# Planet with multiple moons



## Dwarven Gold (Feb 24, 2011)

The world that I'm creating has three moons.  One is small.  In most other ways it's like Earth.

What are some possible consequences of this?


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## Legerdemain (Feb 24, 2011)

Dwarven Gold said:


> The world that I'm creating has three moons.  One is small.  In most other ways it's like Earth.
> 
> What are some possible consequences of this?


 
First, assuming their orbits can never collide, the tides of the water will be thrown ALL out of kilter compared to our planet.  Lots of water movement, potentially more storms, definitely more choppy water.

Secondly, probably a different calendar system to reflect all three moons, possibly their "day" system as well, as we now have "Mon(Moon)day and Sunday" in our weekly mashup, so maybe more days for moons depending on how they do their calendar and time.

It would look pretty sweet imo to have three moons in the sky.  May have religious significance to the people of the planet.  

Who knows, there are tons that could be done with it.  The tides are what strike me most, but it also amplifies sunlight, so if all three moons are full it could be a "midnight day" if will.  Have fun, I think that's a neat idea.


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## Behelit (Feb 24, 2011)

All excellent ideas provided by Legerdemain. I concur with the entire post with the exception of the last paragraph. I don't know if 3 moons would generate enough light to be near day lit but I get your point, maybe I'm just nitpicking.

Just to throw in a few more, yet lesser in significance. I would think you would be more apt to have a lunar presence during the day time. You may have frequent eclipses between the moons. Considering their orbits aren't colliding, you'd, obviously, have one moon closer than the other, that would also effect the tide. Your planet should be considerably less likely to be struck by any space debris.


You might want to research some theoretical ideas behind a planet with multiple moons and just how significant the tidal effect would be. I think that would be the greatest of consequences.


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## Ravana (Feb 25, 2011)

You don't need to worry about the orbits "never" colliding–all that matters there is whether or not they're going to collide during your story. Half the Solar System has more than a single moon, and none of them are going to bounce off one another within the next several million years at a minimum. It's actually quite possible to have permanently stable systems, as long as the moons occupy certain positions relative to one another (look up "Lagrangian point" if you feel a need for the details). 

As for the other issues, the answer is "it depends": on how large the moons are, how dense they are, how far away from the planet they are, and what their surfaces are made of–and atmospheres, if any. Specifically:

Brightness: a large moon has more surface area, therefore reflects more light; a nearer one will appear brighter than a more distant one; a moon surfaced in water ice will reflect more than one covered in carbon soot; an atmosphere of just about any composition will be brighter than none. And, yes, there are moons in the Solar System with atmospheres… and quite a shock it was when we discovered this, too.

Tides: this will be a function of the gravity each exerts, which in turn is a function of size, density and distance. The larger and/or more dense, the greater the effects of gravity; the farther away, the lesser. Note that unless the moons are in permanently fixed positions relative to one another, the tides will increase whenever the moons line up with one another (on the same or opposite sides of the planet), whereas at other times they will cancel each others' effects out to a certain extent, possibly even completely. Note that the position of the sun also matters (or, actually, which part of the planet is facing it): while the precise answer is more complex, basically tides will be higher around noon and midnight–when the sun is directly overhead or underfoot. So the highest tides would occur around noon or midnight on days where all three moons are arranged in a straight line pointing toward the sun… and note that this doesn't mean they're all on the _same_ side of the planet, just that they're all in a line.

Calendar: you're free to do whatever you want here. Easiest would be to ignore the concept of "month" altogether. Next easiest would be to make one moon "primary": it's the one that months are based off of, with the others possibly playing subsidiary roles, possibly no roles at all.

Daytime visibility: this won't necessarily matter–our moon is in the daytime sky as often as it is in the nighttime one. We just can't see it except near dawn and dusk because the sun's too bright. Oh, by the way: full moons always rise at dusk, new ones at dawn. Why? Because the moon is "full" when we can see the whole of its illuminated surface… which we can only do when it's on the opposite side of us from the sun. At other times, we can only see part of the illuminated surface: this is what causes lunar phases. 

Eclipses: they may be more frequent, less frequent, or never happen at all. This will depend mainly upon orbital inclination–that is, how close the orbit of each moon is to being in the same plane as that of the planet. If the orbits are significantly tilted, they may cross directly between planet and sun rarely or perhaps even never (though they'd have to be pretty far off for it to be "never"… and there are good physical reasons why this is unlikely to be the case). On the other hand, if one or more is flat-on with the planetary orbit, eclipses would happen every time that moon was new. Also, depending on size and distance (and therefore how large the moon _appears_ to be, from the ground), the moon may be too small for an eclipse to be noticeable. You should, however, at least remember that solar eclipses can only occur when the eclipsing moon is new, while lunar ones can only occur when the moon is full.

"Dark" side: this didn't get mentioned yet, but it's worth noting. Our moon is in a relation to us that's called "tide-locked"–an astrophysical term meaning it no longer rotates relative to us: the same side always faces us. This does _not_ mean that this is the only side that ever receives light (in fact, the opposite is true, as you should have gathered from the mention of phases: the moon is "new"–completely dark to us–when all the sun's light is falling on its far side). What it does mean is that we never get to see the features on the other side. There's no particular reason your moons have to behave this way, nor any reason that they should all behave the same way. Want a really complicated calendar? Have it take into account not just orbital periods, but also rotational ones–that is, a particular high holy day happens not only when that moon is full, but also when the correct _part_ of it is facing the planet. 

