Greyspace is the common name given to the planetary system of Oerth.
Unlike most planetary systems, Greyspace is geocentric; a terrestrial body—in this case, Oerth—is the center of the system, about which other celestial bodies orbit.
The crystal sphere
Greyspace, like all planetary systems in the Spelljammer universe, is encased in a vast sphere of an indestructible substance. At 16 billion miles across, Greyspace's sphere is one of the largest known. From the outside it is as reflective as polished steel, and from a distance beautiful colors, like the iridescent hues in mother-of-pearl, can be discerned. From the inside it is night-black and completely nonreflective. It is, in fact, the night sky itself. It cannot be touched, exactly; it is perceivable only as a barrier beyond which nothing can go, except through portals opened magically into the chaos outside.
The "stars" that can be seen on clear nights are actually titanic gemstones somehow buried in the surface of the sphere. It has been surmised that they contain portals within them to the Positive Energy Plane or the Quasielemental Plane of Radiance, or that they were mined from the gods from the stuff of the Positive Energy Plane. The heat from them can only be felt from less than a hundred feet away (where it is deadly), but their light can be seen (obviously) for billions of miles. The constellations are described as occupying the midpoint between Law and Chaos; patterns can be discerned, but the greater order behind them, if any, seems elusive. Oerthly sages identify 12 Lairs of the Zodiac and have given names to many other constellations as well, such as the Druid, the Dancer of Swords, and the Swan.
The vacuum of space between the worlds and stars of Greyspace is cooler than that of Realmspace, but not as frigid as Krynnspace. It is almost always 40 to 45 degrees Fahrenheit, except close to the sun.
The major celestial bodies of Greyspace, from the center out, include:
- Oerth: Terrestrial planet and home to the Greyhawk campaign setting.
- Kule: Tidally-locked terrestrial body. Covered with the ruins of a humanoid civilization, but lacking any air whatsoever. A magical cataclysm is suspected to have caused the depopulation. Also known as Celene, one of Oerth's "moons."
- Raenei: Terrestrial planet, covered with earthlike features, including animals and monsters, but no sentient life. Also known as Luna, one of Oerth's "moons."
- Liga, the Sun: Star. Has many small lakes of elemental water to keep it from burning too hot. Larger than Oerth, but orbits it anyway.
- The Grinder: Spherical asteroid belt.
- Edill: Gas giant, populated primarily by dragons.
- Gnibile: Dark-red gas giant, with blobs of lava and shards of ice in the interior. Contains a gate to the Negative Material Plane. Populated primarily by undead.
- Conatha: Elliptical water world with two solid cores that orbit around each other. Populated by many piscine creatures, including merfolk and sahuagin.
- Ginsel: Crescent-shaped world that resembles a sphere with a giant bite taken out of it. Has an atmosphere on the inside of the crescent. Basic civilized fantasy world.
- Borka: Spherical asteroid cluster with a common atmosphere. Formerly a planet inhabited by orcs, goblins and other humanoids, it was devastated by the magic of the elves.
- Greela: Another spherical asteroid cluster with a common atmosphere, although many of the asteroids bear huge forests. Home to elves.
- The Spectre: A flat, circular disc of rock that appears to vanish and re-appear as it is seen edgewise, then rotates to show its full face. Also called The Wink. Has an atmosphere and water, but no life.
Other features of Greyspace
- The Sisters: A constellation of nine star-like objects, resembling a single bright star from Oerth. They are surrounded by a region of reverse gravity and dead magic. Those who pass between them end up far removed in space or time.
- The Great Sargasso: A region of dead magic a million miles in diameter.
- The Habitat: A mile-long ellipsoid of some dark blue-gray metal (perhaps superficially resembling the far smaller artifact in the Pits of Azak-Zil).
- Dark Stars: At least three objects similar to Spheres of Annihilation, but several miles in diameter.
Other possible features of Greyspace
- Atropus, the World Born Dead, is a small moonlet that travels through space, erasing all life it encounters.
- The Dragon's Tear comet, which appears in the sky every 27 years, opening the Vast Gate to the Far Realm with the enormous reserve of potential magical energy it builds up during its long interplanetary journey.
- A fiery red comet heralds the approach of Ragnorra.
- The Rock of Bral, a vast city-state located on a huge asteroid, is (optionally) located in The Grinder.
- The floating castle of Sardior, which stays forever on the night side of the world away from the sun. On the nights it can be seen, it has been mistaken for a small red moon. Periodically it moves between planes.
- Cordell, Bruce. The Gates of Firestorm Peak. Lake Geneva, WI: TSR, 1996.
- Grubb, Jeff. Spelljammer: AD&D Adventures in Space. Lake Geneva, WI: TSR, 1989.
- Findley, Nigel. Greyspace. Lake Geneva, WI: TSR, 1992.
- Henson, Dale "Slade." The War Captain's Companion. Lake Geneva, WI: TSR, 1992.
- Miller, Andy. "The Good Oerth: Constellations of Greyhawk." Oerth Journal #22. Council of Greyhawk, 2007. Available online: 
- Schwalb, Robert J. Elder Evils. Renton, WA: Wizards of the Coast, 2007.