The effect of multiple moons on tides

[vindemiatrix]

So I have a fantasy world that has six moons of varying degrees and sizes. (One is a large white one. The others are of varying sizes and colours but none appear bigger than our Moon.) This has led to two questions:

1) Is it feasible for a roughly-Earth-sized planet (it's a little larger) to have six moons?
2) What effect would these moons have on tides and weather? And if it's unnecessarily complicated, how can I reduce it?

It's culturally important for this world to have six moons: I've built many cultures, societies and religions with the six moons as a factor (it makes one hell of a difference!), and even the name of the world in many predominant languages means "Seventh Earth" or "One of Seven" or something similar.

Also, can a moon have smaller bodies orbiting it that would be visible from Earth? I'm thinking something like the Tears of Selune in Abeir-Toril, where the moon trails asteroids behind it. But with orbitings. ^_^

[adelaidejane]

Well. I don't know much about fantasy worlds or space. But I do watch a lot of Futurama ^____^ I would imagine there wouldn't be any problem with multiple moons. Trisol, from season 1, has three suns and there doesn't seem to be that much of a difference except it's hotter. I wonder how other planets in our Solar System are affected by their multiple moons -- praps do some research on those??

[vindemiatrix]

The thing is, most of the planets in our solar system with multiple moons (ie: more than two) are gas giants, thus irrelevant. Even the terrestrial planets with more than one moon, which is actually just Mars now I consider it, don't have any water.

I mostly want to know how it would affect weather and seafaring. ^_^

Three suns?? O_O I wanted to write something set on a planet that orbits a binary star system, but THREE? Crikey.

The thing is, our Moon has such an influence on our tides that I can't help but think that several would fuck shit up. >__< I hate being physically inaccurate, you see, and it's something I ALWAYS pick up on in books.

[Hippie]

I would think that it would DEFINITELY depend on the placement of the moons... Maybe see if you can talk to an astronomer in Belfast?

Let's see... there's a space observatory in Dublin (http://www.astronomyclubs.com/5/84/0/0/club.aspx). And an astronomy club in Northern Ireland (http://www.eaas.co.uk/) somewhere.

You could also go to the local university and talk to a professor there? I think this might be something you'd want an expert for. :D

[vindemiatrix]

Wow. That's actually a really good idea. Thanks, Hippie! I may try the NaNo boards too, it's worth a look. But if anyone else here has tips or thorughts or even a link or two, I would muchly appreciate it! :D

[Aquamonet]

Firstly there are two objects that create our tides. The moon and the sun although the sun has significantly less effect because it is further away.

As for the effect of multiple moons. I have had a look on wiki and come to the following conclusion. Each moon would create it's on tide so the more you had the more the tides and they would be weaker. Each moon would have 2 tides a day so if you had 13 moons you would have 26 tides, they would be so regular and weak that they would barely be noticable. Of course this would depend if the moons were the same distance from the earth or not.

I read the wiki entry on tides and found the following paragraph particularly interesting:

"It was the universal theory of gravitation due to Isaac Newton (http://en.wikipedia.org/wiki/Isaac_Newton) that first enabled an explanation of why there were two tides a day, not one, and, via calculation of the forces, offered hope of detailed understanding. Although it may seem that tides could be predicted via a sufficiently detailed knowledge of the astronomical forcing terms, the actual tide at a given location is determined by the response of the oceans to the astronomical forces accumulated over a period of many days and to calculate this requires a detailed knowledge of the shape of all the ocean basins. Instead, the procedure is pragmatic. At each location of interest, measure the tide heights over at least a lunar cycle (to capture the spring - neap tidal response) and then analyse the differences from mean sea level with respect to the known astronomical frequencies and phases of the tide-raising forces on the expectation that the tide height behaviour will follow the behaviour of the tide force. Then, because astronomical states can be calculated with certainty, the tide height at other times can be predicted. The main patterns are the twice-daily tide, the difference between the first and second tide of a day (due to the moon and sun being north or south of the equator), the spring-neap cycle in amplitude (due to the relative positions of the moon and sun), and the adjustment of spring tide heights due to the perigees of the moon and sun. The Highest Astronomical Tide is the perigean spring tide when both the sun and the moon are closest to the earth."

Full page here (http://en.wikipedia.org/wiki/Tides#Tidal_forces)

[vindemiatrix]

Ah! Cheers ever so, Aqua. If that's the case, then I could just eliminate tides, right? They'd be weak enough to just cut out altogether. Or it's possible that the one large moon would create noticeable twice-daily tides, but the smaller ones would not have much of an effect.

