# planets, moons, satellites and stuff



## pixallmighty (Mar 8, 2014)

I have this SF/Fantasy project that I'm working on. A few months ago I started to do some world building and I've gathered almost everything I need, expect for one thing. 

I want my story to take place on another world (duh!) where the sky tells the reader/characters that they're no longer on Earth. I was inspired by the Halo games, where you can walk around without noticing you are on a alien world until you look up. 

I though about it for some time and I came up with some things, but I need some more informations to make my world complete. I would like the story to take place on a moon smaller than Earth, but larger than Ganymede. The moon's planet would have other, smaller moons; it would also have rings. 

All of this is good, but I need to know a few more things:
-How can I know the lenght of the days/nights? 
-How can I know the time it takes for the moon to complete its orbit?
_How can I know how long a year would be?   
-How often my world would pass under the planet's shadow, and for how long?
-How big the planet would need to be?

I don't have, at this point, specifics requirements about the days'/nights'/years' lenght. I just need to know how to make them plausible but different from what we're used to. I would want you to give me some advices on how to calculate theses things.


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## Penpilot (Mar 8, 2014)

Honestly, you could probably get away with just picking numbers that you think work best for your story. Since these things aren't key plot elements, then they really won't affect your story much. The difference between a 8hr day or an 20hr day scientifically may be significant, but story wise not so much if they don't play key roles in your plot other than to provide flavor.


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## pixallmighty (Mar 8, 2014)

That's the point: somehow, these numbers will be key plot elements. If the moon stays in the planet's shadow fo 2 weeks or more (like in the Pitch Black movie), there must be some consequences on the storyline. But you're right, I could just pick numbers that suit me and be done with it. On the other hand, if I could have some model to look at (like the Moon: what is the lenght of a day there?), that woud be better for me. These aren't the kind of stats that we find on Wikipedia...


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## chrispenycate (Mar 8, 2014)

To a very great extent you can decide on the desired characteristics and juggle the celestial mechanics to fit. The primary planet, presumably a gas giant, is going to be in the Goldilocks zone (or slightly cooler, there might be some infra red from the planet), no problems, superjupiters been detected closer than that. So, the length of the year will depend on the size of the star. Hotter star, larger surface area, further out the planet, longer the year. But seasons might not match too well, though; Earth's seasons are due to the inclination of its axis of rotation relative to the plane of the ecliptic, while your moon is orbiting a planet, so you've two possible axes to worry about, plus the wobble due to the other moons and the amount the orbit of the main planet diverges from the circular. Complex, but liveable. 

Generally, a moon like that would be tidally locked to its planet, so its day would be its orbital period, and the planet is always be over the same place on the surface. Like Earth's moon, with a twenty-eight period, fourteen days of day, the same of darkness. But that's going to give rather a long day (yes, we can change it by moving the moon in toward the planet or further out, but bring it too close in tidal effects get ridiculous). And nothing says the moon must be tidally locked; it won't have started like that. If we balance it up well, we can leave it spinning, and have any length of day, with the planet duly crossing the sky more or less once a day (like the moon in Earth's sky) showing different phases.

The planet's shadow (eclipses); going to be more frequent than on Earth, obviously; planet's bigger, subtends a much bigger angle. But it all depends on how far off the the axis of rotation the axis of the orbit is; close to equivalent you get an eclipse every orbit. 

Tides are going to be – impressive. I can't calculate how much so without having made decisions about other details, but if the moon is spinning to give a reasonable length day it's going to be several metres. 

Size of the planet? The smaller (and denser) we can make it, the closer we can orbit, and the less excessive the tide-locked day becomes. A rocky planet, rather than a gas giant? Maybe about five times the mass of the Earth, with a moon a bit smaller than Mars, to be able to hold an atmosphere? What would you _like_; let's see how close we can get to it.


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## Ravana (Mar 8, 2014)

As Penpilot said, you're pretty much able to pick whatever numbers you want. In fact, unless you're planning on putting the story on an existing body (i.e. Ganymede), you're pretty much _stuck_ with picking numbers. If there's one thing we've learned over the last few years—courtesy of the Kepler mission, above all other research—it's that a lot of what we previously thought might be rules do not even approach the level of guidelines.

That having been said… once you start picking your numbers, if they're going to be important plot elements you'll want to be able to make sure they all work together, and not create any blatant contradictions. Which in large part means learning to do the math yourself. If decimal places aren't crucial to you, however, there are programs available online (for free, even) which will accept numerical input and produce orbital models. I haven't looked at any of them in quite a while, so I can't even give you name recommendations, and I'm not sure how many bodies they'll model—i.e. you may not find any which will do the orbits of satellites around planets at the same time as doing the planets around the star—but they should at least be able to give you some idea of how your proposed numbers work out in reality. Even if they only point up the catastrophic failures.

