Alright, we learn in, like, Kindergarten that the moon's pull is part of what creates the ocean tides (the other part being the sun, but I'll get to that). Some of you mofos are pretty smart dudes and seem to know a lot of semi-obscure tidbits. So...
...let's say I'm creating a fantasy RPG setting. This planet I'm placing it on just so happens to have two moons: one significantly larger than the other, but the smaller one big enough to affect the oceans as well. What would be the effect? I mean, our tides work the way they do because we only have one moon. Would two create chaos? Would the tides be erratic? Would there be days where the pull is so perfect that the oceans become perfectly still? What about three moons?
Or let's get crazy and say that this solar system has two suns and the planet we're basing this on has three moons? What would the ocean be like? I'm stumped on this bit. Sure, I could hand waive with my magical GM powers, but I also like adding realistic details to my world.
Anyone know what this would be like?
-=Grim=-
I swear I'd read a post on CotI or possibly even here about this...
Very quick Googling reveals these links of dubious utility...
http://www.madsci.org/posts/archives/1999-02/917414217.Es.r.html
http://forums.gleemax.com/wotc_archive/index.php/t-948756
Depending on the orbits of the moons you would get some really funky tides, the tides here on earth are complex enough to require its own specialty branch.
Lets take a 2moon system with 1 large and and 1 small moon both large enough to produce a tide (I have no idea what the size limit could be).
The large moon would be the main component in creating tides, lets say that it has the same period as our moon and produces a high and low tide every 12 hours (at least in the Atlantic, the pacific gets one high and 1 low every 24 hours). The effects of the smaller moon would act as either constructive or destructive to the size of the tide depending on where its at in its orbit. We will give the smaller moon half the orbital period of the large moon.
Moon 1
Hr 0: High Tide
Hr 12: Low Tide
Hr 24: High Tide
Hr 36: Low Tide
Moon 2
Hr0:High
Hr6: Low
Hr12: High
Hr18: low
Hr24: High
Hr30: low
Hr36: High
So if we Combined the moons tides
Hr0: Double High
Hr6: Double Low
Hr12:Half tide (height would be split between the two)
Hr: 18: Low
Hr: 24: Double High
Hr30: Low
Hr36: Half Tide (height would be split between the two)
It gets very complicated very fast. Google the Bay of Fundy for some wicked extremes in tides.
Quote from: KenHR;240715I swear I'd read a post on CotI or possibly even here about this...
Very quick Googling reveals these links of dubious utility...
http://www.madsci.org/posts/archives/1999-02/917414217.Es.r.html
http://forums.gleemax.com/wotc_archive/index.php/t-948756
I think that post was on RPG.net, Ken, but that site is not really searchable.
Basically, you'd have several high and low tides, the height of which would depend both on the moon's mass and it's closeness to the world. You would also have particularly high and particularly low tides when the various moons lined up together, as well as with the sun. When the moons and/or sun were aligned opposite each other in relation to the planet, the tides would cancel each other out to a greater or lesser extent. This wouldn't change the amount or height of waves on a sea - that depends on winds and currents - but it would change the base level of the sea itself.
-clash
That first link is golden.
Continuing this train of thought, I wonder what the tides and atmosphere would be like if the game setting was on the moon, orbiting a large, earth-sized planet, which is also circled by two other, smaller moons. Would such a large body, i.e. the planet, make the moon totally unstable as far as tides, atmosphere and volcanic activity?
*continues musing*
-=Grim=-
Quote from: GrimJesta;240741That first link is golden.
Continuing this train of thought, I wonder what the tides and atmosphere would be like if the game setting was on the moon, orbiting a large, earth-sized planet, which is also circled by two other, smaller moons. Would such a large body, i.e. the planet, make the moon totally unstable as far as tides, atmosphere and volcanic activity?
*continues musing*
-=Grim=-
It would depend if the moon were tidally locked to the parent planet - like our moon - or not. A tidally locked moon always presents the same face to its primary, and would have a permanent high tide. The other moons would affect it the same way as they affect the parent body, just in different proportions as the distances are different. From the point of view of an inhabitant of that moon, the other moons would pursue an extremely elliptical orbit, coming quite close before going very far away.
-clash
GimJesta,
Dude don't laugh when I post this - but the current version of GURPS: SPACE (for 4/e GURPS) has charts and stuff that covers the very thing you are asking about.
I know of several people that DON'T play GURPS who bought that book just for that information.
