Since I've seen no reference to it, I'm curious-- how does the infinite size of the Shadow Prime affect the cosmology of the Prime Material Plane? It would seem contradictory for any single Crystal Sphere to be of infinite size.
Unless, of course... this is part of the reason spelljammers have never discovered Earth.
They don't mean anything. Earth astronomy has been dead wrong for centuries. There is nothing beyond the Oort Cloud, just blurry radioactive lights on the surface of our crystal sphere. 
No, it's just that the Shadow Prime's sphere is very large.
Ask physicists, they'll tell you that our universe, or "crystal sphere" if you prefer, is expanding at a breakneck speed, but it is finite notwithstanding. If it wasn't, the sky would be infinetely bright. So things can be consistent without contradicting physics OR Spelljammer.
This makes me sad.
Hmm. I hadn't thought of it that way. Essentially, beyond the edges of where actual matter has expanded from the central point of our Sphere, you have the borders, which can be accessed via Spelljammer or via normal space travel should FTL ever be discovered. (I'm not keen on doing Galactic Planescape anytime soon, but I don't like the idea of the cosmology shutting down the possibility.)
Of course, this leaves the possibility of other inhabitable worlds within our sphere. Could someone more experienced with Spelljammer tell me if this has occurred with other Spheres?
This gives me an idea: black holes aren't actually super-dense stars-- their trapping of light has nothing to do with gravity. They're vortices to the Negative Energy Plane. (Or, perhaps, Vacuum, with the supernatural lack of pressure is what's actually sucking the light in.)
Yeps. Forgotten Realms has two inhabitable planets in their solar system if I'm recalling right.
I'm not really a modern Planescape guy -- I likes me mythology old-skool -- but my particular take on Earth's curious position is that it's a) in a separate "crystal sphere" (i.e. an infinite bubble in Shadow divorced from the rest of the multiverse) and b) in an almost magic-void region that's so barren of aether/esse/mana/thaums/whatever-you-want-to-call-the-substance-of-magic that any Outsider who makes it there will start asphyxiating almost immediately. This gives two cool historical side-effects: it prevented the larger demons [in the Earthly sense of the word] from entering Earth at all, strangling on their own magnificence, but might support a few lesser grumkins [spinagons, quasits, dretches, &c], and it meant that the function of an Earthly magic circle isn't so much to cage the demon as it is to provide it with a sustainable environment.
Of course, that was the past. What ushered in the modern era? Simple. At long last, the Outsiders figured out the paramortal equivalent of a space suit...
You just wrote "paramortal." Outside The Lady's Cage MUSH crowd, that was never a very popular way of referring to the planeborne.
Huh.
Huh.
Ooooookay...
Is there an implication there I'm missing?
And Sean K Reynolds discusses gravity and black holes in SJ in this article from his site - http://www.seankreynolds.com/rpgfiles/sj_spelljammer/gravitylargeandsmal...
Indeed. That's also why I hope that the cosmology works itself out in a fashion that allows this to be explored further.
Also, I may have spoken prematurely when I said I wasn't keen on getting into it soon-- Galactic Planescape strikes me as a perfectly logical end-point for the Planescape/Star*Drive crossover campaign I've always wanted to run. (I even had it going for awhile, a couple years back.)
All nine of the worlds in Realmspace are inhabited, including the sun.
Suns are the perfect places for conduits to the plane of (fire|radiance|positive energy).
Any chance you could share a few details? I've thought about combining the two in the past... but I just haven't had the time.
Okay, spelljammer and misc. stuff....
1. TSR Published three complete solar systems/crystal spheres: Realmspace, Greyspace, and Krynnspace. In all of these uninhabited worlds were the minority.
2. In my planejammer campaign I place the shadow prime WAY out there. You have to pass throught the Far Realm or very deepest Shadow to get there. I treat the Shadow Prime as adhereing to our modern physics, if you make it to Spelljammer/Planescape the spheres cosmology is what you deal with. Seperate but equal.
3. Wasn't the original idea (Urban PS idea that is) that the earth's space was magically cut off and hidden in the ethereal or somesuch?
4. Here is a wonderful 3.5 treatment of Wildspace and the Phlogiston that is archived on our SJ site. http://lost.spelljammer.org/Knightfall/corecosmo.html It is my baseline for my own campaign series....
Hope that helped!
I have a vaguely related question: say you're on a space shuttle that is in orbit, and you used some magic device/spell/whatever to transport you, and the whole shuttle, to (for instance) Celestia. Where would the shuttle appear? Is there space above/below/beside the normal layers of a plane? what about planes with no layers (like Limbo)?
Well, plaeshifting to Celestia, as I recall, allways ends up with you waist-deep in the Silver Sea(though I don't know whether the water would be inside the shuttle itself). Anywhere else, well, where'd you end up normally? I know someone brought a (illithid)spelljammer into Sigil once, though.
A spelljammer craft brought into Sigil?!?!?!?!?!?!?
Was this in someone's campaign or was it in a rpoduct that I missed?
It was brought it - in pieces and reassembled. It didn't *WORK* of course. But someone did it in canon.
