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Author Topic: Double Star planets?
Chessna
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I was thinking about putting a planet orbiting a double star as the setting in one of my books, but I don't know exactly how that would effect the planet. Would the orbit be a more pronounced elipse, people even be able to survive on such a planet? How would it effect how the seasons change and the people living on the planet?

Any information is greatly appreciated, and thanks for everything people!


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Jaina
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Others have done it and gotten away with it, most notably George Lucas--Tatooine orbits a binary star system. It's a hot, miserable world, but it's survivable. I have no idea how scientifically plausible it is, and I certainly don't recommend treating Lucas with any respect for the scientific plausibility of anything, because most of the stuff he has (lightsabers, carbon freezes, etc) is impossible. But it's something that you can take as a starting point.

A planet orbiting a double star would get double the light (I wonder if it would be possible to have one sun up while the other was down, making an eternal day?), double the heat, and the gravity pull from two stars would definitely do weird things with the orbit, though I can't tell you exactly what. I doubt that there would really be seasons in the sense that most of us think of them. Maybe a dry season and a drier season.

I'm just thinking out loud, here. Feel free to contradict me. I hope this helps a little bit, at least, though.


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Warbric
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Holly Ingraham speaks of this on her site, Suns, Moons, and Rainbows or There's Alien, and Then There's Silly.

The ability of the Coriolis Effect to influence the rotation of water going down a drain, however, is something that has apparently been debunked but the rest of it makes some sense to me, though I'm in the same boat as the Rocketman:

"And all this science I don't understand. It's just my job five days a week"


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wbriggs
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There are 3 ways you can have a planet in a stable orbit in a binary star system:

* Have the suns close to each other, and the planet far away. It'll orbit the center of mass, after all (although it will speed up some when some of that mass is closer. Just as satellites speed up slightly over some parts of earth). I don't know how far it would have to be, but considering it's getting twice the sunlight at any given distance, I'd think it could afford to be further out!

* Have the planet orbit one star, and the other one's pretty far out. Lots of binary star systems are fairly far apart. Sun #2 would look like a bright star.

* Put the planet in one of the Trojan points. I suspect this is unlikely, since Jupiter's Trojan points just have some loose rock, but who knows?

That's a cool link, Warbric!

[This message has been edited by wbriggs (edited August 06, 2005).]


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Chessna
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That really is a good website Warbric, and I enjoyed your comments, Jaina and wbriggs. Thanks! Any other information would be helpful.
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Spaceman
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Many of the points in that web site seem pretty obvious to me. Rob Sawyer has a planet around Alpha Centauri A that every million years or so changes to orbit Alpha Centauri B, causing a severe temperature drop and all life to go into hybernation for several hundred thousand years. the book is called "Illegal Alien."

In Star Wars, if I recall, the twin suns were pretty close together.


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Elan
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The world I'm creating has a triple moon. I wonder what affect that will have, geologically speaking, on the planet. I'm not sure where she found it, but my writing partner actually came up with a detailed calendar showing the daily phases of the three moons and from time to time we make reference to their phases.

I'm assuming the triple moon would make the tides stronger. Is there any additional affect that would be noteworthy?


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Robert Nowall
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Re: the Star Wars references...in the original novelization of the original movie, it was flatly stated that the planet Tatooine had been mistaken for a third sun of the system until somebody entered close orbit around it.

I don't know who actually wrote the novelization (it's credited to "George Lucas"), or who came up with that particular bit, but, really, mistaking a planet for a sun?


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rickfisher
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An earth-sized planet is (probably) unlikely to have even one Moon-sized moon. Three that size would be hard to swallow. In any case, they'd all be in different orbits, so if they were all Moon-sized, they'd look quite different to the eye. You could have them look the same by having them be different sizes, of course.

Probably, for believability, at least two of the moons should be quite small, so to show a visible disk they would have to be quite close, quite possibly closer than geosynchronous orbit, which would make them appear to orbit backwards (i.e., rise in the west and set in the east, like Mars's Phobos). The small ones would probably have little or no tidal effect.

