it will be a crescent (the Sun is farther away than each moons so when they're nearly in line the Sun is shining mostly on the opposite side of the moon). If there's a moon overhead, it will be at about half. If there's a moon in the West, it will be full.

Likewise, at midnight, a full moon will be overhead, and any moon(s) near the horizon will be at about half.

Regardless, if two or more moons are in the same part of the sky, they'll be in the same (or very close to the same) phase.

Ambri said:

Tallent read my mind! Jupiter has 3 moons, or maybe 4, that are always in conjunction. That means, for every time moon A orbits around Jupiter, Moon B orbits twice, and Moon C orbits 4 times. Sorry if I'm kinda vague on the deatils . . . been a while since I took Astronomy classes!

Anyway, cool idea!

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That's not conjunction, I think that's called harmonically locked orbits.

Ben is absolutely right about everything. Since this is a field I work in, I just wanted to add a little more detail...

benbradley said:

Orbital mechanics aside (because that can get really complicated) ... heck if this is a more fantasy-oriented story, just make it all up. Just remember one basic rule, the farther away the moon is, the longer its orbit and cycle will be, and we hardcore SF folk won't scream TOO loud.

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Also remember that the more moons a planet has, the smaller they have to be in relation to the planet. Earth could not have two moons the size of Luna -- they'd knock each other out of orbit.

Regardless, if two or more moons are in the same part of the sky, they'll be in the same (or very close to the same) phase.

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One wrinkle, here -- they can't stay in the same phase for long. They have to orbit at different distances, which means that their orbital periods will be different, so they can not stay in formation as they cross the sky.

You might also have one (small) moon orbit with a period of hours, like Phobos, so that it passes through all the phases every night.

That's not conjunction, I think that's called harmonically locked orbits.

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Yep. But harmonic orbits will create regular conjunctions. This should be the standard for multi-moon systems -- if the moons aren't in harmonic orbits, there should be a reason.

benbradley said:

Orbital mechanics aside (because that can get really complicated) ... heck if this is a more fantasy-oriented story, just make it all up.

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This. Pandora has floating mountains, and nobody really gives a rip about the physics.

caw

IdiotsRUs said:

The Earth has more than one moon - depending on how you classify 'moon' (actually there are at least five thingys that orbit the earth, so the answer to how many moons does the earth have is one or five depending on how you class them)


Just because you have a moon doesn't mean you actually can see it with the naked eye.

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Interesting.

benbradley said:

But the good news is the phase has no relation to how far away from the planet its orbit is. It's determined solely by where it is in the sky in relation to the Sun.

Each moon is a ball (or sphere) in the sky. The bright side is where the Sun shines on it. If you know where the Sun is relative to each Moon, it's easy to figure out what part is lit by the Sun and what phase it's in. If the Sun has just set in the East and there's a moon in the Eastern sky, it will be a crescent (the Sun is farther away than each moons so when they're nearly in line the Sun is shining mostly on the opposite side of the moon). If there's a moon overhead, it will be at about half. If there's a moon in the West, it will be full.

Likewise, at midnight, a full moon will be overhead, and any moon(s) near the horizon will be at about half.

Regardless, if two or more moons are in the same part of the sky, they'll be in the same (or very close to the same) phase.

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RemusShepherd said:

One wrinkle, here -- they can't stay in the same phase for long. They have to orbit at different distances, which means that their orbital periods will be different, so they can not stay in formation as they cross the sky.

You might also have one (small) moon orbit with a period of hours, like Phobos, so that it passes through all the phases every night.

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I think it's important to understand that the "phase" of a moon is meaningless without an observer who makes observations from the surface of the planet. Viewed from a distance, such as above (or below) the ecliptic by many planetary diameters, the moons AND the planet will always look the same.

Unless, I guess, they're not spherical.

I am just a newbie and maybe should be ignored while I gain reputation points, but . . . in my latest title, Sirens, Siren has no moon. It therefore has no tides and very little ocean swell. The planet Preculis, on the other hand, has two moons. The tidal exchange between high and low - ebb and flow - is so violent that the only craft that can travel across the ocean is a hydrofoil.

The hull must remain forty feet above the swells of the ocean. So, be careful how many moons you subject your population to. Plus the oxana clams will just remain untouched in their reefs if the depth devils have their way. but a good harpooner can make all the difference. Bon apetite!

Wig

The big factor with moons is their size relative to the planet they orbit.

In the case of Luna, we've got a king sized moon for how big our planet is. Much bigger and we'd probably be in a tidal locked set up similar to pluto.

With little moons, you could have a dozens of the little guys and they might not influence the planet much. Suppose the earth rather than a big moon, had like 3 or 4, 200km diameter moons in orbit. They'd definitely be visible from earth, but they'd have practically no effect on life down here(their combined mass just would be teeny compared to the moon).

Hell, Luna is too big for us. It's gradually inching away from the earth(back a half billion years ago, tides were like a 100meters high between low and high tide), and some day will probably escape.