Note also that even if certain orbits aren't stable in the long term, that doesn't mean a body can't occupy one in the short term… and a few tens of millions of years will generally cover any time period you're likely to be writing about, eh? 

Other aspects, such as being tide-locked to the primary, are also the result of long-term adjustments between primary and satellite. Tidal forces will depend on distance between the two bodies, as well as to the system's primary. Or primaries: it is entirely possible for multiple-star systems to have planets. Which was one of those surprises I mentioned earlier. Eclipses will depend on whether the orbit is in line with the plane of the ecliptic—as the very name suggests; the satellite might enter the shadow of its planet every orbit; on the other hand, if the orbit is inclined 90Âº to the ecliptic, it might only do so when the primary is itself in a certain position in its orbit relative to the star… and might not do it every single planetary year, depending on how rapidly it orbits the planet.

And so on. Unfortunately, generalizations will only get you so far. Probably, the best thing to do is come up with a basic description and some numbers you think you want, then (a) try to find an orbital modeling program you can feed them into and (b) post them here and see if any of us can poke holes in them.


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## Ravana (Mar 8, 2014)

One thing I can add, based on your original description: if your world is a satellite orbiting a planet, the only way it could itself have moons is if it is a considerable distance from the planet… otherwise, the planet's gravity would long since have plucked them away. Same for rings, only worse, since they're made of smaller stuff. 

In fact, the only way I can think of offhand for it to have rings is if the ring material is being yanked away from the moon by the planet's gravity, and is thus being constantly renewed. Possibly the tidal forces from the planet increase tectonic activity on the moon, and some of the material is ejected far enough into the atmosphere it gets pulled out of it altogether. Most of this would likely continue falling into the planet, but there might be enough left that it creates a ringlike effect. I say "ringlike," because it seems more likely to me that it would create a generalized haze rather than a flat, well-defined annulus. On the other hand, since the material is being affected by the planet's gravity, it would tend to be pulled into a plane representing the ecliptic between planet and moon, so maybe sorta. Note that the ring structures we're familiar with are in fact very small amounts of very small pieces of ice: it doesn't require much mass to make one. So your moon isn't necessarily being eaten away so rapidly it'll vanish in the foreseeable future. Hasn't happened to Io, after all.


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## pixallmighty (Mar 8, 2014)

Thanks for your illuminating help. This is what I thought:

My world (which I will call the Moon, since I have no better name for it) orbits around a Saturn,like planet (which I will call the Planet, for the same reason). The Planet has rings and a few other moons, other than the Moon. I haven't made all the calculations for the exact size of the Moon, but let's say that the story takes place on a large continent of about 45-55 000 square km:  it takes about the fourth of the Moon. So basically, the continent is a little bit larger than Canada. ( I need to make proper calculations about it, but that's the feel I have). The equator cuts the continent in two, one third south, two thirds north.

The Moon's orbit would not be inclined 90Âº to the ecliptic, but would not be parallel to it either. Maybe somewhere between 20 and 45? Eclipses should happen often, maybe each one or two months. Their duration should vary between a few hours and a few days, maybe even a week or two, if possible. I searched Google for some pictures that could represent the feeling of the Moon's sky. That is what I found. It tells you the Planet's size in the Moon's sky (more or less)

http://25.media.tumblr.com/tumblr_m91vu54wuL1qzffw5o1_1280.png

http://www.zastavki.com/pictures/originals/2013/Fantasy__039570_.jpg

I will need to determine the time it takes for the Moon to make a full orbit around the Planet, and the time the Planet takes to circles the sun, but theses are less important considerations: these values can be anything, I don't have specific requirements. These number will only Â«provide flavorÂ» 

I'd rather not bother with seasons, because I don't really need any in my story. I would prefer to play around the day/night duration and its consequences on the world: how people, animals and plants adapt to it, since having one full week with no direct light from the sun would be a big deal. I'm not against the idea a the Moon being tidally locked, but it would be an bold choice since it would be difficult to fit in the story( having a full week of night per month is less constraining than to have it each week). But in the end, I would really like the day/night dynamic to have a fair role in the world building process, since some elements of my story, its mythology and its symbolic already refers to this imagery.


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## CupofJoe (Mar 9, 2014)

If you want/need to have "hard science" behind your world then you will need to get to grips with things like Kepler's laws, Laplace Runge Lenz and Celestial Mechanics in general.
Or find someone that understands some or all of this and can reverse engineer the figures you want...


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## pixallmighty (Mar 9, 2014)

CupofJoe said:


> Or find someone that understands some or all of this and can reverse engineer the figures you want...