Really, you need to figure out how large you need to have them look compared to the Moon, and then figure out how large that would require them to be at various distances, before setting up any tables regarding phases, or figuring out tidal effects.


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Spaceman
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By triple moon, I read it as three moons orbiting a common center of gravity, and that center of gravity orbiting the planet. If that is what you have in mind, it's a cool idea, but almost certainly unstable.

if, on the other hand you mean three moons, go for it.


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NewsBys
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(MAYBE SPOILERS - I can't really remember

If I remember correctly (it's been about 10 years since I read them), Frank Herbert wrote a trilogy set on a planet that had a binary star. The planet had horrible earthquakes and such extreme tidal action that it had little dry ground.
The only dry ground was protected from tidal action by sentient kelp, which the settlers destroyed, dooming them to live on floating cities and the ocean floor.
(Jesus Incident, Lazarus Effect, Ascension Factor were the books. If you want to read them, I suggest starting with Lazarus Effect, then Ascension and go back to J Incident)


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mythopoetic
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Well, Isaac Asimov wrote a book called Nightfall. It was about a planet with multiple suns. They alternated in various combinations of which were in the sky, but the effect was that it was always day. In the story, the inhabitants of this planet have a severe fear of the dark. It drives them crazy. Unfortunate for them is the fact that every thousand or so years all but one of the suns set and the last is eclipsed by an invisible satellite (Due to the fact that there are so many suns so close to the planet) thus creating night for nearly half a day or so. The book focuses on the psycological response of the inhabitants to this eclipse.
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Robert Nowall
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"Nightfall" is a very well-crafted story that I recommend for study to all students of science fiction. A matter of details: the deduction of an unknown dark body in the system, the confrontation with religious belief that carries a tradition of periodic darkness, the introduction of fear of darkness as a universal constant...all done in a manner that builds tension to an incredible point, then wraps things up in a few pages. (I'm talking the original Asimov story, not the Asimov / Silverberg novel.)

But I don't believe Asimov worked out the entire orbital mechanics of his six-sun system. I think he just came up with a few things he needed, and worked from there. (I doubt if I could do it, either, without serious help from someone.)


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Elan
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So here's the question: several of you have backgrounds well-grounded in physics and science. How badly are you affected when you come across something that is a minor point in the story if you feel it is improbable or impossible?

For example, my story with a triple moon. I envisioned three moons orbiting the planet, each visually of different size. They are refered to as the Mother, the Grandmother, and the Child and are viewed by some of cultures as a facet of the triple goddess.

There is no real plot reason to have this triple moon in the story, beyond giving the world an 'alternate other-world' feel, and to provide a vehicle to interject the idea of the goddess culture (which is a source of friction between religions).

If this is not believable, or real, does it ruin your ability to suspend disbelief? As I said, it's a minor point in the story. The characters look up at the sky from time to time, but if the price I have to pay for this little detail is too high, I may wish to rethink the premise.


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wbriggs
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When I come across an obvious scientific flaw, I do get disgusted; it kicks me right out of the story, and I'll only stay in if I was really liking it before then.

That said, I'm not sure a triple moon is a scientific flaw. It would depend on how big it was. Maybe in order to be small enough to be stable, the dots of light would be too close together to distinguish?

Or: how are these moons related? I'd suppose it's one major moon, and then two minor moons orbiting it. Pretty close.


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Elan
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It hadn't occurred to me that the WAY these moons orbited was important. I hadn't planned on developing the concept that far. I have envisioned that the characters will be able to discern the phase the moons are in, whether they are crescent, full, or new. There will be cultural superstitions based on "no moon nights" which will be rare occasions.

Beyond that, I figured it was irrelevant. NOW I'm nervous that I've committed a scientific faux paux!

I might add, the story is fantasy and not science fiction, so I hadn't thought about holding this concept up to the same scientific rigors I might have, had it been a sci-fi tale. Clearly an error in my thinking, as many science nuts also enjoy reading fantasy and I don't wish to lose them, based on making a stupid mistake of probability.

[This message has been edited by Elan (edited August 10, 2005).]


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rickfisher
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When I come across a scientific flaw that would keep the story from actually working, I get really disgusted. When I come across one that the story doesn't really depend on, I just sort of laugh it off. It pulls me out of the story, but it isn't a killer.

