Are you just trying to determine gas giant size, or do you need to figure out how to calculate eclipse time?  In any case, I found this article on gas giants that might be helpful:  http://en.wikipedia.org/wiki/Sudarsky%2 … sification

I personally am in the camp that figures, if it works for the story and setting you want to build, do it without worrying about what's scientifically accurate, as long as it passes the "sounds reasonable" sniff test. That being said, you may want to look at a moon around a gas giant in our solar system. How long is its orbital period, how far away is it from its parent planet, how large is that parent planet. Then you can math it up and find that it wouldn't be behind the planet for much time at all - look at how long one of our lunar eclipses last (a few hours) and our moon is much bigger in relation to Earth than a gas giant's moons are in relation to it.

Also note that the moon likely wouldn't be in the eclipse shade at all but for maybe once or twice a year, unless you go for the contrivance of having the moon's orbital plane exactly match the orbital plane of the parent planet.

As for the color of planetshine - seems reasonable that a blue planet would reflect more blue light while a red gas giant might reflect redder light.

As far as I know, gas giants are well known to exist within different Goldilocks zones in different star systems. Upsilon Andromedae is one I can name off the top of my head (because I wrote a story set there too, ages ago).

Well, in general you want to know enough facts to be able to fake the rest. Unless specifics are needed for story reasons you can be vague about quite a bit. As this is a fantasy world, physics might not even work the same as on Earth.

It's actually more of a pre-tech sci fi. There's zero magic or anything like it. Sci fi because it's set on a very different planet.

Because you said Neptune, I used actual Neptune to do some quick and dirty math. To have a 30 day orbital period our "moon" has to orbit at about 1,000,000 km. This makes our gas giant have an angular diameter of about 170 arc minutes, or ~ 6 times larger than the moon. This would roughly mean that an eclipse would last about 6 times longer, for a max of about 45 minutes when everything lines up and you're standing in the right place. As for how blue it would make your nights (and, potentially, at least a little bit, your days), use your imagination, I feel like you've got a significant amount of artistic license there.

EDIT: And to add, just in case you don't know, your planet is tidally locked to the gas giant. The above only applies to the gas giant facing side of the planet. The other side will have uninterrupted days and very dark nights. And is probably colder, I think. And gets hit by more meteors.

Last edited by ShadowDuelist (2015-01-24 13:15:54)

A more distant orbit would result in a slower orbital period than 30 days. Check every moon in the solar system and in fact the planets themselves. Mercury takes 88 Earth days; Neptune takes 165 Earth years.

Also the orbital period would be thirty hours, not days.
Maybe that would force the moon inside the planet's Roche limit. Or at least inside "we're dead" limit...

Yes. I've been struggling with the necessary label for an iron-age war drama set on an alien planet...dunno I'll figure it out after I've written the thing.

Maybe you're taking the wrong approach here.

Lord Of The Rings, Dragon Age and Final Fantasy are all works of fiction not set on Earth. Yet, they never claim it to be an alien world.

LotR is simply set in the world of Middle Earth. Is it Earth? Is it not earth? One can't be sure, but geographically, it isn't. It's a fantasy world in which men did not descend from apes, and there are other humanlike races around.

Dragon Age is set in the world of Thedas(which, fun fact, is just an abbreviation of THE Dragon Age Setting.), but it's never claimed anywhere that this is even our universe, or what galaxy it's set in. Just a fantasy world that is similar to our planet, and has dragons and magic and other humanlike races.

Final Fantasy is a bit more tricky, as each new game takes on a new setting, but they all have things in common. They're all set in fantasy worlds known as this or that, but never claim to be part of our universe. The all have varying degrees of Space and/or space travel using different technology to get there, but never in the course of now 14(numerical anyways) entries have anyone mentioned the Milky Way galaxy, or even Andromeda.

What I'm getting at is that your world doesn't need to be scientifically correct, nor does it have to be a Sci-Fi. It could simply be a fantasy novel set in a different world than ours.

Your world is a moon set around a gas giant, and that's fine. Reading ShadowDuelist's post, they make enough sense for me to buy the concept, even if he's just bullshitting us all. Not saying he is, but for all I know, he might be. He is, however, making a good enough case of it for me to buy it.

I don't think you should linger on Orbiting distances, Roche limit, arc minutes or planet shine. Instead, focus on getting the basics right: It's a moon orbiting a large gas giant. It's day cycles, measured by the sun, is X amount of hours(measured in earth hours, apparently), and as it's a moon, and tidally locked, one side has more direct sunlight, and longer nights, whereas the other side has constant heat emitting from the planet, and a much bluer(if that's the giant's color anyway) hue in the sky. The latter side also has more stable weather, as the day/night cycles aren't that different from each other, temperature-wise.

..And even that might be going too much into depth.

Take LotR, for instance: It's insinuated, in the films, that the Grey Havens, is simply a place that isn't middle earth. It's also insinuated that this is the realm of the dead, but never specifically explained. Do I care? No. It works, and that's enough for me. I'm sure the books flesh out the details, but having not read them, I don't know.

If you're writing a fantasy-like novel that doesn't have magic, write it as such. Don't pester the reader with techno-babble, or you might lose them fast. I know I'd have a hard time trying to grasp all the numbers and facts if they were to suddenly pop out all at once.

Have it be lore. Something that all the inhabitants know, but do a fair bit of explanation to the reader. Depends on your writing style, obviously. Is it first person, third person? Is there a narrator? If it's the latter, then sure, you could explain all in detail, but if it's the prior two, the characters probably know all of it, and unless your protagonist is new in town, he/she will too. Don't have someone take the time to explain it to him/her. Hell, even if he/she has a sudden case of amnesia, it's not like they'll suddenly think things on their planet work like on earth, and have to have it all explained again. I've met people who have long-term memory loss, but even they won't ask what the big, bright thing in the blue large field high above us is, and why it gives off heat.

The TL;DR version:

Don't get stuck on details. Figure out something that's theoretically possible, and stick with that.

Last edited by Tomahawk (2015-01-25 09:09:22)

I'm going to second Tom and BDA here. It's more important for the details to serve the story then vice versa. Unless you're writing super hard sci-fi, no one is going to get mad at you because your math doesn't check out. Don't feel beholden to my math and speculation if it's getting in the way of your story. I was going to type more here, but I realized I was just repeating what Tom said in different words, so just go read his post again.

Edit: I will add this: It's ok to tell us about your world, the details you give us help bring it to life, but do it in a natural way, and don't overwhelm us. We should learn about your world the same way we learn that Tattooine has two suns, or that Coriscant is all city. No one sits down to have a conversation about it, it just shows up in an establishing shot and everyone just treats it like it's normal, 'cause to them it is. Those details are an important part of bringing a world to life, Tattooine isn't really the same without it's twin suns, but they're backstage to the story taking place.

Last edited by ShadowDuelist (2015-01-25 09:53:32)

....I haven't read a page, and I'm already in love with the concept world we're kinda building here.

Yeah. I love world building (and is one of my current projects for my own writing).

But, to echo what has been said, characters matter more. The details of the mechanics of the planet are important to the author but not always to the audience. Please don't misunderstand me. I like that you are building your world on physics and that you want that to be realistic (relatively speaking). I think that will give you great internal consistency for your world and story.

But, the characters are what draw people in. The essence of Speculative Fiction (note: not science fiction) is the idea of how characters react to a given situation.

tl:dr Love the world building, and happy to help. Make sure characters get the same love