Can anyone confirm?

[Aquamonet]

If you had a large moon near then that would create your basic tides. If the other moons were more like small asteroids far out then they would have no effect.

Obviously someone with proper scientific knowledge could prove me wrong. My head kind of sploded with the science.

[vindemiatrix]

The other moons... varying distances. Some are close but very small, some are larger but far out, but none of them appear larger than our Moon apart from the one big one. I think that it's possible then that the one large moon would create a greater tidal effect than what we are used to on Earth (it's maybe three times larger in appearance?), but would this be counterbalanced by the smaller moons?

I love science, especially astrophysics. I love asking these questions. ^__^

[Hippie]

Ah, see, Aqua had a go at what I was scared to: Looking at the science of it. My brain would have sploded.

BUT! Her argument seems sound, and I really like the idea of seven moons. :D Maybe you should could have two Strong Tides (for your biggest/closest moon), and then four Weak Tides (for the next two biggest/closest moons), and then the rest are almost unnoticeable. Just to make it have a little bit of a difference to us on Earth.

[Kail]

Ah! Cheers ever so, Aqua. If that's the case, then I could just eliminate tides, right? They'd be weak enough to just cut out altogether. Or it's possible that the one large moon would create noticeable twice-daily tides, but the smaller ones would not have much of an effect.

It also depends somewhat on the periods of your moons. Unless your moons all share the exact same orbital period (meaning they're all the same distance from your planet) then you're going to have lunar conjunctions and oppositions periodically. If you had a large number of moons, the tides would vary widely, depending on which moons are in the same alignment. Sometimes you'd have massive tides, sometimes almost none, and it would happen at varying intervals.

It's like if you generated a number of waves of slightly different frequencies; sometimes they'd cancel each other out, and sometimes you'd get huge constructive interference making the waves much more massive than any of them would have been individually.

Anyway, as long as you don't start talking about the periods of the moons in your story, you could claim just about any tidal pattern was possible.


1) Is it feasible for a roughly-Earth-sized planet (it's a little larger) to have six moons?Feasable, but the Earth's moon is (statistically) already quite large for a planet of our size, so they'd probably be a lot smaller. Having a lot of huge moons orbiting the planet would probably put a fair amount of strain on the crust due to tidal forces (earthquakes, volcanoes, that kind of thing). Plus, significantly massive moons cause the planet to wobble (the bodies pull each other's orbit slightly) so a complex multiple large moon system would be a nightmare to keep stable, as they'd be pulling at each other as well as the body they're orbiting.

No known moon has any other moon orbiting it (possibly Saturn's Rhea does, but that hasn't been verified) presumably because this would destabalize the orbit. Some moons do have smaller moons or asteroids following or preceeding them by about sixty degrees in their orbit (at the LaGrange points).

This depends on how small you're willing to go while still calling something a "moon." Larger planets can have a lot of moons because their huge mass prevents the moons from mucking with them too much. But smaller bodies (like Earth or the Moon) are much more heavily impacted by having something big orbiting them. Saturn has about 100 times the mass of the Earth, but it's largest moon is only twice as massive as ours. So, fudging the math heavily, if you wanted a system of moons like Saturn's around the Earth, you're looking at the largest moon being about 2% as massive as our current one (and the others being a few hundredths of a percent).

Edit: Orbital period, not rotational period -_-;

[MichaelB]

Time for my own contribution, yes/no?

I was watching a strangely relevant show on this topic the other day. Apparentlky, when the Earth was still forming, the moon orbited much faster than the Earth spun, so it eventually pulled the earth into a faster spin AND into a tilt because of the way it was formed (giant multi-planet collision).

So if we had no mood, we'd have no summer or winter, no tides, and the day would last a lot longer.
With two moons then, or more, the effect might be increased: unstable tilt of the poles, unstable, fluctuating spin speeds, many tides (if the moons are closely aligned in their dawns and dusks then the tides will be greatly exaggerated). I would think.

[vindemiatrix]

Kail, Michael, this are BOTH really useful posts! Oh man, it looks like having all these moons is going to make for some really awesome, complicated effects. I can hardly not include some of this stuff. Thanks especially to Kail: that was a whole heap of awesome right there.

Looks like whatever happens I'll have to fudge it a bit, but I can definitely factor in incredibly unstable, unpredictable tides and even some axis-wobbling so the seasons vary. Tectonic instability could be fun, too.

This is way cool, guys. ^__^