My problem is this, while very "fantastic" element of my story is explainable through some kind of science, I do not have th eenvironment operate the way it probably should. How far can I go before readers will notice this and begin to complain, let me give you an example.

The story is set on a moon, geographically much smaller than the earth. This moon revolves around a large and rocky plannet.

My moon operates with a 24 hour day and 364 day year, which is clearly would not, but I do this to help the reader relate to time that is pasisng and so forth. I also ignore the fact that the planet would probably eclipse the sun on a regular basis. I tend to treat the planet like a "big moon," similar to Earth's moon but larger, and otherwise have the place act mostly like Earth would.

Though the naturally evolved flora and fauna are somewhat different and certainly more limited.

And there's nothing to rule out a 364-day year, if by year you mean the time it takes for the moon (and its planet) to revolve around the sun. (If you mean the time it takes for the moon to revolve around the planet, then there are problems.)

That's about as much as I can offer. I'm not a big fan of hard SF, and I couldn't write hard SF for the life of me. If you're worrying about orbit, length of days, plant life, etc., then it sounds to me like you're writing hard SF. Though I don't know much about hard SF, I know that your fantastic setting needs to be grounded in fact.

It sounds like you might have to spend some time in the library.

"My moon operates with a 24 hour day and 364 day year"-- I feel like right here, you've lost my suspension of disbelief. There's no reason this can't happen. But on the other hand, what are the odds against this? Even if it does happen here, why should we believe you? Uncomfortable with anything else to show time passages we can relate to? Well, look what OSC does in <i>Xenophobia</i> with the longer days on Path: the "righteous labor" on the longer day becomes a cultural componant in its own right. Or the Pern series and "Turns", etc. You can even tell the reader how many Earth years to a moon year.

"geographically much smaller than the earth." Once again, you're going to lose my suspension of disbelief unless you're careful about it. Volume and thus density and thus *total mass* all vary with size, and smaller sizes can mean much smaller total masses because of how you calculate volume. So right away, unless your planet has a super-dense (and I mean super-dense) core, which is unlikely, you're going to have lower surface gravity. You're going to want to address this somehow if you mention the size thing.

Again with the mass: if the mass is that much smaller than Earth, how is it holding onto its atmosphere? Mars, well, doesn't really have much, and I'm guessing your moon is smaller. (It's probably also too small to have much in the way of active tectonics-- again with the Mars comparison-- and thus no carbon cycle, but that's one you can probably ignore.)

Moving on with rotation and orbits: Is your moon tidally locked (like Earth's), or does it rotate independently? When do the eclipses occur? This isn't something I feel you can convincingly ignore: it's going to affect how much sunlight arrives where, and therefore the course of evolution on this planet, where things can survive and grow, society, climate, weather, everything. On a tidally locked moon like ours each day can be two weeks or more, and that's certainly different from a rotating moon.

Your moon's going to have to deal with tidal stresses, which will depend on the planet it rotates around (see below) and how close it is. Look at Io: tidal stresses produce the heat to regularly resurface the entire planet. This kind of thing could make a difference.

"This moon revolves around a large and rocky plannet." Well, this is probably pretty rare. I know we seem to be an example-- but current opinion holds that the Earth-Moon system was very very unlikely, having resulted from a major collision late in Earth's formation. We're the only terrestrial (rocky inner) planet with a large moon, and that moon seems pretty depleted in iron (and thus mass) as a result of the formational collision. Your moon would be more likely (at least from what we know now of extra-solar planets, which is isn't much) if it orbited a gas giant. Of course, gas giants can't form in the innner solar system-- the volitiles all get blasted out to the outer system in one of the later stages of stellar formation-- but we see them in the inner systems in some of the extra-solar systems, which we think means they migrated in. Your planet could have migrated in and picked up the rocky moon on the way. This also would let you have a much bigger moon-- maybe the size of Earth or Venus-- which would solve the mass-related problems.

And you probably want your planet in the inner system, because otherwise you'll have to deal with very low insolation, low energy input from the sun, and it's pretty hard to justify plants or humans living out by Jupiter (or equivalent) or so without space suits.

I'm not trying to pick things apart to be annoying. I'm picking things apart because if you write science fiction, you're going to have people like me-- people who like hard science-- in your audience, and they're probably going to see the things I just said. Maybe they don't agree with, say, Hoffman's findings on the Snowball Earth, but they're still going to be willing to suspend disbelief for it. It's when you haven't looked far enough to read about Hoffman that they're going to be much more likely to fling the magazine against the wall.

I just finished the first third of Gene Wolfe's "The Fifth head of Cerberus," and Wolfe very casually lets the reader know that the planet the first story takes place on has a much longer year than earth's, but since such knowledge doesn't effect the story it is not elaborated on.

