Author Topic: Two Moons (Read 575 times)

The Evil DM · « **on:** May 01, 2008, 12:48:56 PM »

What effect (other than it looks cool in a scenic shot) does having Two moons have on a planet? I know there is something to do with ocean tides, but anything else?
I'm not much of a science guy.
Thanks,
Jeff

Fritzs · « **Reply #1 on:** May 01, 2008, 01:22:16 PM »

flood tides become absolutely unpredicable and if it's small planet, or double planet, like earth... the moon will sooner or later collide with the planet or another moon or fly away...

The problem is that while behavior of two objects is predicable, when there are more of them, it's hard to predict anything...

The Evil DM · « **Reply #2 on:** May 01, 2008, 04:25:50 PM »

Ahh Ha, That helps me.
Thank you sir!

jhkim · « **Reply #3 on:** May 01, 2008, 05:10:17 PM »

Quote from: The Evil DM

Ahh Ha, That helps me.
Thank you sir!

Well, if you have two moons, most likely they'll be much smaller than Earth's moon. Mars is an example. In that case, the setup is likely to be stable, but there won't be much tides at all.

Fritzs · « **Reply #4 on:** May 01, 2008, 05:16:34 PM »

jhkim: SDince tides are considered to play crucial part in "creation" of live from non living matter, there won't probably be many observers to enjoy view at theso two miniature moons thata are very hard to see without some sort of binoculars...

Anyway, in most cases, fictional worlds, if they have more than one moon tends to have large moons, not small ones...

Maybe it can be stable with one huge and one very small, but I am reall ynot sure about this...

Ian Absentia · « **Reply #5 on:** May 01, 2008, 05:17:10 PM »

I was in the middle of typing up a big ol' response, when I Googled up this response to a similar question. The basic gist is, How big are the moons in question? And how close are they? If you're, say, adding another moon to a planetary system much like ours, but smaller and in a more distant orbit, then the physical effect won't be nearly as pronounced.

Interestingly, the author of the linked response brings up cultural effects related to the relative apparent size of our moon to our sun (which allows for total eclipses of the visible disk), the effect of reflected light, etc.

!i!

John Morrow · « **Reply #6 on:** May 02, 2008, 01:04:38 AM »

Quote from: The Evil DM

What effect (other than it looks cool in a scenic shot) does having Two moons have on a planet? I know there is something to do with ocean tides, but anything else?
I'm not much of a science guy.

I hate to burst your bubble (it burst mine) but the "real science" is that two large moons probably won't work. How do I know this? Because in simulations of how Earth's Moon formed, sometimes the simulations produced two moons but they didn't stay in orbit for more than about 100 years or so. A few years ago, when I learned that the simulations had produced two moons, I sent Dr. Robin Canup an email asking her about it and she had a research assistant send me a paper on it. In fact, I found a copy of it in Postscript format online: Evolution Of A Terrestrial Multiple Moon System

The problem is that their gravity interferes with each others orbit and either (A) crash into each other or (B) one crashes into the Earth and the other goes flying off into space. About the only way it might work is to artificially balance the two moons on opposite sides of the planet in the same orbit but even that's probably not stable over the long term. Even our own Moon's orbit isn't entirely stable and it's moving away from the Earth (it was once quite a bit closer than it is now). This all makes perfect sense just thinking about it. The problem with most simple computer simulations is that they don't simulate the interference of the moons with each other and only track the moons in relation to the primary planet.

Over the years, there were a variety of theories about how our Large moon formed and the only one that seems to work is a collision between the proto-Earth and another Mars-sized proto-planet at an oblique angle. Theories about captured moons and so on all fail to work in some critical way and they currently think that even Pluto's moon Charon formed that way because, again, it's the only theory that works. So if you've got a large moon that was formed scientifically, that's probably how it was done. Pluto does seem to have two tiny moons so, in theory, multiple little moons are possible but probably only one big one works, not two or more. Multiple big moons simply mess each other up.

John Morrow · « **Reply #7 on:** May 02, 2008, 01:15:55 AM »

Quote from: jhkim

Well, if you have two moons, most likely they'll be much smaller than Earth's moon. Mars is an example. In that case, the setup is likely to be stable, but there won't be much tides at all.

Yes, you can have several moons that are effectively little asteroids orbiting the planet. But I think Fritzs is spot on correct. Most fantasy world have large moons, not little asteroids-moons like Mars, multiple large moons aren't stable (they'll collide with each other, the planet, or fly off into space -- as our own Moon is actually slowly doing), and Pluto shows that in theory you can have one large moon and some small ones. Alas rings apparently aren't stable around Earth-sized planets, either.

John Morrow · « **Reply #8 on:** May 02, 2008, 01:21:54 AM »

Quote from: Fritzs

jhkim: SDince tides are considered to play crucial part in "creation" of live from non living matter, there won't probably be many observers to enjoy view at theso two miniature moons thata are very hard to see without some sort of binoculars...

While the tide theories are interesting, I think the more critical contribution from our moon comes from it's formation that re-liquified the planet, gave it an added dose of heavy radioactive elements and iron that sank to the core, and the Moon's tidal forces help keep the mantle and core liquid and the core spinning all of which help preserve our magnetic field which is critical to life and which both Mars and Venus largely lack. I think it's no coincidence that the one rocky inner planet with the big moon still has a large healthy magnetic field while Mars and Venus don't.

Blackthorne · « **Reply #9 on:** December 27, 2009, 03:21:33 PM »

One of my DMs had a world with 12 moons. Pretty interesting.

Another DM had a world with no moon. We didn't get into that much, because we focused on other details...until he was reading aloud from a store-bought module and mentioned a full moon, and the entire gaming group, at once said, "FULL MOON??" It was pretty funny.