On a related note in campaigns - Shemmie's game - our inn is called the Portal Jammer for a reason. We sorta stuck a spelljammer *through* our inn. The thing didn't have a helm when we found it and it looked cool. 
I would absolutely love to see more info (start a new thread or email me privately) on this cannon instance of a ship in Sigil as well as anything you care to tell me about th Portaljammer in Shemmie's game.
http://www.enworld.org/showpost.php?p=1752217&postcount=210
Our first day in the inn, setting up the jammer and um... discovering that our inn is armed.... Later in the game we get some permanent animate objects going on the sails - and rig up pirate flags.
I love it! I will have to use the Portal Jammer in my own game! The look on my PCs faces when they find an SJ ship in Sigil with a Xaositect and her pumpkin/gith crew taunting them from above! Fantastic!
BTW, is there anywhere online where I could find details/game stats on your PCs and the Portal Jammer? I promise to keep y'all posted if I use them as NPCs......
<rubs hands together and cackles with malicious glee>
I'm not looking for any particular PC details, but I am interested in any possible stats and information on the Portal Jammer.
In the Cage: A Guide to Sigil mentioned someone in Sigil who meant to order some squid (for his restaurant? I forget) and ended up with a squidship instead. Anyway, it's in that book.
In the Cage' detailed a person who imported an illithid spelljamming vessel into Sigil. The vessel itself was eventually sold to some idealistic Doomguard wannabe pirates.
However the 'Portal Jammer' inn in the Clerk's Ward first showed up in a campaign of mine (the one for my storyhour). So details as they unfold in there, or I could at some point writeup any pertinant details you'd like (or have one of my players do so).
*bats 'lothy eyelashes at her players*
Fyrehowl's working on it...
I would be most indebted for a write up. It will fit very well in my own game, and I could use the assist. Most of my development time has been spent getting the reboot of the Planewalkers Guild together here, and working on an SJ/PS rewrite of the Celestian Religion for spelljammer.org While both are major fixtures of my game, it leaves me with little in the way of free time to develop other things.
I have greatly enjoyed the glimpses of Shemeska's game that I have gotten through the storyhour and commentary from some of his/her players. I think that the Portal Jammer might need to get written up for Sites to Skeg. But then I think that Dead Kennedys and Ludwig Van Beethoven make an excellent counterpoint to each other......
Loking forward to it in whatever form we eventually see it!
I know this thread is very old, but it popped up on the random posts screen (I love that feature) and I have a solution to how an infinite space can be presented inside a crystal sphere of finite size.
From inside the sphere, imagine distance being measured by concentric rings from you, so that there is a line at 1 mile out, 2 miles out, 3 miles out, etc., on off to infinity. (Our universe might be spatially infinite, although this is unlikely; however, this solution would work for any such prime.)
From the outside, if these imaginary rings were visible inside the crystal sphere, one would see them not evenly spaced, but getting thicker and thicker as one looks deeper into the sphere. All of the physical objects of the Universe would appear to be crammed at the center of the sphere, untold distances away (and hence would require greater teleport to get anywhere) once you passed the wall. From inside, you would perceive yourself to be at the side of an infinite perfectly flat plane of crystal -- the inside of a sphere of infinite radius.
That's right -- you would be at the Source Wall.
It's basically like [url=http://en.wikipedia.org/wiki/Gabriel's_Horn]Gabriel's horn[/url], which is technically infinite surface area in a region of finite volume, but it's the same idea.
My tastes run more toward one of two possibilities: that Earth is a mythic, magical place inside a fairly standard crystal sphere, or that "our" world is in a different universe entirely, still accessible from Sigil but not part of the Spelljammer cosmos.
I actually think (and canon supports this, for what it's worth) that there's more than one Earth, and both are true. Probably many more than one Earth. There might well be an Earth for every one of Earth's pantheons, and the Gothic Earth of Masque of the Red Death, as well as the modern Earth of Urban Arcana with its hidden magic, and the not-Magical-at-all Earth of "City Beyond the Gate" and The Immortal Storm. And then there's our world, beyond the fourth wall.
Please forgive the impertinence of a clueless, but I stand with ripvanwormer on the notion that 'Earth is a mythic, magical place inside a fairly standard sphere' and so on. And certainly the concept of being accessable through Sigil (our Norse myths and legends are plastered across various articles withing the planescape setting).
There are a great many similarities between the portals chanted about in the cage that start on Earth and dump just out of reach of the dustmen, like Mark Anthony's 'Rune Lords' saga or even Lewis' Narnia stuff. There are older literary masterpieces like 'complete enchanter' by L.Sprague de camp and Fletcher Pratt that dips in to Norse and great Earth myth related prose such as Faerie Queene.
Without the metaphysics and general belief in the realm of Faerie, the inception of TSR, Dungeon & Dragons and its further releases such as Forgotten Realms, Dragonlance and, of course, our beloved Planescape; would never have been spawned.
As for Spelljammer, It is something that I have heard mentioned every now and again but not something that I am overly familliar with so, I thank you all for your insights here.
I would like to add that it has been a wonderful experience for me to read through some of these forums, the very mention of old TSR merchandise makes me tingle with excitement. Kind regards, all.
Or maybe they have and are the basis for all those UFO stories...