That was pretty much the point of posting all this here 

I don't need Â«hard scienceÂ», just to build a system that is scientifically coherent. My questions may seem to be require complex answers, but they don't. I won't have to use precise numbers and exact inclination degrees to describe anything. As long as there's no contradictions in what I said in my last post, I will be able to go on.


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## Ravana (Mar 9, 2014)

As mentioned, tide-locking is optional. So Moon still rotates relative to Planet, giving day and night. These can literally be anything you want them to be. From a strict astrophysics standpoint, the younger the system is, the shorter days would be; these would lengthen over time, as Moon's rotation progressively slowed due to tidal forces. While this would normally give a maximum "day" equal to the orbital period of Moon, it would even be possible for the day to be _longer_ than the orbital period–if Moon were a captured body, say, rather than one which formed at the same time Planet did, and thus whose rotation had nothing to do with that of Planet and its other orbiting bodies.



pixallmighty said:


> let's say that the story takes place on a large continent of about 45-55 000 square km:  it takes about the fourth of the Moon. So basically, the continent is a little bit larger than Canada.



That would make Moon pretty small. Canada may look big on a flat map of the Western Hemisphere; on a globe, not so much. Also, you're going to want to check your figures: Canada's actually close to 10M km[SUP]2[/SUP]. You mentioned that Moon would be slightly larger than Ganymede: the surface area of Ganymede is 87M km[SUP]2[/SUP]. So even a Canada-sized continent would occupy only about 1/9 of the surface. Also, big though Ganymede is in terms of satellites, its surface area is only 17% that of Earth's. Don't think that affects anything you want, just adding it in case it does.

None of that matters other than in not giving your readers numbers that'll be instantly recognizable as incorrect. The continent can be Canada-sized, overlapping the equator by a third, and the rest of the world can be _luna incognita_ if it has no effect on the story… so, for instance, it won't matter if your continent is 1/4 of the total surface area, 1/9, or any other number. All the _relative_ size will affect is how far away the horizon is. If you expect to someday bring the rest of Moon into the story, then you will want to nail both relative and absolute sizes down.



> The Moon's orbit would not be inclined 90Âº to the ecliptic, but would not be parallel to it either. Maybe somewhere between 20 and 45? Eclipses should happen often, maybe each one or two months. Their duration should vary between a few hours and a few days, maybe even a week or two, if possible.



This will depend on the orbital period–i.e. how long a "month" is–as well as inclination. Basically, if the inclination is 0Âº, eclipses happen every month, and last for whatever percentage of the orbit can be covered by Planet's shadow. Note that this puts a sharp limit on the possible length of any eclipse: unless Moon is _very_ close to Planet, the amount of time it can spend in Planet's shadow is going to be a minuscule percentage of its total orbital period–and no matter how close you put the two, the amount of time inside Planet's shadow will still only be a small percentage of the period. This is one of the few factors you don't have free choice in: no matter how you arrange things, Planet can only be so big, and Moon's orbit needs to be considerably larger–or else it isn't orbiting, it's crashing. You can lengthen the eclipses by slowing Moon's orbital period, thus keeping it in the shadow longer, but that would also increase the amount of time between them by lengthening the amount of time it isn't in shadow. You _might_ be able to vary the length of eclipses by increasing the extent to which Moon's orbit is elliptical (i.e. away from exactly round); I'm not sure how much of an effect this would have, though, since while Moon would remain longer in an eclipsing position relative to Planet at aphelion (the "pointy" end of the ellipse), this would also put it that much farther away from Planet, and since the eclipsing shadow is conical, the farther from Planet, the smaller the cone of totality (though the larger the cone of partial eclipse). You'd have to really crunch numbers to figure out how to get an eclipse of greater length based on slower movement through a smaller cone compared to more rapid movement through a larger one when the eclipse took place during perihelion.

Incline Moon's orbit, and you end up with the time between eclipses becoming pretty much anything you want it to be; the lengths will still depend on how near Moon is to Planet and Moon's orbital period, and will (I'm pretty sure) be independent of inclination.

Keep in mind that eclipses in the Terra/Luna system last only a few hours–and that Luna is _much_ larger in relation to Terra than any other satellite in the Sol system is relative to its primary.

One possible nifty alternative is that Moon is outside, or at least between sets of, the rings, and that the shadows of the rings create "eclipses" as well: these would be far from total blackouts, but would still create significant shadowing–look up pictures of Saturn. These could happen far more frequently, and for longer periods, than eclipses from Planet itself. Note that this would require Moon's orbit to be inclined relative to the rings–which is the same as saying inclined relative to Planet's rotation, as the rings ought to form parallel to Planet's equator. So that's not a problem.

For information on bodies with axes not perpendicular to their orbit, see Uranus and Pluto.



