Crystal spheres are actually opaque - they're black, and the stars and galaxies are painted or mounted on it. What seems to be a greater universe beyond our solar system is only an illusion.
This explains a lot about the sometimes inconsistent results astronomers report back.
Oh,I forgot about that :oops: .The only Weird part is Why are so many settings in SJ and at the same time ignore them.FR is huge when it comes to this the only way off the prime into the flow is through a couple of portals in undermountain.
This explains a lot about the sometimes inconsistent results astronomers report back.
Yeah... I still prefer to go with the other way of looking at it.
But if we're going to get technical, (and who doesn't enjoy that! 
) if the solar system were actually incased in an opaque orb, it would act as a Dyson Sphere to prevent light and other energy from escaping. Which isn't necessarily a bad thing, but would make things a heck of a lot brighter 
Anyway, I make these arguments not because I want to punch holes in Spelljammer, but because I've been working out some creation theories for use in my particular incarnation of Planescape wherein the prescence of a phlogiston rather than an empty space vacuum would be troublesome.
) if the solar system were actually incased in an opaque orb, it would act as a Dyson Sphere to prevent light and other energy from escaping. Which isn't necessarily a bad thing, but would make things a heck of a lot brighter Not if the sphere absorbed all the energy and channeled it into the phlogiston.
I see the Crystal Spheres and Phlogiston as being the main part of the Prime Material, encased in the Plane of Shadow with other, smaller bits of Prime Material floating out in the Shadow. Eberron touches the Ethereal, Shadow, and Astral Planes officially. I think that it could easily be a very isolated world, part of the Prime Material but cut off from the Crystal Spheres by the Plane of Shadow. The thirteen planes that orbit it don't strike me as corresponding directly to the Outer and Inner planes, and I think it would cheapen the setting, and confuse the way that they influence Eberron, to make them just the same as the Great Ring. The Planes are infininte -- why can't Eberron have these pseudo-planes orbiting it in the Astral? Having the Region of Dreams in the Astral instead of the Ethereal is a little weird though, I'll grant that.
This also allows for other, very different Prime Material worlds that don't have Crystal Spheres or Phlogiston to be hidden away in the Plane of Shadow as well. Our universe, for example. And isn't Aebrynis/Cerilia surrounded by a "Shadow Plane" as well? Maybe that is the Plane of Shadow, and Aebrynis is another isolated Prime Material pocket.
This doesn't necessarily have to rule out an Ordial, either. The Plane of Shadow could be one massive demi-plane, a barrier that separates Prime Material pockets within the Ethereal. Or it could be considered a part of the Prime Material itself, the substance in between the Prime pockets, just as the Phlogiston is the substance in between Crystal Spheres. And somewhat like the Phlogiston, the Shadow is not connected directly to the Ethereal or the Astral.
But my knowledge of Planescape is rather shallow still, so if I said something really obviously dumb, please forgive me. 
That's because crystal is see-through
Pants of the North!