Seriously.
The back third of that book is a gold mine of stuff thats useful for world creation.
- Ed C.
Quote from: Koltar;240896The back third of that book is a gold mine of stuff thats useful for world creation.
Ahha! Something to add to my buy list!
Quote from: Koltar;240896Dude don't laugh when I post this - but the current version of GURPS: SPACE (for 4/e GURPS) has charts and stuff that covers the very thing you are asking about.
I did laugh. Just cause it's funny that you're
THE GURPS dude. But I'll check that out. I have a dozen or so GURPS books myself for the very reason you're getting at: they have a shitload of good information in them. Hell, GURPS Horror is one of the best gaming supplements I've ever read.
Ever.So I'm gonna have to look into that book. Thanks.
-=Grim=-
Quote from: GrimJesta;240684AThis planet I'm placing it on just so happens to have two moons: one significantly larger than the other, but the smaller one big enough to affect the oceans as well. What would be the effect?
You'll probably just need to hand wave your magical GM powers because multiple large moons (large enough to cause tides) probably wouldn't be stable. The short answer is that the moons also pull on each other and that pull will make their orbits unstable, either causing them to crash into each other or one will crash into the planet and the other will get flung out into space.
You find a more detailed paper on how simulations of dual-moon formations worked out here (http://www.iop.org/EJ/article/1538-3881/117/1/603/980139.text.html) (in Postscript format for printing here (http://www.boulder.swri.edu/~hal/PS/cls99.ps)) and a summary of possible outcomes here (http://www.lpi.usra.edu/meetings/LPSC98/pdf/1842.pdf).
Quote from: John Morrow;240938You find a more detailed paper on how simulations of dual-moon formations worked out here (http://www.iop.org/EJ/article/1538-3881/117/1/603/980139.text.html) (in Postscript format for printing here (http://www.boulder.swri.edu/~hal/PS/cls99.ps)) and a summary of possible outcomes here (http://www.lpi.usra.edu/meetings/LPSC98/pdf/1842.pdf).
AHHHHHHHHHHHHHHHHH! MATH! *
sign of the cross with fingers* EVIL! HISS!
-=Grim=-
Quote from: GrimJesta;240940AHHHHHHHHHHHHHHHHH! MATH! *sign of the cross with fingers* EVIL! HISS!
Just look at the pictures. They have pictures, too. Honest.
John - not all of us were Physics majors - okay?
Thats why I recommended the GURPS book, the author is pretty good at breaking that stuff down to a manageable and gameable level.
Heck, I've even seen him answer questions both on the SJG and RPGnet forums when people had questions about astrophysics, cosmology, star system creation and all that stuff.
John F. Zeigler and James L. Cambias are the authors of that book.
- Ed C.
Quote from: GrimJesta;240934I did laugh. Just cause it's funny that you're THE GURPS dude. But I'll check that out. I have a dozen or so GURPS books myself for the very reason you're getting at: they have a shitload of good information in them. Hell, GURPS Horror is one of the best gaming supplements I've ever read.
Ever.
So I'm gonna have to look into that book. Thanks.
The current crop of GURPS books are generally hardcovers, and cover a genre as a whole, rather than the old very specific softcovers of the past. However, the new versions of GURPS Space, Fantasy, and Infinite Worlds cover their topics in astonishing and very useful detail. Some of the best game purchases I've ever made. All contain very, very good lists of resources, and each is an idea mine for its genre. I was most surprised by Fantasy, because it covers such a broad spectrum, rather than just the bog standard of the original version.
GURPS space (1e, I think, maybe 2e) was the first, and for a long time the only GURPS book I ever owned.
Alas, I've misplaced it over the years. But I remember some absolutely fantastic worldbuilding stuff in it.
There's a book floating out there... 'Habitable Planets for Man' that has a lot of useful information on it (such as water escape velocity in upper atmospheres and its likely effect on planetary atmospheres). I first saw it in the early 90s, so it might be way out of date, given some of the wacky planets that have been discovered.
Man. When I was a kid and through college, extrasolar planets were all theoretical.
Wonderful time to be alive.
I actually asked my uncle Steve -- an astrophysicist -- this question once, in relation to ElfQuest's World of Two Moons, and he said that the simple answer is that the more moons you have, the smaller the moons have to be, so the less gravitational force they exert. This means smaller tides that happen more frequently. So basically the difference between low tide and high tide is lessened, and you have more high and low tides per day.