> I'd rather not bother with seasons, because I don't really need any in my story. I would prefer to play around the day/night duration



You will or will not have "seasons" based on Moon's rotational period, since if its "days" become long enough, they'll look more like seasons anyway. Very short ones, perhaps, but long enough that the two concepts will bleed together: some things one normally associates with seasons, such as growth cycles, will come to be dominated by the light/dark cycle of the "day." Of course, if the days are short enough, this won't matter: it will only work out that way if you do have extended periods of light and dark. You can also _optionally_ have seasons based on the inclination of Moon's axis to Star (for lack of better name…  ) and the thickness of Moon's atmosphere–the same factors which give Terra its seasons. The more distant Planet is from Star, the less this effect will be… a point to be considered independently of anything previously discussed.

Note that the inclination of Moon's axis, relative both to Star and Planet, are also independent of anything mentioned thus far, though if Moon and Planet formed at the same time, their axes will be fairly similar. Barring some catastrophic astrophysical occurrence such as a rogue body passing near Moon at some point in the past and yanking its axis in a different direction, at least. If Moon is a capture (or a victim of such an occurrence), then its axis can point absolutely anywhere, including parallel to its orbit, rather than the normal near-perpendicular… which would have the interesting effect of making what faced Planet at any given time dependent on both rotation and orbital position: when the axis was pointing at Planet, it would be the same as being tide-locked (that half of Moon always facing Planet regardless of Moon's rotation), whereas when the axis was oriented 90Âº from Planet, you'd have "days" equivalent to Moon's rotational period. It will also make the "days" partially independent of Moon's rotation: these would be determined more by where Moon is in its orbit–just as if it were tide-locked. Axial inclinations of values between these will produce similar effects to whatever lesser degree the axis is tilted. This will considerably complicate the calculations involved, but that may be just the sort of weird mythology-generating circumstance you're looking for. 

You said Moon has satellites of its own. This would probably require Moon to be a good distance away from Planet (so that Planet hasn't dragged them in); also, it would probably require that these be captures–I don't consider it likely that such bodies could have formed as part of normal Planet/Moon formation. I could be wrong on that, though. There are no known examples of this in the Sol system. Apparently there _is_ some non-visual spectrum evidence that Rhea, one of Saturn's moons, has something resembling a "ring," though subsequent photography in a flyby failed to image one; the phenomenon remains unexplained, but while investigating it number-crunchers determined that it would be possible for small bodies to possess stable orbits around Rhea… so there's something resembling a precedent at least. If you want the physics determining distances at which stable orbits can exist, look up "Hill sphere."

Just to toss it in: there could also be _non_-orbiting satellites of the combined Planet/Moon system at the L[SUB]1[/SUB] and/or L[SUB]2[/SUB] points. 

In any case, these bodies would have to be small, which would also make them irregularly shaped, since their own gravity would be insufficient to cause them to round on their own. Tidal heating (melting and refreezing) can cause such bodies to become round over time, so maybe even then. As observed from the ground, their irregularity or lack thereof may or may not be evident: probably it would be, since they'd have to be awfully close to Moon for reasons already stated. The best models for these would probably be Mars' satellites Deimos and Phobos, or, again, the Pluto system. Look up "astronomy on Mars": the rovers up there now have allowed us to make actual observations of what its skies look like from the surface. (Which is totally awesomely cool to be able to do. Find the "you are here" pic.…  )


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## Ravana (Mar 9, 2014)

P.S. Cheap plug: look at the bottom of the page, under "Similar Threads," and find "A Tale (or two) of Two Moons." It's something I put together for a shared world, involving a "catastrophic flyby." It only bears the vaguest of connections to what you're doing, but it still might prompt some ideas. I did work my way partially through the physics involved, enough to assure myself that what I was presenting wasn't _prima facie_ impossible.

Unlikely… well, that's another story. The odds of something happening as presented are, both literally and figuratively, astronomical. But every story, taken in isolation, is unlikely.


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## Edankyn (Mar 12, 2014)

A tool like http://www.transhuman.talktalk.net/iw/Geosync.htm will give you most of the information you want. It allows you to choose how long the day is. It includes geosynchronous orbital distance and geosynchronous orbital distance which you can use to compute how long it would take for your moon to orbit the planet. Years will be dependent on the planet, not the moon. If you plan to use it, I'd recommend making one random large gas giant and then specifying the details for the moon since the moon is more critical to your story.


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## chrispenycate (Mar 13, 2014)

Thank you for inquiring about our habitable moons offer. 