But you know what? The odds are high that you'll have some errors no matter how hard you work not to. So don't let something that you don't feel sure about slide, thinking that it will be the only thing. It won't. Find out for sure. For example, if you want all three moons to have visible disks, you can't have one of the be the size and distance of Mars's Deimos.

Earth's moon, by the way, is only about the apparent size of a green pea held at arm's length. (It seems bigger, doesn't it?) Your moons could have a substantially smaller apparent size and still show a disk. But you should figure out the sizes you want, and some combination of actual size and distance that will give you that apparent size, and from the distance figure out the period of revolution for each (at least if you want to do the calendar thing with their phases).

[This message has been edited by rickfisher (edited August 10, 2005).]


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Elan
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lol.... it is clear to me that there is a wide gulf separating us, rickfisher! I am no more able to calculate the moon's orbits than I would be able to fix the engine of a car. (Which, means zero ability.) Nor do I really want to. I've got a chart that randomizes the moon phases. My understanding is that the biggest problem in fiction is the writer who seems to think the moon(s) are full each and every night of the year. The story makes only brief mention of the moons in the entire novel-sized draft, and each time the moons are in different phases.

As long as it's POSSIBLE to have three moons, I'm not sure how the reader would benefit from me spending time making scientific calculations that have only a marginal success of being right. After all, I never got past basic algebra in school. (Too busy in art class.)


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rickfisher
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Ha! Well, it wouldn't really be that hard, if you assume the planet resembles Earth. For relative distances, you could (for example) stick a dime, a nickel, and a quarter to a wall (or a window if you want to see them against the sky) and back up until they seem like the right size, and measure the distance. Then if an actual moon was to be (say) 10,000,000 times that far, it would have to have 10,000,000 times the actual diameter. It's pretty easy then to pick three reasonable numbers (like, each moon being double the distance from the planet of the previous) and from that you can calculate the actual sizes.

If you do decide to do that, tell me their distances and I'll tell you their orbital periods. This isn't terribly important unless you want to try figuring out tidal effects, though. But if you make the ratios of their periods (full moon to full moon) in the ratio of 1:2.8:8, you'll have a reasonable system.

quote:
My understanding is that the biggest problem in fiction is the writer who seems to think the moon(s) are full each and every night of the year.
Yeah, and another impossibility is when a cresent moon is high in the sky at midnight, or a full moon near the horizon at midnight. And if two moons are near each other in the sky, they will be in the same phase; if they're far apart, they won't.

[This message has been edited by rickfisher (edited August 10, 2005).]


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webm0nster
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Kind of depends on your audience to a degree I would think. I believe the Dragonlance world has white, red and black moons. I doubt people reading Dragonlance quibble a whole lot about flawed physics. In fact in fantasy, who is to say you can even nitpick physics at all? On the other hand if the book is hardcore science fiction you may be in trouble with your audience.
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rickfisher
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Fantasy readers are a lot less likely to quibble over bad science, but a lot of SF readers read fantasy also. And, although it's quite true that in fantasy you can make the laws of physics anything you want, it's assumed that anything you don't explicitly change works the same as in the real universe--and if you do change something, you have to be consistent. (For example, if you change the rate the a moon at a given distance orbits the [earth-sized] planet, then you're changing the gravitational constant, and you also have to change the rate at which your warriors accelerate as they fall off the castle walls.)

As I say, in Elan's case it depends on whether she wants to deal with tides, and have a believable calendar. If neither of those things are important to the story, and the phases of the moons are not mentioned enough that the reader can keep track of them, then it doesn't matter. But Elan, if you WANT to know which moon has the most effect on tides, let me know and I'll help you figure it out.

[This message has been edited by rickfisher (edited August 10, 2005).]


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Survivor
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People who can't fix car engines...can someone like that really exist?

If you want to create a working planetary system and see what the stars and moons would look like from the surface, I recommend Celestia. It doesn't have any restrictions to let it calculate orbital velocities, you just have to do that on your own. But it isn't that hard, once you make up masses for your stars and planets (or you could just assume they have the same masses as more familiar objects).