The setup is very important for the actual plot. The planet it orbits is most like Venus, volcanic--rocky, not espceially large but sizable compared to the moon, which is somewhat larger than earth's moon.

The reason it is set on a moon is to serve a number of purposes:
-tie together the planet and the moon
-explain the small and limited geography
-explain the absense of nuclear weapons in a superior technological environment. (they don't want to screw up their orbit and the effects of fallout would mean problems...)

There are other reasons so the setup is really non-negotiable. I can; however, change the time referencing, it isn't too late for that.

I was merely wondering if I could get away with it, there are lot of things that are "incredible," in my world, but I prefer it that way.

Based on the sun, the time period of day (one rotation of the moon on its axis, appearing to make the sun rise, set, then rise again) would be used. The time period of a year (measured by the sun's position relative to the stars that are close to the horizon prior to sunrise) would probably be used, especially if the moon's axial tilt leads to seasonal changes. (Seasons would be another unit of time.)

Since the largest object in the sky would be the planet, its motion through the sky would have phases that might be very significant in terms of measuring time. Assuming a similar orbital period as our moon had around Earth, your civilization could have a time period similar to a month, and based on moon phases might have a time period similar to a week.

Some civilizations here on Earth used a lunar calendar instead of a solar one, and with the planet being larger in the sky of your moon than the Earth's moon is in our sky, I think it more likely that their calendar would be based on the planet's movement through the sky instead of on a solar year.

As I mentioned in my earlier post, solar eclipses would not necessarily be very frequent, so while they might have cultural significance, I do not think the occasional reduction in sunlight would have much effect.

However, if the planet and moon were close enough that the moon went into the planet's shadow frequently and for long periods, then it would have an effect, and the amount of sunlight reaching the planet at other times would have to be higher.

As for the question of whether a day of about 24 hours and a year of about 365 days would be too coincidental, I think the anthropic principle applies:

1. For life similar to that on Earth to develop, the planet (or moon) would probably need to rotate on its axis in a period of time reasonably similar to one Earth day.

2. For life similar to that on Earth to develop, the planet (or moon) would have to orbit its sun at a distance that would give it about as much sunlight as Earth receives from the sun. For a star similar to the sun, that would be around 93,000,000 miles, and the orbital period would be somewhere in the neighborhood of 365 Earth days.

It may seem like circular reasoning (and I guess, to some extent it is), but a place where life similar to ours could develop would have to be a place similar to ours.

Which brings us to the major problem with your setup: gravity.

"-explain the small and limited geography"
Maybe the planet has a far higher ocean to land ratio. Maybe the planet's land mass is almost entirely achipelagos. Maybe the planet is further away from its star, and only the equator is warm enough to be habitable. Maybe the planet has other large continents, but they're inhabited by imicable sentient fungus or the long-dead former alien masters polluted them beyond livability.

"-explain the absense of nuclear weapons in a superior technological environment. (they don't want to screw up their orbit and the effects of fallout would mean problems...)"

I found this confusing. First, I feel like the effects of fallout mean problems on Earth, and we still have bombs knowing full well what happens when we use them.

Second, there's a staggering amount of intertia built up into the mass of a planet. To give you an idea: The largest bomb in the US arsenal was around 25 megatons (refrence). The Chicxulub impact crater-- the one that killed off the dinosaurs and most other living things-- was 100 *million* megatons (reference). And the Chicxulub crater killed off most living things and made the Earth all but uninhabitable for quite a while-- without significant orbital perturbations. Nuclear bombs or comets mean quite enough difficulty without worrying about the orbit.

Remember, the Earth - moon system has a pretty large distance of seperation, and the moon is incluined about 5 degrees from the ecliptic. The result: lunar eclipses do not happen every month.

If Alias inclined his moon just a little bit more, centuries or millenia could go by between eclipses.

I've heard several science fiction writers, who've been making a living at their writing for quite a long time, say more than once that except for the things that are more or less taken for granted in science fiction (like faster than light travel in space opera, for example), a writer should only have one or two "impossible" things in a story and let all of the rest of the stuff be "possible" (or at least feasible).

Of course, I've also heard writers, who have been making a living at science fiction for a relatively short time, say that nowadays you have to take into account the fact that if one "impossible" thing is accepted in a story, there may be other "impossible" things that would happen because of the effect on the world from the first "impossible" thing. They point out that you don't change reality in a vacuum, and that if you change one thing, that's going to change other things.

So, you might want to ask yourself, first of all, how many "impossible" things do you think you can get your readers to accept; and second, how does one "impossible" thing connect to and cause the other "impossible" things. The more you manage to connect them causally, the less trouble your readers may have in accepting them.

Don't just throw a bunch of "impossible" things into a story because they'd be cool to have there. They need to be there for a reason.

(This isn't officially Card's law, but I've heard him say this enough times that it ought to be: Everything in a story should have a really good reason for being there. If the story works without it, get rid of it.)

