A question for you brainiacs out there.

[Dimwhit]

So I've toyed off and on with the idea of creating my own campaing world (in case I ever DM again). I always enjoyed the romanticism of a world with two suns or multiple moons. But that's when I start thinking a little too much, and I come up with more questions than anything.

Multiple moons. Anyone have any idea what it would do to a planet to have, say, two moons? It of course depends on each moon's orbital pattern, but just think what that would do to the tides, among other things.

Multiple suns. This gets even more hairy. I can't remember the terminology from my college astronomy days, but a planet can have different patterns in a binary star system (if it's even possible to have a planet in one). But I imagine having two suns for a planet could really screw with the environment as we know it.

It's always bugged me when a stories/movies/etc. have multiple moons and suns without considering these things. Though maybe I'm just being weird. I know Tattooine has two suns, but all Lucas came up with was that it was a desert planet (which may be all there is to it). Honestly, I'm more interested in ideas with multiple moons.

Anyway, there's probably a brainiac or two out there who knows a lot about this stuff. If you're bored, please give me your thoughts on how a fantasy planet would be altered to fits either option.

 

[Umbran]

Okay, as you guessed, things get very complicated.

A planet can have many moons and not make it complicated - if they are very small compared ot the planet, or very far away. Having multiple moons the size of our Moon (which is anomalously large for the moon of a rocky planet), tidal patterns will get very hairy. Unless the orbital patterns are very simple, the resulting tides are likely to be "chaotic".

Typically, the larger the moon(s), the more tectonically and volcanically active the planet will be.

As for multiple suns - again, this situation is not too difficult if the two suns are close together compared ot the distance from the planet. If you can essentially think of the two suns as one body in the center of the solar system, you're okay. If the distance to the planet is not very large compared to the distance between teh stars, you can have problems - the planetary orbit may not be stable.

Mind you,that's considering real-world physics. This is a fantasy world. You don't have to give a whit for real-world physics. Just slap it together any way you want. Make it do what you want to do, and ignore the physics.

 

[Ranes]

Sadly, my interest in astronomy is hampered by my limited scientific knowledge. I can only suggest you taking a look at Brian Aldiss's Heliconia series for a treatment of a habitable planet in a binary star system.

As for multiple moons, a lot depends on the mass of the moons and their orbital distance. It could be that neither have any substantial effect on tides. If the orbital velocity of a moon (that probably isn't the appropriate phrase but I hope you understand my meaning) is slower than the rotational period of the planet it orbits, its motion across the sky may appear retrograde (ie it could appear to traverse the sky from west to east when it actually travels east to west). If you decide your planet's moons influence magic in some way, this illusion may make it very difficult for students of magic to understand how the moons affect things the way they do.

Totally out of my depth, wishing you luck.

 

[Centaur]

On earth, both the sun and the moon have an effect on our tides, when the two line up, our tides are often at their most extreams (full moon and new moon). Of course, the sun has less of an effect on the tides as does the moon.

If you have more than one moon, you can expect realy strong tides and some unusual tide patterns. When the two moons line up together, you will have some realy insane tide levels.

This generaly means that it will be dangerous to live as near the water as some people on earth do now. People will tend to build further back from the ocean.

More than one sun would produce some realy interesting weather patterns. Our weather is caused by a combination of the earths rotation and the warming action of the sun. As the earth gets warmer, the weather extreams get more intense.

What you could do is have your plannet orbit one star much like our earth orbits the sun. Then have another smaller sun orbit the same star a little further out. If you assume that the the primary star heats the planet much like our sun heats the earth, then that would produce similar weather much of the time.

However, when the orbit of the smaller star comes close to the plannet, much like mars does every couple of years, things would get much warmer, more rain, more flash floods, more extreams.

Then there is the concepts of magic. If you assume that the stars or the moons have any influence on level of magic in the world, it could get real interesting. If the second star repesents a specific god, say a chaotic one, and it's followers get more powerful as he gets closer, things could get fun....

Hope this helps.

 

[PowerWordDumb]

I don't know about the "brainiac" part - it is monday - but I'll give it a shot.

Multiple moons: Most of the time I believe you'd have highly-fluctuating tides of relatively low magnitude, due to destructive interference of the various gravitaitional attractions. Rarely, you'd hit a period of constructive interference, and you'd have either super-massive high tides which could go hundreds of miles inland (think of the campaign possibilities!) or low tides that push the seafront out several miles, stranding entire fleets, wrecking fishing and shipping industries, and depriving entire areas of drinking water (assuming drainage to the sea of most rivers).

Lots of good fodder there for historic accounts of devastation that you can have sprung upon your characters to profide campaign motivations. What do people do for food & water after such a catastrophe? What happens to power structures - who's been hiding in the woodwork looking for a chance to seize control? What, if anything, can anyone do about it? What if someone found a magical way to lock the moons in their position in order to continue a condition beneficial to them?