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franc li
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Spaced Penguins at bigidea.com gives some interesting explorations of multiple star orbits, and the like. Though in a case where two stars were close enough to setup up a figure 8 orbit, they would tend to pull into one another. Unless there was some other orbital force keeping them apart. Dunno. I guess a set of moons could be orbiting a large star so they are all in orbit around the big star and the planet is in orbit relative to the moons. It would be a 3 d orbit instead of a 2 d orbit.
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Spaceman
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Or if you want to spend a lot more, you can buy Starry Night Pro Plus. I bought it because I need to determine distances between different stars and know their spectral class. It'll drive a telescope, too, if you care.

As for scientific goofs, it's irritating, especially if it's something obvious like starting a fire with a near-sighted person's glasses or having a geosynchronous polar orbit for a satellite. (concave lenses disperse, and the closest you can get to a polar geosynchronous orbit is what the Russians use, a Molnya orbit.)


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Elan
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Oh, I can't resist. Rickfisher, if you are willing to make the calculations, I'd LOVE to know that I have that little detail right in my world. No one will ever care but me, but it always gives me a feeling of satisfaction to have things as accurate as possible.

Here is the calender we've BEEN using in our story. I have no idea where the original calendar came from, only that the player who developed this plucked it off the net somewhere.
http://www.a2zgorge.info/kaldara/Calender1.htm

The theory in the story is that the planet they are on, Obara, has a 36 day cycle. There are 432 days a year, 36 days a month, 6 days a week, 36 hours in a day. (Six being a sacred number).

None of those details are particularly significant in the story, except the days of the month which we've tracked carefully as events unfold.

The three moons are referred to as the Mother Moon (primary moon and largest), the Wife Moon (secondary moon, middle sized), and the Daughter Moon (third moon, smallest moon) within one of the cultures.

This is about all the detail we've come up with. I've had in mind the Daughter moon would still be large enough to see the phases, but very small to the eye.

I'll be intrigued to see what calculations end up looking like for our moons, and having some idea of how intense the tides would be would very possibly factor into my story at some point. There is an underlying theme of pending ecological disaster in the story, and I'd love to build greater emphasis around that thread.

Thanks for any help you can give me!!


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Spaceman
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If everything relies on the number 6, why 432 days in a year? 420 days at least adds up to six when you add the digits together.
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rickfisher
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Elan, I looked at your calendar and see two major problems. The first is this thing about 6. It works out TOO WELL. Look at Earth: There aren't an integral number of days in the year, or days in a lunar month, or lunar months in a year. Things just don't work out that neatly. To have it work out with 3 moons is absurd. I would be willing to buy the number of days being a multiple of six (but with occasional need for a leap year [or maybe a "stumble" year, when a day is dropped instead of added]). I would also buy an official "month" lasting exactly 36 days, but not that any of the moons have exactly that period. One of them might have a period between 35.5 days and 36.5 days, but NOT exactly 36. The other two should not come in multiples of 6. It's just too unbelievable. Remember, the moons will not have cycles that are multiples of 6 just because the people think it's a holy number.

Second, these moons are way too close to each other. If they were little captured asteroids, it might be possible, but they would never form this way (its dubious that any moons this size would form around an Earth-sized planet--our Moon seems to be a bit of a freak--but we'll ignore that). If moons this size did exist in orbits this close, they would quickly find some new equilibrium, either by crashing into each other or ejecting at least one of them from the system.

If you're willing to redo it, I'd suggest this: Stick with the largest (in appearance) moon being the (approximately) 36 day one. Give the smallest one a cycle of about 13 days, and the medium one (in appearance, but actually the largest) a cycle of around 100 days. Since the days of the month would still be numbered from 1 to 36, this might not require any change in your calendar dating. But there is no way you could predict the phase of any of the three moons just from knowing the day of the month (although the main moon would change only slowly from month to month).

If you're locked into the current system, you'll just have to live with having knowingly included an unbelievable setup (but one that most people won't think twice about). If you're willing to (and want to) change it, let me know and I'll proceed.

