A question for you brainiacs out there.

[UrathDM]

I've done some of this, sorta

I have done the multiple moons thing for my own campaign world, so here are a few thoughts on the matter.

If you are running a standard D&D-style fantasy world, you can stop worrying about the physics. In a medieval-esque fantasy setting, the interaction of moon and tide are not understood (though they have probably been observed) -- and there may not even be one. Gravity is understood as "things fall", not necessarily "bodies attract each other".

So... your world does not need to be a sphere (it can be a dish on the back of a titanic elephant, as at least one ancient religion pictured it)... it does not need to have climate bands like the real world (though it is better to have some sort of consistent variation so that players have a sense of reality in the fantasy)... it can have multiple moons or suns without worrying about erratic orbits... and so on.

Look to common knowledge and conventional wisdom of the middle ages for guidance, and use mythology as well.

In Europe, it was commonly observed that lands got warmer as you moved south. In a fantasy world, you could put a "hot zone" and a "cold zone" on opposite ends of the main land mass. It could as easily be East-West as North-South. Move toward one, and the weather gets colder; go the other way, and it gets warmer. Frost Giants are native to one, Fire Giants to the other. Or use other monsters.

Many ancient religions thought of the sun as a vehicle traveling in the sky, with stables or docks as appropriate in the east and west. The moon was often treated likewise. Perhaps multiple suns and/or moons race each other across the skies of your world ? Perhaps the moons are trying to trap the sun(s) ?

The suggestion of moons affecting magic is good, but it has been done. Try making different types of Lycanthropes respond to differnt moons. Give each moon a different period to run through its phases.

Far more important than the effects of multiple moons on the tides in a magical medieval society would be the effects on astrology and the effects of coincident lunar phases.

"The yellow sun is in the House of the Helix, your majesty, but the red sun is in the House of the Serpent. The kingdom is doomed."

Two moons are full ? What happens to the lycanthropes ? Is that a GOOD thing, because the dark-loving monsters will fear to come out; or is it a BAD thing, because animals all become wild and attack their owners ? THREE moons full, or dark, at once ? FOUR !!?

You DO want to be careful not to make the world too fantasy rich. The players need some things to be "earth-like" to have a recognizable frame of reference. The skies should probably be blue, the grass green, and so on, so that there is at least some familiar material to measure the differences against.

If letting go of the physics bothers you too much, you should probably be playing a SciFi oriented game, anyway.

Hope that helps.

 

[Agback]

G'day

For a planet to have a fairly stable orbit in a multiple system, it must either orbit one star much closer than that star orbits the others, or else orbit a closely-orbiting pair at a distance that is large compared with their separation. Anything in between results not only in huge climatic ranges and giant tides, but in a chaotic orbit that will either send the planet out into space at escape velocity, or else crash it into a sun, in a time short compared with the time it takes a planet to develop.

In the first case the extra suns are basically big bright starts that move very noticeably over decades or centuries. In the second case the multiple suns will always stay close to one another in the sky.

As for multiple moons, Mars and the gas giants do fine with them: Mars because Phobos and Deimos are small enough that their gravitation is negligible, and Jupiter and Saturn because their larger moons are fairly well spaced. Earth's comparatively huge moon is something of an anomaly, and if you want your planet to have several such you are going to have either to space them well apart or put them in one another's Trojan points (very implausible). If you space them plausibly far apart, the closer one will produce noticeable tides and the further one will produce negligible tides (tidal 'force' drops off with an inverse cube).

Before you comit yourself to anything, Google 'Roche Limit'.

Regards,


Agback

 

[Snapdragyn]

I have to agree with those voting for turning to fantasy to augment the physics. Granted, using fantasy to explain everything in a world can easily produce something so foreign that there will be no sense of 'realism', but D&D is a system with magic & fantasy, after all. Why not use it?

I'm currently working up a game setting which draws on elements of Niven's "The Smoke Ring", Weis & Hickman's "Dragon's Gate" (specifically the first volume), & Reeves's "The Shattered Sphere". The 'world' is a series of shards of what was once a typical fantasy planet in the Prime Material Plane; thousands of years ago it somehow shifted to the Plane of Air & was sundered into these shards which now orbit a glowing fiery ball (the planet's original core? a bit of elemental fire? no one knows).

I wanted the shards to have 'normal' gravity with a 'top' & a 'bottom'... so they do. No one in the setting knows why they do, nor do they even think it odd that they do (they're not exactly familiar with Newtonian physics). If someone from outside of their cultures were to ask why this was so, they'd be met with puzzled looks, or at best the supposition that this was simply an inherent property of the original world's material.

Now, if the campaign continues long enough, the players may discover some things unknown to the regular inhabitants of the Shards, such as exactly how their unidirectional gravity is maintained (not that I've bothered to flesh that out yet since it isn't relevant to early levels).

 

[Umbran]

What about an earth like planet that is a different size than earth: like one the size of say Mercury or Uranus? How would that effect things?

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That depends on the composition of the planet.

Robert Silverberg has a series fo novels set upon Majipoor - a world that is much, much larger than Earth. It manages to have something like Earth-normal gravity by being far less dense than Earth. So, in the end, Majipoor is pretty much like Earth, except that there's lots more territory (most of it ocean), and rather poor in metals.

A rocky world or Earth-like density smaller than Mars would have the problem of not having a strong enough gravitational field to hold an atmosphere (this is part of Mars' problem today). A rocky world much larger than Earth would have very high gravity - make it too much higher, and life as we know it becomes problematic.

"Gas Giant" is as much a matter of composition as it is of size. Gas giants are largly composed of gases, rather than solids.