Unfortunately, there are some slight difficulties in putting a gas giant as small as Saturn into the Goldilocks zone. Basically, gas giants consist almost entirely of hydrogen and helium, which are small, light molecules, and easily achieve escape velocity. (At any particular temperature any gas molecule has the same energy. Energy equals em vee squared, so velocity is higher for lower mass)  Why there's practically no helium in Earth's atmosphere. So a small gas giant like Saturn might have difficulty forming in high temperatures, like the melting point of ice, as it's very low density, and its escape velocity is too easy to attain at those temperatures.  A bigger gas giant, say a couple of Jupiters in mass, would have an escape velocity high enough to prevent this, and a bigger shadow, too, and might even be persuaded to generate some fusion energy at its core, thus slowing the cooling of its moon during eclipses (it would subtend a huge angle at the moon's surface, which, even if ir were not emitting much energy would not be absorbing it as fast as open sky).

Of course, we can always eliminate the problem by using something other than water as our principal solvent; ethanol would be nice (whisky rain, anyone?). But the most convincing would be to follow our solar system model; Titan. Methane, ammonia; develop a lifeform based on these, rather than oxygen and carbon dioxide; and we know that that combination works with Saturn. 

Now, for that extended eclipse. Does that have to be a total eclipse, or will a partial eclipse suffice? Because the way to prolong the eclipse is to slow down the orbit, lengthening the 'month' moving the moon further away from its planet. Unfortunately, the shadow of the planet is two cones, one, the penumbra cone giving partial eclipses and spreading wider than the planet (apex at the star; I'd add pictures but haven't got attachment privileges yet. Anyway, I'm a lousy drawer), and the other, the total eclipse, getting narrower as it gets further out until it diminishes to a point and is no more. So you can't move too far away and slow the month way down, because you move out of the total shadow cone completely. Juggle with the figures as I may, the most I can get of black is an hour or two - not even a day, let alone a week. And I'm not that unhappy about that. With several days without the sun's energy warming the moon the atmosphere would be freezing out. Even if the planet's a lot warmer than background, it's not going to emit enough infra-red to keep its moon warm; even an Earthly eclipse of the sun, which lasts a few minutes, you can feel the air cooling down.

Accelerated time sense for the residents, and spin the moon as fast as possible while maintaining some gravity at the equator? If you made the day last a few minutes, and accelerated their reflexes and senses until this seemed reasonable (no, I have no idea what chemical reactions could be used for a metabolism like this; nitro glycerine, perhaps?) then a one hour eclipse would seem several days to the inhabitants, without the difficulty of freezing them solid. (Like Forward's 'Dragon's Egg' but a few orders of magnitude less. With the difference that, in a gravity field lower than that of Earth, 'falling' becomes almost a null concept, eclipsed by inertia. Your cup of tea's too hot? Just let go of it, and go and clean the living room, and when you come back the cups only a handspan lower - and still not cool.) 


Sorry about being so slow; it's not thinking out details that's time consuming, but writing them down. And remember, the readers don't need to be told any of this. The ones like me will work it out for themselves, the others don't want to know.


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## psychotick (Mar 13, 2014)

Hi,

First I can't see your moon having satellites of its own - not if they orbit it in the same plane as it orbits Saturn. They would be pulled out of orbit and towards Saturn as they orbit on the planet side of the moon, and would fly off into space on the other side.

Tides on your moon won't be related to the moons orbiting it, they'll be linked to your moonworlds orbit of Saturn. Think of it as being like if our moon had free water it too would have tides and they would likely be a lot bigger than the ones we see here on Earth - but they'd also only be four weekly.

A day on your moon would be it turning on its axis, and the obvious thing people would see would be the huge planet in the sky above them - much as we see the sun.

The big problem you have with a moonworld is that its planet - Saturn - doesn't provide any warmth. What it does do is provide shade from the sun. So every time the moon is behind the sun it freezes. (I mean really freezes.) And if you look at our moon thats for a week or two at a time. And while you could try to negate that by having the moonworld orbit Saturn at sort of right angles to the way Saturn orbits the sun, in practice that doesn't happen. Everything fairly much orbits the sun in the same direction and same plane - the ecliptic. Anything not travelling in the ecliptic tends to smash into stuff travelling in the ecliptic and ends up as rocks and dust.

So your moonworld is going to ultimately freeze and burn - pretty much like our moon does. I don't know how you get around that, but part of the answer might be that if it has a very thick atmosphere it may be somewhat protective. But of course over time the planet will steal that atmosphere from it.

Cheers, Greg.


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## chrispenycate (Mar 13, 2014)

Hello, Psychotick; was it you that sent me here?



			
				Psycho said:
			
		

> What it does do is provide shade from the sun. So every time the moon is behind the sun it freezes. (I mean really freezes.) And if you look at our moon thats for a week or two at a time.


No, the cycle from full moon through new and back to full is nothing to do with the shadow of the Earth, it's simply the luna day/night cycle, roughly every four weeks. Since the moon is tidally locked to the Earth, one face always toward it, this corresponds to the moon's orbit, but not all moons, even in the solar system, are so locked. So we can have a reasonable length day, just by spinning the moon up a bit.

