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 (in Postscript format for printing here) and a summary of possible outcomes here.

Quote from: John Morrow;240938You find a more detailed paper on how simulations of dual-moon formations worked out here (in Postscript format for printing here) and a summary of possible outcomes here.

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.