Being that it's a fantasy setting that I'm working on, I'm willing to ignore reality at least to some extent. I still like to have an idea of how certain arrangements would scientifically work though; it's nice to at least have the world seem as though it's simulating some sort of reality...even if that reality is exotic and different from our own.
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Originally Posted by StevenH
The two suns circling the planet is indeed dynamically unstable, and wouldn't work from a purely scientific viewpoint. But there is nothing stopping a deity from setting it up as he/she/it sees fit, depending upon how real/active your deities are in your universe.
As for the paired moons orbiting the planet...don't know. I would suspect that it would be possible, with the right mass and distance from the primary. Although I would think that from the point of view of someone on the planet surface, both moons would have about the same phase, since they would necessarily always be close together. (If they weren't close together, I would think that the two moons would just orbit the primary instead of each other. Gravitationally, the primary must see the pair of moons as a single source of mass for the concept to work. People with more skill at astrophysics than I have* are of course encouraged to elaborate!)
*I estimate that I have Astrophysics-3 (on a good day, without overcast) ;-)
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You probably have a better Astrophysics skill than I do, so no worries. I figured there were people here on the boards who would have better skills than I.
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Originally Posted by Humabout
L1, L2, and L3 aren't stable, but are where gravitational forces either cancel each other out or cancel out centrifugal force. L4 and L5 are both stable; these are 60 degrees forward and behind the orbiting body in the same orbit. Not only are these stable, but several bodies in our solar system are at the L4 or L5 point of other orbiting bodies. Saturn has several such moons (called trojans), and I believe Ceres or Orcus has its own trojan, though I could be mistaken. I get the dwarf planets, plutoids, and KBOs all mixed up.
Theoretically, if two moons orbited a point outside themselves, their center of mass could probably orbit a planet. I don't know of any instances thereof or how it'd come to be, but considering the nature of so many of hte planets being found of late, I hesitate to discount any occurance, no matter how bizarre. I mean, they've found planets orbiting pulsars and white dwarfs that formed after the star died, and they've found proto-planetary disks around brown dwarfs.
Another interesting, though improbable system might be that of a binary star system with one main, hot star being orbited by a brown dwarf or suitably small, dim star (like a white dwarf) with a planet chasing it at L5.
Another option I used once for a fantasy setting was that of a planet with one large moon similar to our own and one tiny black hole orbiting the planet further out. As long as the gravitational fields canceled out it would remain stable, though I sort of ignored all the radation the black hole would be spewing out.
With ragard to planetary rings, don't discount such visions as Arthur C Clarke's in The Fountains of Paradise, either. It could be possible to artificially manufacture a planetary ring and even connect it to the planet with several space elevators. Such a world would look much like a wagon wheel and also be plausible (provided materials science is sufficiently advanced).
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A lot of what I'm looking at is theory (since, as you said, the set up I mentioned isn't one which is known to exist.) I'm mainly trying to figure out if -in theory- it would be possibly... or at least be feasible enough to create the illusion of being possible in a fantasy setting.
I could have the two moons just orbit the planet, but I was trying to do something a little more exotic. Also, I wanted some reason for the tidal waves to be a little more extreme and for ocean travel to be a little more trecherous than on Earth... a way to explain why, even after long periods of time, certain races and societies still hadn't come into contact with other races and societies who were based on other continents or land masses.
From my understanding, having two moons on opposite sides of the planet (which is one idea I had considered) would make for less severe tides because the two opposing forces would somewhat cancel each other out. It was then that I started looking at the possibility of two moons which were closer together; as such, they would be more or less both on the same side of the planet as they orbit. My admitedly limited understanding of astrophysics leads me to believe that this set up (having two moons which are more-or-less on the same side of the planet as they orbit) would create more severe tides.
It was then that I contemplated how being closer together would impact the relationship of the moons to each other. This lead me to consider the more exotic arrangement of having the two moons orbit each other; with the pair orbiting the planet together... like two dancers circling a ballroom floor if that helps to give a mental image. I've been trying to mentally figure out how the phases of the moons would look from the surface.
In my imagination there would come a point when at least one of the moons would be clearly visible (i.e. a full moon) due to the moons creating a sort of eclipse of themselves. What I'm trying to figure out is if there would ever be a time when both moons were in the full moon phase. I'm not sure if there would be because -as Steven H mentioned- they may appear as though they were roughly in the same phases. I figure this would be due to the moons casting a shadow on each other...?
Part of world brainstorming might also involve considering what the terms "day, month, and week" mean concerning keeping track of time. There is some good information located here:
http://library.thinkquest.org/29033/begin/time.htm
There is also some information pertaining to months here:
http://en.wikipedia.org/wiki/Month
...weeks here:
http://en.wikipedia.org/wiki/Week
...days here:
http://en.wikipedia.org/wiki/Day
..years here:
http://en.wikipedia.org/wiki/Year
I'm sure some of the information there is viewed as somewhat common knowledge, but it's still good information, and it invokes some thought concerning how time might be viewed differently within a variant cosmology.
In the two moon world I've been mentioning, I envisioned that the sun would be slightly bigger than our sun, but the planet would also be slightly further away which would roughly equal out to having roughly the same amount of heat as does Earth. From looking at the maps I've made, the world also seems to be slightly bigger than Earth, but that aspect of the world was a fudge job instead of based on science; a way for me to have more room to world build.
I wish I could remember the site I used to figure out the calculations, but I can't; I just remember there was a world building site which I used at one point to punch in the numbers and figure out some of the details. IIRC, the world I'm working on would have roughly the same climate as Earth, but slightly cooler overall. I'll try to find the site again if I can and provide a link; at any rate, the information I gathered using the site lead me to come up with a different calendar for the world. The calendar is broken down as follows:
1 Year = 400 Days
1 Year = 10 Months
1 Month = 4 Weeks
1 Month = 40 Days
1 Week = 10 Days
1 Day = 24 Hours
So, what does this mean (if anyone cares)? The world takes 400 Days to travel around the sun of the setting. The moon(s) of the setting take 40 days to complete their process of going around the planet, and this happens 10 times for each time the planet orbits the sun once. Days are the same length though, so, if the planet is slightly bigger than Earth, that would also mean (I think) that the planet would need to spin slightly faster than Earth.
I realize that the way we keep track of time on Earth isn't perfect; some days, months, and years are slightly longer or shorter than others due to certain variances which occur, but for the sake of keeping things simple, I ignored such things in the setting I'm building. Days, months, weeks; etc are all always the same length all the time.
If I've done my math right; if you're curious how old you'd be on the world I'm building, multiply your age by .9125.