The deepfreeze shadow from the planet gives eclipses of the moon, total or partial, and we know how often they arrive. Certainly the angle subtended by the planet at the moon's surface is much greater than that of the star, rather than roughly the same as on Earth, so eclipses are more frequent and go on longer (and no sparkly corona around the rim while they're in progress, either), but depending on the inclination of the moon's orbit relative to the ecliptic, not over frequent.



> First I can't see your moon having satellites of its own - not if they orbit it in the same plane as it orbits Saturn. They would be pulled out of orbit and towards Saturn as they orbit on the planet side of the moon, and would fly off into space on the other side.


As I read the first post it's the planet that has extra moons, and a ring, not the moon. But it is possible to orbit a satellite; both American and Russian craft have done orbits of the moon. The arithmetic gets very hairy, but leaving bits moving in the right general vicinity for a few million years can generate stable solutions faster than me with an abacus. It's only one body more complex than star – planet – moon, after all.


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## psychotick (Mar 13, 2014)

Hi Chris,

I think we're mixing up two things here. First the lunar day does correspond to the lunar orbit because of tidal locking. So yes the moon if it was spinning on its own axis at a different rate could have a normal length day. However the cycle of new moon, full moon etc is linked to the moons orbit of the Earth. For a fair chunk of the time the moon - especially if in orbit around a Saturn sized planet - would still be in shadow.

Just did some checking. Our moon has two full lunar eclipses per year each lasting several hours. The limited number of full vs partial eclipses is due to the size of our moon re Earth and the fact that the moon's orbit is 5 degrees off eliptical. The scenario given is a Saturn type planet.

Cheers, Greg.


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## chrispenycate (Mar 14, 2014)

Psycho said:
			
		

> the fact that the moon's orbit is 5 degrees off eliptical


  Angled five degrees to the 'ecliptic' (the imaginary plane orthogonal to the axis of rotation of the sun, in which the orbits of the planets lie, except when they don't) not the elliptic (the shape of the orbit of just about everything, like a circle with two foci instead of one. With the Earth, the two foci are very close, making it almost circular, while a comet goes decades out of its way).

The gas giant, as it looks bigger in the sky than the Earth (that's all 'subtends a greater angle' means in this case) will produce a bigger shadow, more frequently per 'month' (as a moon makes one orbit of its planet in a month, OK?); if the orbit's divergence from the ecliptic is only a few degrees you could get a total eclipse every full moon - of differing lengths, but always there. But I can't drag one out further than hours, playing with masses, orbits, positions, whatever.


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## psychotick (Mar 14, 2014)

Hi,

Yes, used the wrong word. What else is new?

Actually if the moon angled at five degrees gives two full lunar eclipses per year each lasting several hours and a number of partials I would think the scenario would yield many more full eclipses and they would last much longer.

Cheers, Greg.


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## Wormtongue (Mar 14, 2014)

I admit I didn't read all of the responses.  I will just make a couple of observations.

The moon will not be in the planet's shadow for two weeks.  The closer to the planet or the bigger the planet, the faster the orbit.  The slower the orbit speed the farther from the planet or the smaller the planet.

To see this in action you can look at Jupiter's moon transit times.

But to save you the trouble the longest likely time the moon would be in shadow is less than five hours if you want to use genuine celestial mechanics.

There is a scenario that would allow a much longer shadow time but this would be an unlikely arrangement.  On the other hand, unlikely isn't really a hindrance for fantasy, is it?

If the moon was cycling through the L2 Lagrangian point it could be in shadow for weeks at a time.  Although objects at the Lagrangian points are typically not thought of as moons.

Plus, the planet would always be near the sun from the perspective of the "moon" and therefore you don't get the huge beautiful planet filling the night sky.


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## Inglorion (Mar 14, 2014)

I was once part of a World-design project on another forum, and I recall someone there who had great knowledge of astronomy who stated that it was impossible for a satellite of a planet to have a satellite of its own.  But I can't find that old thread now, and he might have just been talking about an earth-sized planet's moons or have been wrong.  However, if you already have a huge planet in the sky, and that planet's rings, do you really need a moon in the sky as well?  It would be overshadowed by the planet itself.


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## chrispenycate (Mar 15, 2014)

> If the moon was cycling through the L2 Lagrangian point it could be in shadow for weeks at a time. Although objects at the Lagrangian points are typically not thought of as moons.


 Because they are not actually orbiting the planet, right.

But, apart from 'Trojan asteroids', how would you specify such bodies?




> someone there who had great knowledge of astronomy who stated that it was impossible for a satellite of a planet to have a satellite of its own.