Note that the above is just a bare-bones outline. Unless you're a serious stickler for scientific accuracy, I'd suggest ignoring the issues and handling the moons any way you feel like. Yes, I know that sounds like I'm contradicting some of my other posts. But this is fantasy; were it SF, I'd tell you that you _would_ want to do the research–and know that, for instance, orbital periods aren't arbitrary, but are determined by distance and mass. Or that real-world orbits are never perfectly circular, though it will be far more convenient for you to treat them as such: if you don't, then tidal forces vary–as, in fact, tides on Earth do–as the moon gets nearer and farther away (the force exerted by gravity is based partly on distance, remember?). If we're talking about a world where functional magic exists, and possibly overt divine activity as well… then, hey, who's going to expect the moons to follow the laws of physics? 

By the way: I _have_ created a world that has three moons. Now. I can also tell you exactly what happened to the other two. And none of it has a thing to do with orbital mechanics.


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## Ravana (Feb 25, 2011)

P.S. My spouse just brought up another item commonly linked to lunar cycles: menstrual periods. (Whether this linkage is in fact causal or not, I make no judgment on.) 

Which, through whatever odd mechanism my mind works by, also triggered the thought of lycanthropy–_which_ full moon causes you to turn into a raging beast? Or do all of them do… and are the effects cumulative if more than one is full? (Assuming you link this to lunar cycles at all.)

Socially: any activity that relies on or is impeded by light/darkness might be affected. If the full moon(s) provide enough light to work by, some tasks could be extended well beyond daylight hours, at least some of the time; likewise, criminal and other nefarious activities might be hindered in the same way. (Imagine how much less often a ritual, say a sacrifice or summoning, that has to take place "in darkness" could be performed if you have to wait until all three moons are new.)


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## Chilari (Feb 26, 2011)

You could certainly have a lot of fun with religion with three moons, particularly if a rare astronomical event occurs during the story, like the moons lining up or creating a certain pattern in the sky. Festivals could be dependant on certain arrangements of the moons. Cosmology has been part of religion on Earth for thousands of years - sites set out so the setting sun on midwinter's day shines through a gap to illuminate a particular image, structures built to reflect the arrangement of a constellation.


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## JGSTYLE (Mar 21, 2011)

Also consider that the length of your day/nighttime will be affected.

Even though they orbit the planet, their collective gravity be slowing down the rotation of the planet.

Our moon is 1/4 the size of Earth. I don't think there's any other moon that big in relation to it's planet. If your moons add up to a similar size or bigger you could play with that.

Also, consider the magnetic field of the planet. If we didn't have our moon, Earth would be spinning three times as fast. That means our magnetic field (generated by the spinning molten metal in the core of the planet) would be *three times as strong*.

You could expect a lot darker and colder days, since a lot of sunlight and radiation would be blocked by the stronger field. Plus less plant life (or at least _differently adapted_ plant life) and much more powerful and more frequent storms.

Anyway, those are just a few things you can play around with for your story...


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## Ravana (Mar 23, 2011)

Actually, those are only considerations if you plan on setting your story _on Earth_, but with different moons. If the planet is entirely fictitious, there's no reason it wouldn't (1) have whatever rotation period is convenient or (2) have whatever magnetic field is convenient. (Nor is the second completely dependent on the first: it doesn't matter how fast it's spinning if its core is nonmetallic–or even primarily nonferrous–or if it's cooled to the point where most/all of it is solid.) 

In fact, I wouldn't go making any too extravagant claims as to what the Earth would be like without Luna. I'm not sure where you get your figure of Earth spinning three times as fast if Luna weren't there; I'd be interested in seeing the source, if you can locate it. (The numbers I have close to hand would put it nowhere near this, but it's too late at night for me to go digging any more.…) Venus, with no satellites, has a 243-day rotation period… and a magnetic field so weak that even its slow rotation doesn't suffice to account for the differences between it and Earth; Mars, with two insignificant satellites, has a rotation period only marginally longer than Earth's; Mercury, with a 58-odd-day rotation period (and no satellites), has a stronger magnetic field than either Venus or Mars (though this isn't quite as problematic as Venus's case). All of which goes to show just how strange and interesting the universe is… our own system has yet to cease providing us with surprises: imagine what we'll find when we get out where the _really_ neat stuff is.…  (Unless you're a keen follower of astrophysics, I doubt you can truly appreciate the astonishment when we discovered that _Ganymede_ has a magnetosphere–three times as strong as Mercury's… and Ganymede _doesn't rotate_–it's tide-locked!)

Even if you did want to set it on an Earth with multiple or no moons, all you'd have to do is set it in a time period sufficiently removed for these factors to have adjusted… though, depending on what you are doing with the moons, you may be looking at a time scale of billions of years. At which point, it seems kinda silly to still be using Earth as your setting… though I could think of some reasons.

And, yes, Luna is the largest satellite relative to its primary in the Solar System–now that Pluto's been demoted, at least, and ignoring other non-"planet" bodies with smaller bodies orbiting them.


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