Below are some random bits of knowledge that have been gleaned from the spelljammer lore recovered from the moon base:

Vespalen Sarillien and his elves are diligently translating as much of the works as they can. The grey elves were contacted, and appeared within an hour. (They were a bit cranky because Lord Sarillien would not allow them to take the whole bundle back to their secret mountain city of Ti'ye Simosk to study, but they are being reluctantly helpful.) 

Some of this information is supported as well by the Kapurniak and Ceravan mages' guilds, who continue to fly various boats about the land and into space.

Our solar system is encased in a huge black globe of unimaginable size. The planets whirl around in the empty globe, and the stars are fixed to the inside of the globe's black surface. The globe itself is referred to as a "crystal sphere". 

The sun, named Suryn, is the centerpoint to the globe, and all other celestial objects seem to orbit Suryn at various distances and speeds. The vast, empty space that the planets orbit through (and you know as 'outer space') is referred to as "the void" or 'voidspace' in the texts.

There are apparently other solar systems out there, because the texts make references to multiple crystal spheres. Much like sea ships travel from continent to continent across the oceans, spelljamming vessels can travel from sphere to sphere. (This would explain a great deal about the legends and wizardly theories of "alternate Prime Material Planes", perhaps.)

To continue the boating analogy, from descriptions in the materials it seems that these myriad crystal spheres themselves float in some sort of universal ocean; a strange vaporous substance known as "phlogiston". Spelljamming ships exit a crystal sphere, and float through this phlogiston to another crystal sphere. The texts call this ocean of phlogiston "The Flow".

At Cy'Pren's behest, the god Corellon Larethian sealed off the crystal shell, and caused the elves to forget all they knew of the destruction of their people and the war they'd been fighting. This would bring their souls peace and allow them to start their lives anew. However, the god Lebolas Enorath, god of history and knowledge, certainly didn't want the elves to completely forget who they were, where they came from, and what they'd been through. So Lebolas petitioned Corellon, who decided that when the Thirteenth Generation of elves native to Cy'Prenspace was born, those elves would have the ability to discover and their roots and history, if they were so inclined.

The sprites know of this, of course, since a very few of the immortal fairy creatures were there at the time and are still around, but the sprites were sworn to secrecy by their gods in the heavenly Seelie Court.

It wasn't until Reyn Swift and Rafton discovered the DwimmirChamok tome and began visiting elf sages that the sprites knew that the prophesied Thirteenth Generation was clever enough to figure out their history.