 Unlikely, maybe. But I can quite easily specify a stable orbit for a 'satellite of a satellite' (stable for a few million years, at least). I wouldn't like to do it for the Jupiter system, littered with moons, but on a simple 'one star, one planet, one moon, one sub moon' system I could specify orbits and make a reasonable explanation of how it condensed like that out of the original protoplanetary disc


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## pixallmighty (Mar 15, 2014)

I thought about what all of said, made some decisions and found what was important to me. I had a lot of ideas that were just cool, but weren't necessary: so I have some numbers to work with, and if I have to give up some ideas, than so be it.

-One day and one night last 28 hours.

-The diameter of the moon is between 4700 and 6300 km (smaller than what I originally said)



> However, if you already have a huge planet in the sky, and that planet's rings, do you really need a moon in the sky as well?



Maybe I said it wrong, but the moon DOES NOT have moons. As Psychotick said, they wouldn't be necessary.



> Plus, the planet would always be near the sun from the perspective of the "moon" and therefore you don't get the huge beautiful planet filling the night sky.



That is the point of having my story on a moon. In the end, it doesn't need to be a gas giant, but since my moon is more or less the size of Titan, I hope that something would be possible.

-For the eclipses I think a few hours are okay. If I must choose, I prefer to have fewer and shorter eclipses during a year, but to have the planet in the night sky.

-I problem I encountered was the moon's gravity, that I need to be similar to the Earth's. If I understood things correctly, that mean the moon's density, because of it's smaller size, would have to be two times the Earth's,. Would that be plausible? It wouldn't matter if the moon's gravity was inferior to the Earth's, maybe 0.9 G (?). 

-Humans must be able to live on the moon. 

I think it sums up all the things I really need; the rest is accessory. And as Chrispenycate said:



> the readers don't need to be told any of this. The ones like me will work it out for themselves, the others don't want to know.


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## Wormtongue (Mar 15, 2014)

chrispenycate said:


> Because they are not actually orbiting the planet, right.
> 
> But, apart from 'Trojan asteroids', how would you specify such bodies?



No, trojans do not orbit the body they are in synch with.  I suppose if a planet was at a Lagrangian point it would be a trojan planet.

My world has a trojan asteroid at the L2 point.  Obviously the inhabitants don't know what asteroids are, much less what the Lagrangian points are.  To them it's just a star that always rises at sunset and sets at sunrise.  They call it the Watchstar.


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## Ravana (Mar 15, 2014)

pixallmighty said:


> That is the point of having my story on a moon. In the end, it doesn't need to be a gas giant, but since my moon is more or less the size of Titan, I hope that something would be possible.



The Kepler mission has discovered numerous gas giants very close in to their stars… far closer than we previosly believed possible, as chrispenycate gestured toward. Whether or not the gases escape will depend not only on proximity to the star, but also on the mass of the planet, the amount of radiation coming off the star, the extent to which the planet's magnetosphere helps deflect said radiation… probably a few other factors not coming to mind right away. Above all else, though, the process still takes place over time, so you can simply put your system at a younger age and say the gases haven't all been blown away yet.




> -I problem I encountered was the moon's gravity, that I need to be similar to the Earth's. If I understood things correctly, that mean the moon's density, because of it's smaller size, would have to be two times the Earth's,. Would that be plausible? It wouldn't matter if the moon's gravity was inferior to the Earth's, maybe 0.9 G (?).



Plausible, or possible? Earth's pretty dense, as far as observed bodies go. It could be denser… if, say, the amount of lead in the planet far exceeded the amounts of iron or silicon. Also, density might be greater if the moon has fully cooled rather than having a molten core (and is thus smaller). I can't actually think of a way this would be _likely_ to happen–and I'm pretty sure that the younger the system, the less likely it would be–but that doesn't make it physically impossible. The universe is a strange place.

Note that a hyperdense moon might also imply a greater density at the core of the planet it orbits (if they formed at the same time, and the planet got its own greater-than-normal chunk of heavier materials), making it easier for the planet to keep its gaseous envelope longer.


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## Wormtongue (Mar 15, 2014)

pixallmighty said:


> -The diameter of the moon is between 4700 and 6300 km (smaller than what I originally said)
> 
> -For the eclipses I think a few hours are okay. If I must choose, I prefer to have fewer and shorter eclipses during a year, but to have the planet in the night sky.
> 
> ...



I don't know what your motivations are behind the size but why not just make it near earth size?  That's the easiest way to get the gravity and atmosphere you need for humans.

As far as the eclipses, depending on the inclination to the plane of the ecliptic the eclipses could be every day or only a few eclipses a year.  If only a few times a year they would happen in clusters twice a year.   The clusters would have a pattern of short, medium, long, medium, short.  The number of eclipses in each cluster could be any number you choose but both clusters would be about the same and it would not vary much year-to-year.