Since the Sun also exerts a tidal influence, about half that of the moon, the smaller moons will be overwhelmed by the solar influence very quickly. So you can easily justify having Earth normal tides by simply saying "the moons are too small to overcome the solar influence."
I'd also recommend picking up the The Writer's Guide to Creating a Science Fiction Universe (http://www.amazon.com/Writers-Creating-Science-Fiction-Universe/dp/0898795362/ref=cm_lmf_tit_3_rsrsrs0), or a similar reference. Very useful stuff for anyone who worries about science in their game.
Quote from: Jackalope;240994I actually asked my uncle Steve -- an astrophysicist -- this question once, in relation to ElfQuest's World of Two Moons, and he said that the simple answer is that the more moons you have, the smaller the moons have to be, so the less gravitational force they exert. This means smaller tides that happen more frequently. So basically the difference between low tide and high tide is lessened, and you have more high and low tides per day.
Ummm. No. While that may be true of a majority of collision moons, captured moons can easily take on some very eccentric orbits similar to orbits of comets that swing into the solar system from the oort cloud. With this, One would have long periods of years where the tides are relatively stable, followed by short spans of months when the moon is in close orbit containing extreme tides that extend the tidal zones significantly from the established norm.
QuoteSince the Sun also exerts a tidal influence, about half that of the moon, the smaller moons will be overwhelmed by the solar influence very quickly. So you can easily justify having Earth normal tides by simply saying "the moons are too small to overcome the solar influence."
Binary and Trinary solar systems would also have some interesting tidal variations.
Quote from: Koltar;240945John - not all of us were Physics majors - okay?
- Ed C.
And some of were Ed. ;)
Good stuff John.
Edit to add: That last paper has all manner of goodness in terms of setting possibilities.
Bill
I'm no physics major, but I've got a physics-minded brother who loves this stuff. He can lend a hand in interpreting this. Thanks, John!
Quote from: HinterWelt;241082Good stuff John.
Years ago, when I first read an article about the moon formation simulations producing multiple moons about a third of the time, I send Robin Canup an email asking her about what happened to the two moons and she had a research assistant send me that paper in hard copy. I'm glad it's online now.
Quote from: Koltar;240945John - not all of us were Physics majors - okay?
That's why I provided a summary of the issue first, and then the papers. The papers were for people who wanted to see the support for what I was saying. Again, the bottom line is that multiple large moons will generally gravitationally interfere with each other.
Quote from: GameDaddy;241012Ummm. No. While that may be true of a majority of collision moons, captured moons can easily take on some very eccentric orbits similar to orbits of comets that swing into the solar system from the oort cloud.
I'm not sure that anyone has gotten a captured moon scenario to work for a moon planet pair like the Earth-Moon system. When the masses of the two bodies are that close, the potential moon just doesn't slow down enough to be captured.
Aeia, my fantasy world has three moons. According to legend they are houses of great spirits that watch humanity (positive spirits) because when all three are in the eky their relfected glory leaves a bit less fearful darkness than the night without them. Tides (and storms) are fiercer than on Earth, and mountains are often higher in comparison to "flat" lands. Now "why" there are three moons in a reltively "realistic" fantasy scene, is because I like them, and feel this is different. The gravity dymanics are simply not a concern to the coolness of it all.
Quote from: Silverlion;241356Now "why" there are three moons in a reltively "realistic" fantasy scene, is because I like them, and feel this is different. The gravity dymanics are simply not a concern to the coolness of it all.
You know what?
Thanks for reminding me about that. Seriously. Sometimes I get so caught up in plausibility that I often forget that the imagination should be about cool shit, not scientific mojo (at least not all the time). Huh. I get so wrapped up in my own bullshit that I forget that multimple moons = cool and science be damned.
:)
-=Grim=-
Quote from: GrimJesta;241737and science be damned.
The first rule of good writing!
Quote from: GrimJesta;241737You know what? Thanks for reminding me about that. Seriously. Sometimes I get so caught up in plausibility that I often forget that the imagination should be about cool shit, not scientific mojo (at least not all the time). Huh. I get so wrapped up in my own bullshit that I forget that multimple moons = cool and science be damned.
:)
-=Grim=-
You could also justify it with some kind of crazy super sorcery/science- which would make a great adventure hook later on.
"The blood crystals are tainted, soon the moon will crash into the sea and destory us all!"
"Gods of Rimjohb! We'll have to venture into the Klotswamp and steal some... er... fresh crystals!"
