Some astronomical calculations -- help needed!

[Desdichado]

So, I started thinking about some more tweaks to my homebrew. I had an idea; what if the world is actually a large, vaguely Earth-like moon of a gas giant, or even a low-grade brown dwarf. Because of reradiated or contraction heat from the gas giant, the system can be in a larger orbit than that of our earth while still maintaining a somewhat Earth-like climate.

This lead to more thinking; what if there were actually two such moons, in seperate orbits, one a bit closer and thus fairly similar to an earlier period in the Earth's history when conditions were warmer than they are now, and one similar to Mars if it had an Earth-like atmosphere to buttress against the extreme cooling.

When I started thinking about things like the length of the day, the position of the gas giant in the sky and what it does over time, I quickly found myself floundering in waters way over my head, though. Does anyone have a good source of information like this, or can approximate it in their head, or can otherwise help me out in any way?

 

[Altamont Ravenard]

Why not use the characteristics of Venus for the moon that's closer and of Mars for the moon that's a little further away from the star?

You seem to be complexifying the situation when in fact you're basically describing our neighborhood in the solar system

AR

 

[Desdichado]

I'm not sure what you're saying. Venus obviously wouldn't be a good idea, since it's completely uninhabitable. Also, it doesn't help me in the least with the question I asked, since Venus and Mars don't orbit a gas giant that in turn orbits the sun, I can't use the characteristics of either to figure out the frequency of solar eclipses, the likelihood of having a big, full, gas giant shining in the night sky (or the day sky, for that matter) or any of those other kinds of things that I'm actually trying to get answered.

I also fail to see how this describes our neighborhood in the solar system in any reasonable way...

 

[Hand of Evil]

You may want to see if there are any planet generators on the web that will allow you to plug info and get a result. You use to see them for space games but not sure of the detail the provided.

This is something I placed in software some time ago.

http://www.enworld.org/forums/showthread.php?t=63534

 

[kipling]

You want World-Building

So, I started thinking about some more tweaks to my homebrew. I had an idea; what if the world is actually a large, vaguely Earth-like moon of a gas giant, or even a low-grade brown dwarf. Because of reradiated or contraction heat from the gas giant, the system can be in a larger orbit than that of our earth while still maintaining a somewhat Earth-like climate.

This lead to more thinking; what if there were actually two such moons, in seperate orbits, one a bit closer and thus fairly similar to an earlier period in the Earth's history when conditions were warmer than they are now, and one similar to Mars if it had an Earth-like atmosphere to buttress against the extreme cooling.

When I started thinking about things like the length of the day, the position of the gas giant in the sky and what it does over time, I quickly found myself floundering in waters way over my head, though. Does anyone have a good source of information like this, or can approximate it in their head, or can otherwise help me out in any way?

You want the book World-Building by Stephen L. Gillett; he discusses exactly this scenario on page 118. (Well, for one moon of a gas giant.) The book also contains the formulae you'll need for length of day and the position of the giant in the sky and the number of degrees of arc it subtends.

Basically, the moon is almost certainly tide-locked, and you'll need to adjust the orbital tilts very carefully to get earth-like seasons and avoid serious tumbles.

Melissa Scott, in her "how to write SF" book called Conceiving the Heavens, makes an oblique reference to this, since she did basically that, but I don't recall that she gives sufficient information for you to suss it out.

Poul Anderson has used this gimmick in at least one story and might have provided astronomical notes, but I haven't any better memories than that.

You'll need to work out the Hill radius for the gas giant to determine the minimum safe separation for the two moons, if you want two moons.

 

[Desdichado]

I really ought to just buy that book. I've checked it out from libraries in the past, but it's really more of a reference book than a "sit down and read it" book, and my local library here doesn't own a copy. Referring to Gillett's World-building, that is.

I was hoping to avoid the tidal-locking, as it is less interesting, but I suspected that it was unlikely. Two moons shouldn't be a problem. After all, it's not like Jupiter, Saturn and Uranus don't have several large moons.

 

[DrZombie]

I don't want to spoil your fun but, imho, if you don't have any astrophysicists as players it doesn't really matter, and even if you do it shouldn't really matter as well.
It is more important to think about the consequences it will have in trade, magic, religion on the different moons and the main planet, and to make that logically sound (wich is a lot harder then it sounds) then to let boring physics interfere with a nice idea. I mean, magic isn't explainable with "regular" physics either. You can make the phases of the moon variable and let them interfere with magic, create omens or have religious significance.
As a player I'd rather play in a fantasy world wich is thought out well, is interesting and feels "alive" but wouldn't be able to exist with planetary physics as we know'em , then play in a world wich is physically totally kosher but is boring as hell, with illogical economics and religions that don't care wether or not the moons are in conjunction.....

 

[Desdichado]

Let me just say that what's boring to some is an essential detail to someone else. Just because you don't care about astrophysics doesn't mean someone else doesn't. And I hate that claptrap "it's fantasy so it doesn't have to be explainable" wishy-washy excuse.

 

[DaveStebbins]

The World-Building mailing list (sign up at http://www.nocturne.org/world/) has some serious math-physics people subscribed. You could ask your question there.

 

[MarauderX]

I suppose eclipses from the gas giant would be a common occurance, say like every other day or so, and depending on the size ratios and location of the planet and how it rotates (planet's axis and plane of rotation vs. the gas giant) it would give the whole planet complete darkness for a few hours. During the night, your 'earth' planet could be on the sun-side of the gas giant, creating a gigantic lantern for seeing at night, creating a twilight until dawn or until the 'earth' planet disappears behind the gas giant.

Of course, you could just have a couple of orbs in the sky that appear regularly, one of the a giant that lights up the night every so often, and have a few days of total blackout a month. The players/PCs don't have to understand what is going on, they just have to know when it is going to be light, dark or twilight. Explaining the whole thing could be quite confusing to them, so you might just want to say "the gods did it this way" kinda thing and move on.