[QUOTE=archer73]
Background: I’m writing a fantasy tale and am trying to explain scientifically how it’s possible (unless it isn’t, of course) to have a planetary system with four moons (from innermost orbit to outermost: X1, X2, X3, X4, all of which orbit Planet X which in turn orbits the sun) on one of which (X2) exists life (e.g., liquid water, Earth-like climate, etc.). Basically, I always thought it’d be cool to see three moons and a mother planet overhead but I don’t want to wind up with really nasty tidal situations or other scientific difficulties that would get in the way of my story. Going a bit further here, I wanted each of the planetoids to take up about ½ a degree of visual space in the sky (which, as I understand, is about the same as our moon). Is this all just wishful thinking on my part or could it be possible?
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First of all, minor & pedantic nitpick but your questions actually pertain to celestial mechanics and planetology rather than astrophysics, which is the study of stellar-class and larger phenomenon. (No charge for that one; it’s a freebie. 
)
Back to your question: insofar as the little we know about planetology–since there’s only one solar system we can observe in any detail our emperical data set is naturally limited–there’s certainly nothing that prohibits the system you’ve proposed. A habitable moon would have to be large enough to retain an atmosphere, so we’d assume something at least roughly Titan-sized or larger with sufficient surface gravity to hold low molecular weight molecules like O[sub]2[/sub] and N[sub]2[/sub] orbiting in a habitable zone. For a G2 star like Sol that would require a much more massive inboard planet; a more luminescent star could have your hypothetical Planet X about a much more distant orbit (and in a presumably wider habitable range). We’ve seen supermassive planets in close orbit of nearby stars, so this isn’t entirely out of question. For the purposes of discussion we’ll assume that your Planet X is, like Jupiter, seriously more massive than the system it anchors so that we don’t have to place significant consideration on orbital couplets (as with the Pluto-Charon system).
The issue of other moons is somewhat more complex. Multiple moon systems fall into what are called orbital resonances, which can be fiendishly difficult problems to work out. This restricts the range of long-term stable orbits about a body. Having three moons big enough and close enough to take up 1/2 angle of arc slice of the sky is iffy, methinks, though perhaps not impossible, particularly if the moons are in a close orbit. However, orbital resonances are going to contribute to tidal locking (reducting and ultimate elimination of independent rotation as with The Moon) which would probably be detrimental to a stable climatological system on your forest moon. I’d guestimate that so many large bodies in close proximity a) probably wouldn’t develop naturally, though they might possibly be captured later, and b) would ultimately become unstable owing to their influence upon each other. But that’s just a total, back-of-the-gum-wrapper guess with no math behind it whatsoever.
[QUOTE=archer73]
Second question: If our moon had liquid water, would the earth’s greater mass relative to the mass of the moon do more serious things to the moon’s tides than the moon does to the earth’s tides?
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This is more complex than a yes or no question; the force of gravity upon the Moon as imposed by the Earth is the same as that which is seen by the Earth from the Moon; nominally, the tidal forces are the same. (The tidal forces by the Sun on either body are minor, although not insignificant, in comparison.) The more significant factors are the depths of the ocean–a shallow ocean and larger littoral area (that is, the shallow coastal areas) are going to see vastly more effect from tides than a moon with deep oceans and small shelves–and the particularly fact that the Earth’s Moon, with its tidal locking, is going to see a more-or-less constant tide with only minor fluctuations in accordance with its orientation to the sun and tidal rocking.
[QUOTE=archer73]
Third question (unrelated to those above, but pertinent to my tale): What would happen to our Earth-moon relationship if the moon’s mass were suddenly (instantly) increased to three-and-a-half times its current mass? I assume they would come together with a great bang and hasta luego to life as we know it—is that about right? And what if the moon’s orbit were farther away than it currently is? Could I come up with some work-around here?
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Pedantically speaking, that’s two questions, so I’m going to have to charge you double. 
If the Moon’s mass were “suddenly” increased by a substantial factor (ignoring the dynamic acceleration and jerk effects of said increase) the only significant change you’d see, at least immediately, is somewhat more exaggerated tides, which would be significant for low-lying coastal areas, but you wouldn’t significantly change the orbit of the Earth-Moon system. Kepler’s Laws don’t really care about mass; the characteristics of orbits are predicated upon period (or speed) and eccentricity. Of course, a much larger mass is going to force the barycenter–the common point about which the Earth-Moon (or whatever) system rotates–outside of the Earth’s surface. That’ll have some long-term impacts, and aside from that, the increased mass will exacerbate the effects of tidal locking and rocking, but it’s not going to cause worlds to collide or anysuch. In celestial mechanics, order of magnitude changes are required to seriously affect massy objects in orbit; these aren’t easily perturbed systems.
Sorry, that was kind of a disoganized glurge. I’d put it into a more formal order but I’m running late to attend a pointless meeting to explain the obvious to people regarding why they can’t stuff 170kip transporter onto a C-17 and fly it up to Alaska. Compared to that, orbital mechanics is a piece of cake. Anyway, I hope that helps some. A quick piece of advice for you regarding writing, though; don’t sweat the details. Heck, Niven had the Earth rotating the wrong direction in the first edition of Ringworld, and yet he still managed to crank out a sizeable volume of saleable novels after that. (Never mind the whole mess regarding the questionable stability of the Ringworld and his ad-hoc Engineers patch job.) It’s commendable that you desire techincal accuracy, but don’t get lost in the details.
Good luck to you,
Stranger