Would a "Death Star" style space station the size of our moon screw up a planet's gravity well? Kinda like a regular moon would?
-=Grim=-
Quote from: GrimJesta;243680Would a "Death Star" style space station the size of our moon screw up a planet's gravity well? Kinda like a regular moon would?
-=Grim=-
Depends a lot on its mass. I presume since the Death Star moved, and since spaceships in Star Wars have gravity that they use some form of Anti-Grav/Grav devices; that is, if I cared about explaining it with SCIENCE!
The death star also was, a lot hollow, as you can tell by the one destroyed at Endor.
Quote from: GrimJesta;240940AHHHHHHHHHHHHHHHHH! MATH! *sign of the cross with fingers* EVIL! HISS!
-=Grim=-
Dude. You ask a question that roots straight into hard astrophysics; you don't get to hiss at math...
Bottom line, though, what folk have said here is solid. You don't get multiple large moons around an earth sized planet. You hardly ever get ONE large moon around an earth sized planet - the Earth/moon system is something of a freak show in that regard. When you get more than one, they eventually screw up each other's orbits. Sooner or later one gets pushed inside the Roche limit and gets torn apart. This happens with smaller moons too, it just takes longer.
Someone mentioned highly elliptical orbits as a workaround, but they aren't any more stable - if anything, less.
Now I'm wondering if you can have a ring of equal sized moons in stable orbit around a planet. It might self-synchronize. But it would *scream* "artificial construct".
For the curious - if you like the moon, enjoy it while you can. Eventually it ends up inside earth's Roche limit - millions to billions of years, I think - and shatters. Earth gets a ring at that point, except of course for the massive moon chunks that pelt the planet, probably wiping out all life. Or so I read a couple decades ago; they might have more accurate models now.
Quote from: Ikrast;244133Dude. You ask a question that roots straight into hard astrophysics; you don't get to hiss at math...
Hrmm. Well played. You win this round. I was so proud of my Lugosi-inspired hiss and everything. ;)
I thought the moon was moving away? Or is that Roche thing further out?
-=Grim=-
Quote from: GrimJesta;244166Hrmm. Well played. You win this round. I was so proud of my Lugosi-inspired hiss and everything. ;)
I thought the moon was moving away? Or is that Roche thing further out?
-=Grim=-
It is moving out - but that costs energy (taken from Earth's rotation, if I remember) and eventually there's no more available. At that point it stops spiraling out, and sometime thereafter it starts to spiral back in. Sometime after that there's a Roche moment. It should be really cool to watch happening, except I don't think anyone will be living on this rock when it happens, which is just as well, because some of that lunar mass will be coming down hard.
It's all coming back to me. Wow. The moon raises tides in the ocean. But that water is rotating, like everything else on the surface of the earth, so even as it heaps up, it's heading east. That is, slight tidal bulge is spun a little off-center to the east, with respect to the earth moon axis. That shifts the earth's apparent center of gravity very slightly up and east, from the moon's perspective. Different center of gravity, different orbit: that allows the moon's orbit to creep just a little wider, in effect dragged into a faster, higher orbit by the ocean's slightly off-center mass.
The energy doesn't come from nowhere. The earth loses a little rotational speed from all this. Basically, the earth is powering the moon's outward spiral.
Eventually, that rotational speed loss slows the earth's rotation to the point where it is turning once a month, not once a day. At that point, the earth's rotation is locked to the moon's orbit, and ocean's tidal bulges stay nicely under the moon, so they don't haul on the moon to go faster any more. But you still have solar tides (about once a month now) and the usual ocean friction that tides always cause, so the earth keeps slowing. Now you get an offcenter tidal bulge again - lagging to the west this time, pulling against the moon's orbit. Now the moon starts to drift in instead of out.
Eventually, the earth's gravity pulls the moon apart, simply by pulling harder on the near bits than it does on the far bits, since gravity works that way. When the difference in pull mounts up enough, rip. Pieces of moon tear free, grind, and spread out - some forming a pretty ring, some forming an utterly lethal hail of vast moon rocks. I think the earth's surface probably ends up molten from the energy of the bombardment; at any rate the oceans should do a nice boil.
Wait long enough, and the Earth/sun system should play the same game, except I suspect the earth vaporizes from the heat before it shatters. Unless the sun goes out first, I don't remember. We're toast either way by then.
Have a nice day.
Awesome dude. That rocked.
-=Grim=-