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Remember, the Earth - moon system has a pretty large distance of seperation, and the moon is incluined about 5 degrees from the ecliptic. The result: lunar eclipses do not happen every month.

If Alias inclined his moon just a little bit more, centuries or millenia could go by between eclipses.

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Not only would light gravity be fun, but (assuming an atmosphere, liquid seas, and a heavy mass of the primary planet)

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Thank you for pointing out the obvious, which I clearly missed. Well light-ravity wouldn't be consistent to the already set environemnt and how it functions, so it there a science-bull way of pulling myself out of it?

Would a super-dense planet core be feasible, I know density plays an influence on gravity and that in truth we, scientists, don't really understand what gravity as. As spoken best by Newton, "I cannot feign a hypothesis,"

The bad news is that it would need to be made up mostly of platinum and/or iridium.

With a body the size of Saturn's moon Titan, a core of mainly tungsten carbide would do. A mercury core would get you to 99% of Earth's gravity.

You can play around with various densities and radii for your moon here:
http://www.ericjamesstone.com/weird_stuff/gravitator.htm

By "non-exotic materials," I meant there would be no need for the "super-dense" materials Alias mentioned as a posibility.

I believe that the general geography, planet's relationship with its solar neighbors, and the fact it has a mercury core, (or other extra-dense material) is enough to set it aside.

But I would suspect that if enough of the planet were made out of a very dense and conductive metal (including iron), the magnetic field would tend to be quite weak. I suppose the only thing to do is try it (and give me the planet when your finished )

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Fortunately, since my story focusses most heavily on the story and is not a scientific text, there are many things I can ignore.

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That doesn't mean that you have to get the science right for all time, but it does have to be pretty convincing to any audience that would care whether or not the story is happening on a moon.

And that audience is here, now, telling you that to convince them, you have to use a bit of science.

The other audiences simply won't care that this is set on a moon, for the very same reasons that they don't care how is can be set on a moon.

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I recognize that there's a lot of people who don't care about the science. We're just saying that these are things the hard sf audience will care about if you mention enough to get it wrong.

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People can still enjoy something they can call you on for scientific accuracy, but why would you want to be called on the accuracy in the first place?

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The big quibble in Star Wars, of course, is "there is no noise in space". How did GL get away with it?

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But the real answer to Wraith's question has already been made -- he wasn't writing hard SF, but soft SF with elements of fantasy. (I suppose you'd call that sceince fantasy, but I'm not sure what that means.)

But the other issue -- not the issue of scientific accuracy, but the issue of garnering a large following -- is even easier to explaing. Star Wars is a very good, highly entertaining, story. Sure, we can quibble about how bad the prequells are, but they didn't spark the excietment. Star Wars and The Empire Strikes Back did. (Jedi more or less rode the coat-tails of these movies.) And it seems to me that the vast majority of fans care more about the story than about scientific accuracy or plausibility. Even Survivor has admitted to liking Michael Crighton's work, whose science, according to Survivor (I wouldn't know a thing about it), is suspect.

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And it seems to me that the vast majority of fans care more about the story than about scientific accuracy or plausibility.

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Ah, so that there would be time for a space battle to destroy the Death Star after it dropped out of hyperspace and before it blew the rebel base to bits...remember?

The second Death Star battle also takes place near a green moon for purposes of extending and refining the imagery of inhospitable giant planet, smaller life bearing planet, and Death Star. But that was possible because we already have a clear story reason for the first Death Star/green moon/gas giant combo.

Eric has a point about the noises, I always thought that it was just interference caused by the engines and blasters...but a deliberate simulation would make sense too.

Back to my point, though.

If you don't have a clear reason to set this on a moon (and so far your reasons have been flimsy at best), then don't do it.

I know, you say you have a good reason....

I don't really have a strong opinion about how scientifically and technically accurate your fiction has to be, but this Star Wars discussion jogged an interesting idea:

As you're reading published Sci-Fi stuff, look specifically for the science that isn't explained, but is believable. Is it because the descriptions are vivid, the characters well developed - and they believe it?

For example in the Ender series, ansible communication is absolutely critical - yet OSC never even tries to explain how it works, it's just there, a relic from Bugger civilization.

This might even make a good seperate thread:
Actual examples of believable, yet unexplained fantasy science.

--edited (twice) to make italicswork.

[This message has been edited by danquixote (edited March 19, 2004).]

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That doesn't mean that you have to get the science right for all time, but it does have to be pretty convincing to any audience that would care whether or not the story is happening on a moon.

And that audience is here, now, telling you that to convince them, you have to use a bit of science.

The other audiences simply won't care that this is set on a moon, for the very same reasons that they don't care how is can be set on a moon.

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I was skimming, like you, and didn't notice you'd made the same point earlier.