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.

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:

* 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!

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.

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).

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?

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.

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).

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.

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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.

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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.

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.

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:

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

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

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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

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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!!

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.

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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.

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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.

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

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Maybe I should start a topic about it.

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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.

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:

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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.)

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But, Eric:
Its Maaaaaagic!

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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.

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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.

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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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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.

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