By the way, in your current system, your "rare" moonless nights (expect several in a row each time it happens) would occur four times every year (so, maybe twelve such nights each year). Is that the kind of rarity you were after?

Survivor said:

quote:
People who can't fix car engines...can someone like that really exist?
Oh, yes, we exist. I'm not sure I even believe in car engines. Someone told me once that each car had one under the hood, but I've always been afraid to look.

[This message has been edited by rickfisher (edited August 11, 2005).]


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EricJamesStone
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> Remember, the moons will not have cycles
> that are multiples of 6 just because the
> people think it's a holy number.

No, the people think it's a holy number because their god designed the moons to have cycles that are multiples of six.

In science fiction, having the numbers work out into nice multiples is extremely unlikely, and shouldn't be done unless you can provide a scientific explanation. But in fantasy you can have such things happen without much difficulty.


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rickfisher
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Well . . . I wouldn't believe it in fantasy, either, without an explanation. I'd accept: "The gods made it that way" as an explanation, though--as long as the gods are real forces in the story. But I wouldn't believe it if it's a matter of religious belief in gods that seem to have no real presence in the fantasy world.

[This message has been edited by rickfisher (edited August 11, 2005).]


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Elan
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quote:
If everything relies on the number 6, why 432 days in a year? 420 days at least adds up to six

There is a correlation between this calculation and the "D" I got in 9th Grade Algebra.

I am not married to the calendar the way we have it. I'd love to have it reworked so that it's accurate.

The only thing the story needs is for the size of the triple moons to reflect small, medium, and large, and for the phases of the moons to be visible. I would LIKE to keep the 36 days per month, 12 months per year, six days per week. (Our "new year" starts roughly equivelent with November 1, so the end of the old year is the end of harvest season). Adding in leap years/leap days would be fine.

The fact that the number "six" is sacred is relevant to the story... there are six castes in the society, which ties into the creation mythology we are using. The lunar cycles would create justification for the "six" as a sacred number.

If you can make a pie out of that mish-mash of ingredients, I'd love to see what you can come up with!!

[This message has been edited by Elan (edited August 11, 2005).]


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EricJamesStone
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> Well . . . I wouldn't believe it in
> fantasy, either, without an explanation.

For me, it would depend on whether the fantasy seemed to be "hard" or "soft."

I recently wrote a story in which the sun is carried around the world on the shoulder of a glass giant. That's not just what the world's inhabitants believe, it's actually true (in the story, of course.) There's no explanation of the giant's origin, etc. It's just the way the world is for the purposes of the story.

Luc Reid has a marvelous story in WOTF XIX, "A Ship That Bends," which takes place on a version of Earth where the world is flat, and there are people living on the other side. The story contains no scientific explanation for how gravity works in that situation -- which is one reason why it's fantasy instead of science fiction.

Sometimes I think some people have gone a little overboard in demanding rigorous explanations of the fantastic in fantasy.


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Kathleen Dalton Woodbury
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quote:
Yeah, and another impossibility is when a cresent moon is high in the sky at midnight, or a full moon near the horizon at midnight. And if two moons are near each other in the sky, they will be in the same phase; if they're far apart, they won't.

I can't emphasize this enough because writers getting the phases of the moon(s) wrong is one of the things that drives me crazy in stories.

Maybe I should start a topic about it.


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Elan
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quote:
The story contains no scientific explanation for how gravity works in that situation

It is a well known fact that gravity sucks. Thus, denizens will be held to the surface of the world no matter which side they live on.


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Elan
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quote:
Maybe I should start a topic about it.

Seems like we already have a good topic rolling on this one. Perhaps adapt the topic header to: Double Sun, Multiple Moon . This is becoming an informative thread, particularly due to the links and knowledge of our more scientifically inclined friends.

People like me are simply trained monkeys when it comes to orbital physics and all. I do what someone else tells me works. I have been trying to account for the phases of the moons in my story, but it never occurred to me that there are people out there who actually calculate this stuff for FUN!! Me, I'd rather be beaten with a whip than to do all that math...


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EricJamesStone
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> Perhaps adapt the topic header to: Double Sun, Multiple Moon

This is now becoming very familiar ground for me.

