Gravity plane

A gravity plane was a geometrical plane generated by certain large, non-spherical objects towards which other objects "fell". Gravity was what held an atmosphere around planets and spelljammers and what allowed an inhabitant to walk on the surface of a world or a spacefarer to stand on the deck of a ship.

Most residents of wildspace recognized that every physical object had its own gravity and assumed that the direction of the force was always towards what was most convenient for sentient life. For planets, this was always toward the center of mass, but for spelljammers and other space objects, such as asteroid cities, the force was usually perpendicular to a horizontal plane. More knowledgeable scholars of wildspace recognized that gravity planes were not natural occurrences, as was the gravity of planets, but rather the result of the special magic of spelljamming helms. (Interestingly, despite their truly astronomical size, crystal spheres were thought to be without gravity; other sages, however, explained that the sphere's plane of gravity was compressed to a single point at the very center of the sphere, where the sphere's primary rested. )

All gravity planes known shared the same gravitational force, roughly that of standing on the surface of the planet Toril in Realmspace. This force was constant over the full range of the field, and the field extended as far as the vessel or asteroid's air envelope. Outside the air envelope, someone would cease to feel the pull from that gravity plane.

While under the effect of a gravity plane, a person or object accelerated and moved with that object.

Unlike normal gravity, a gravity plane extended along the two horizontal axes of the massive body producing it. The plane did not terminate with the object but extended out past it such that objects fell toward the empty space surrounding the body. A falling object passing through the plane would be slowed down by the suddenly opposing force in the opposite direction until it reversed direction and crossed the plane again. This would continue until it would eventually come to an equilibrium at the plane. An object resting in an empty region of the gravity plane like this would bob up and down, much like something floating on the ocean,  and slowly drift away from the source of the gravity until the outer reaches of the plane, which typically extended outwards from the inducing massive object by a distance equal to that object's width. (See the figure above.) Once drifting outside the air envelope, the object would be left behind in wildspace unless recovered.

Gravity planes had two sides; that is, they attracted objects from both the bottom and the top. It was thus possible for someone to walk on the bottom of a spelljamming vessel "upside-down".

Some vessels took advantage of these two sides in their craft designs, but most sentient creatures preferred to have only a single "up" and did not build inhabitable decks on the "bottom" side.

One of the main reasons for this "single-side" preference, was the fact that the force from a larger object's gravity plane would overcome and negate a smaller object's plane. Entering a planet's gravity well, for example, would cancel the effects of a spelljammer's gravity plane so that only the planet's gravity could be felt. Anyone walking on the bottom of a spelljammer when nearing a planet would thus plummet to the ground.

The same issue occurred when a smaller spelljammer entered the gravity plane of a larger vessel. Crew and cargo might find themselves hurled sideways or toward the ceiling if the larger ship's force of gravity took over. This was even used as a ship-to-ship combat tactic. Known as a shearing attack, larger ships would intentionally approach close to smaller ones, forcing them to reorient or risk loosing crew or cargo to falling. The dwarves, with their particularly large citadels, particularly favored this tactic.

Proper diplomatic etiquette—and the safest operating procedure—when approaching another vessel was for the smaller vessel to first align itself to the larger one, such that the gravity planes were parallel. This avoided any issues once the two ships were close enough for the larger one's gravity plane to take over. It was considered an insulting breach of conduct to align one's ship parallel but upside down.

Similarly, a ship docking on an asteroid city, such as the Rock of Bral, would have to approach carefully to orient the ship parallel to the asteroid's gravity plane. Docks were typically built at the level of the gravity plane so that vessels could "land" and "float" upon the plane and dock while the helm remained unmanned.

Projectile weapons were often fired from at the level of the gravity plane, because objects here could "float" upon the plane.