Multiple suns: Depending upon the binary system and orbital paths you decide upon, this can get complex. It could range from a relatively earth-like daily rotation (both suns follow each other closely) to something bizarre, with wildly-varying day/night lengths, or no nighttime at all. Seasonal changes could be either muted entirely, or extreme.

Lots of good stuff there too to draw upon.

I'm sure much smarter campaign-builders than I can give you much better help, but maybe this'll get you started.

 

[Greatwyrm]

I'm no astronamer, but here's my 2cp.

As far as multiple suns, this is probably easier than it sounds. If you remember the scene where Luke is looking out over Tatooine at sun(s)set, the suns were pretty close together. I'd imagine that from the point of view of a person on the planet, they probably rise and set within a few minutes of each other.

This would probably be true unless you were really close in orbit. So, multiple suns, probably just make it really stinking hot most of the time, unless the stars themselves are pretty old.

Also keep in mind, different colors of stars together would produce some interesting colors at sunrise and sunset.

Multiple moons are another matter entirely. Unless you've got a physics degree and too much time on your hands, you may just want to add a few extra tidal cycles each day and say that's what happens.

You will also need to come up with ideas for what happens to lycanthropes and other moon-dependent magics with multiple moons. Are they keyed to a specific moon? Do they wax or wane in power based on some formula of all the moons' phases?

 

[Centaur]

the only way that two suns would remain in close proximity to each other as they traced across the sky would be for them to be a closely orbiting binary star system and for the plannet to be in a wider orbit than typical of earth.

This means that depending on the radiation emited by the suns, the planet need not be any hotter or colder than earth. Also, because of the the higher gravitational pull of those suns, the planner would need to orbnit quicker and therefore the duration of the year wouldn't need to be all that much longer.

The closer the planet is to a binary system like this, the greater the apparent distance between the two stars and the hotter the planet, You could have a desert plannet or you could have one beset with hurcanes year round. It all depends on the amount of water on the planet.

 

[takyris]

Shoot, there's a great book out there -- it's aimed at writers, but works well for DMs, too, if you're interested in scientific plausability.

I believe that multiple moons have been covered. For multiple suns, it depends on whether you're having the world orbit a mass of close-together suns or swirl between a bunch of far-apart suns (like doing a figure-eight between two suns in a binary system). The former is easy, although you'd probably be so far out that the suns wouldn't provide a ton of heat, OR moving so fast in order to not get sucked into the suns that you'd have veyr short years. The latter involves a lot of time when you're moving between suns, not close to either (or too close to either), and that's gonna mess things up.

I did a binary star system with one planet moving in a figure-eight between the suns. The inhabitants never figured this out -- they just knew that the odd-numbered years were warmer than the even numbered years, that the Watcher was the brightest star in the sky, and it moved around during the year, and that on New Year's Eve, the sun wanes and vanishes, and there is a two-day period where the sun doesn't shine, and only the blessings of the gods keep the cold from killing everything. The odd/even thing was because one sun was larger, the Watcher was whichever sun was farther away at the moment, and the Day of Darkness (New Year's Eve) was when the planet was moving from one sun to the other. It was a pretty cold world.

 

[der_kluge]

I would imagine that your players would be less interested in the effects you dozen moons have on the tides, and more interested in the effects they might have on the magic in your world.

Many planets in our own solar system have multiple moons.

 

[Steverooo]

I agree with most of what's been said, above. So, let's look at the simple solutions...

For the suns, the two simplest solutions are to assume either two (or more) suns, fairly close together (but in a stable orbit) in the center of the stellar system, and then a world orbiting in the "Life Zone". The effects are roughly Earth-normal. The other solution is to have a Sol-like sun around which the planet orbits, and the binary far away, with little affect on the system.

In the first case, a Yellow-white G type star, and a pair of small red ones might work. In the other case, a large, massive Blue-white O, B, or A type, around which (distantly) orbit a Sol-like G, with a planet a little farther out than Earth, the temperature difference made up for by the larger, distant star.

As for the moons, as long as they are smaller and relatively distant, or larger and in fixed orbit relative to each other, tides should be no problem. In the last case, two moons (or even three) in the same orbit, sixty degrees apart, would be stable ("Trojan" Orbits). Tides would be high in two/three places, at once, but regular and predictable. In the first case, sprinkle some small "diamond" moons wherever you like. They'd look like moving stars, with maybe a few larger ones.

From there, you can get stranger and more exotic, as you like. I'd really suggest looking at some old Traveller:2300, 2300AD, or just plain old Traveller Sci-Fi books for stuff like that. 2300, especially, strove to be more "Hard Science", and discussed the effects of various such worlds.