In July 2003, in the Literary Boot Camp 2003 private forum, we had a topic called "The Two Sun, Two Moon Question."

The discussion in that topic eventually reached the point where I posted this:

quote:
What if nightfall came only once in a thousand years? Woops! That one's been done.
Actually, what Isaac Asimov did in "Nightfall" is instructive. Now, I haven't read the story in years, but I don't recall him going into complicated descriptions of orbital mechanics. I have no idea whether it's possible for a habitable planet to exist in a stable orbit in a system with six or seven stars, so that on rare occasions only one star shows in the sky and, by coincidence, a low-albedo moon that no one has seen before will cause a lengthy eclipse of that one remaining sun. (And do it at regular intervals of over a thousand years.)

My point is that you can probably just assert things like "The twin moons eclipse the twin suns but once in a generation" or "Once the second sun rises it will be too hot to continue traversing the desert." Make the astronomy conform to the needs of your story, not the other way around.

Unless it's absolutely necessary for the story, you needn't goo into too much detail about the astronomy. Just let us know there are two suns and two moons, and only give us details as needed. Since your solar system is so different from ours, chances are you won't make a mistake as bad as they did in the movie Ladyhawke, in which a solar eclipse takes place the day after a full moon. (Absolutely impossible.)



To which someone replied:
quote:
But, Eric:
Its Maaaaaagic!


My reply:
quote:
I can believe that a man can turn into a wolf at night and a lady can turn into a hawke at night. Lustful, corrupt priests have the power to do that.

But for the moon to be full one night and to eclipse the sun the next day, that would require the moon to move through half of its orbit in about half a day.

Consider that the radius of the moon's orbit is approximately 400,000 km. That means that half of its orbit would be pi * 400,000, or about 1,256,000 km. (Of course, you could shave some of that distance off by taking as close to a straight-line course as possible, having the moon scrape along the surface of the earth for a bit. But that would cause some major problems of its own, so it's perhaps better to follow the normal arc of the orbit.)

Now, the mean orbital velocity of the moon is 1.023 km/s. In order to get the moon around half its orbit in twelve hours, you would have to increase the mean orbital velocity to 29.074 km/s.

Leaving aside the fact that this is far above the escape velocity needed to permanently leave earth (see "Space: 1999"), since you have to both accelerate and decelerate the moon, the most efficient way would be to accelerate for half the distance and decelerate the rest of the distance. So you will actually be getting the moon up to a maximum speed of 58.148 km/s.

Thus, the rate of acceleration needed is about 1.3 m/s^2.

Fortunately, that rate of acceleration is not very high (For comparison, acceleration due to gravity here on earth is 9.8 m/s^2.)

Unfortunately, the mass of the moon is 73,490,000,000,000,000,000,000 kg. Which means the total amount of force needed to accelerate the moon is about 56.53 * 10^21 newtons. Multiply by two because you need the same amount of force to slow it down and you get 113.06 * 10^21 newtons.

(Which is a lot, considering Newton hadn't even been born when Ladyhawke is supposed to be taking place.)

To then find the total amount of energy (joules) needed to move the moon, we multiply the force (newtons) times the distance (meters.)

113.06 * 10^21 newtons * 1.256 * 10^9 meters = 142.00 * 10^30 joules.

The explosion of a one megaton nuclear warhead corresponds to 4.185*10^15 joules.

Therefore, the total amount of energy needed to move the moon into the correct position for the eclipse would be equivalent to the explosion of about 34 trillion one-megaton nuclear warheads.

Of course, it doesn't happen all at once. Spread out over the twelve hours, it's only like 787 million one-megaton bombs going off every second. Or, if you prefer to think of it this way, one 787-teraton bomb per second.

Now, obviously getting the necessary energy from nuclear fusion is rather inefficient. I will leave it as an exercise for the reader to determine how much matter would need to be converted directly to energy in order to accomplish the task. (Hint: E=mc^2)

We're talking some pretty powerful magic.

--Eric

P.S. I think I did the math right. But I wouldn't guarantee it.