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## Wormtongue (Mar 15, 2014)

Wormtongue said:


> As far as the eclipses, depending on the inclination to the plane of the ecliptic the eclipses could be every day or only a few eclipses a year.



Can't edit my post.

What I meant to say is that the eclipses could be every orbit, not every day.  That could be as little as a few days or up to a month or more depending on the orbit.  It's not practical to have an orbit shorter than 3 days due to the extreme tidal forces and probability of being tide locked.  Plus the radiation close to a gas giant is lethal.

And speaking of tides, a moon around a gas giant is going to experience some spectacular tides.  Sea travel would be much more challenging.  Harbors as we know them would not be possible.  Think Bay of Fundy X10.


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## Ravana (Mar 17, 2014)

Wormtongue said:


> Plus the radiation close to a gas giant is lethal.



Possibly. Depends on how the planet's magnetosphere works: how strong it is, how far it extends, etc. Compare the magnetospheres of Jupiter and Saturn, for instance. Certainly, it's better to bet this way rather than otherwise, but very little is impossible, only less probable. Regardless of these factors, the moon's atmosphere will still shield the surface to a considerable extent… though the radiation might produce some interesting effects in the upper atmosphere. Global auroras, anyone? 

Note that not only will distance be a factor here, but depending on the inclination of the moon's orbit to the planet, the moon may pass in and out of the magnetosphere. Which itself could make a fascinating plot point… possibly even replace what pixallmighty had initially been hoping to achieve with eclipses.



> And speaking of tides, a moon around a gas giant is going to experience some spectacular tides.  Sea travel would be much more challenging.  Harbors as we know them would not be possible.  Think Bay of Fundy X10.



Will depend in part on the distance between the planet and moon: the differences between Io and Callisto, for example, are pretty dramatic (as are the differences of the effects from Jupiter's magnetosphere). Again, large tidal forces would be the default assumption, though.

Note that even Callisto is regarded as an "inner" moon of Jupiter: move farther out, everything drops off all the more. So ultimately distance will be an overriding factor.


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## pixallmighty (Mar 18, 2014)

Ravana said:


> Note that not only will distance be a factor here, but depending on the inclination of the moon's orbit to the planet, the moon may pass in and out of the magnetosphere.



Do you have some examples of things that could happen? You mentioned auroras, but do you know what would be the impact(s) on the moon of such phenomenon?


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## Ravana (Mar 21, 2014)

I don't know offhand what effects might be expected. Apart from the pretty lights, at least. So I'll speculate. 

One would expect a significant increase in ionizing radiation bombarding the upper atmosphere; possibly, some of the radiation might reach the lower atmosphere, causing… whatever such radiation might cause. Dramatic electrical storms, possibly even noticeable ground discharges (that is, ones that follow along the terrain, not ground strikes), if enough of the charge reaches the moon… perhaps channeled by the moon's (weaker) magnetic field toward its poles, creating differences in static levels between poles and other regions. 

The upper atmosphere might see a significant increase in the amount of ozone, which would help shield the surface but which could have follow-on effects such as greenhousing. Note that the ozone increase will be short-term, as it will break down into normal O[SUB]2[/SUB] over time; depending on how often the moon passes through the planet's magnetosphere, this may create cyclical effects–more likely if this only happens every few orbits, since otherwise, the ratios would probably remain fairly steady (it takes time for the ozone to break down). The normal ozone-oxygen cycle is caused by UV radiation, which both creates and breaks down ozone (yeah, it's strange); I'm not sure whether something similar would happen in this situation. Probably not… unless you can find a way for the magnetosphere to be concentrating UV rad from the star onto the moon. Which is probably possible.

Depending on how you set things up, passing through the planet's magnetosphere might be a respite rather than a bombardment, as solar winds are redirected by it around the moon. If the period within the magnetosphere is minimal (i.e. the moon's orbit is a distant one), this probably wouldn't be of much use as a plot point. If the period is large (due to a closer orbit, a larger magnetosphere, or both), and the moon only _left_ its protection occasionally, then you'd see an increase in bombardment of all radiation types emitted by the star. And the transition period, as the moon passes through the highly-charged outer edges of the magnetosphere, could be potentially dramatic as well.

Check the following terms for a start on what might happen: ionizing radiation, ionosphere, stellar/solar wind; perhaps cosmic ray. (Plus magnetosphere, if you haven't already.) After that, follow whatever links seem most promising. I imagine the _possible_ effects are pretty broad, depending on how you want to set things up. Unfortunately, you'll eventually start hitting heavy science, so if that isn't your thing, you may just want to find a few nifty potential effects, say they happen, and hope no one reading your book bothers to work through the science to find out where you cut corners.  Which, honestly, is a pretty safe bet.

Besides, this is fantasy, so there may be unexplained phenomena involved.


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