It was that post which led me to write "The Man Who Moved the Moon," my second story sale.


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Survivor
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Well, Celestia won't drive a telescope for you, but it will resolve problems of creating realistic moons and all.

By the way, Elan, 432 does divide by six, twelve, thirty-six, seventy-two, etc. The problem is still that it shouldn't. Unless the god's just made it that way (and are keeping it that way too, tidal effects slowly change the length of the day on any planet with significant tides). But if the gods are keeping the moons like that, then there is no problem. And no need for science to play a part.

By the way, it is obvious that the original story of LadyHawke must have involved a lunar eclipse...during a midnight mass. That's why the full moon was visible the night before. They just changed it later because the image of a great big werewolf tearing bunches of priests to bits in the middle of a mass was too disturbing


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Spaceman
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quote:
In science fiction, having the numbers work out into nice multiples is extremely unlikely, and shouldn't be done unless you can provide a scientific explanation. But in fantasy you can have such things happen without much difficulty.

It is also very unlikely to have a moon with a disk the same optical size as the star. But we have that.


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rickfisher
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I can get you something that will leave you with 6 days/week, 36 days/month, and 12 months/year without too much trouble. (And of course the hours/day is totally dependent on the human element.) If you haven't heard from me in 36 hours, 6 minutes, and 12 seconds, email me.

By the way, EJS, I agree completely that you can posit anything you like in a fantasy story, as Luc's story and others demonstrate. But in those cases, the posited change from reality is the point of the story. "What would happen if the world were flat?" sort of thing. But if the story is about quests or battles between mythical kingdoms, and there just happen to be three moons that behave in unbelievable ways, that's when I think an explanation--whether rational or magical--is needed.

[This message has been edited by rickfisher (edited August 12, 2005).]


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Elan
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I thank you for your efforts on my behalf, rickfisher. If you ever need similar advice on making gemstones into necklaces, or writing metaphysical mumbo-jumbo, I'd be happy to help.

That's what I love most about this forum... and about critiques: having the eyes of other writers spotting potential issues before they become a public embarrassment. You guys rock!


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Mechwarrior
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I have to agree with some of the posts mentioning 'fit the astronomy to the story and not that story to the astronomy'. Unless the orbits and the suns and planets are a major part of the story I'd say use some artistic license. Anyone that feels it necessary to work out the orbital physics should be prepared to simulate at least a few hundred million years. Otherwise, someone just as obsessive will come along and say, " by my calculations, the orbits of moon X and Z would collide after 1.2 million orbits. Assuming a short 1 billion years to evolve intelligent life the moons should have created a ring of debris/collided with the planet/spun off into space/been eaten by giant space rats......."

We all know something that when handled poorly in a story will kick us out.

Please disregard all comments when applied to hard SF. The day Gregory Benford puts flying space monkeys from the 7th moon of Fizzywinks in his books is the day he loses a fan.


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Spaceman
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You can always respond with chaos theory. The critic's initial conditions were not entered with sufficient decimal places.
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Mechwarrior
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LOL. Whatever happened to chaos theory? It had a big heyday in the late 80's then seemed to fizzle out. I always liked the concept of "strange attractors", sounds so much like a lot of my old girlfriends.
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Survivor
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It became a standard part of high-definition mathmatical models, and thus was no longer a separate field.
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Spaceman
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High definition? I ought to invest on one of those. I'm still using an old black-and-white mathematical model with rabbit ears.
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Mechwarrior
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Score: +2 to Spaceman. Funny stuff.
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yanos
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EJS, why are you assuming it was the moon that moved? Now I want to see calculations for the sun changing position in one day, and then for the earth to change positions.
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Survivor
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The calculation for the Earth moving is the same as for the moon, just with the mass of the Earth plugged in where the mass of the moon went in the other equation.

If you want to do it with the sun, then...that's different. Very different.


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yanos
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As for simulations, I used to work on computerised solutions to fluid dynamics problems. Take enough steps, include enough parameters and variables, and voila! Floating point error. You can't avoid inaccuracy in such things. And the small errors magnify over the number of steps you do. So, just like life, the more perfect you want your solution the less likely you are to get one.
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