In sticking with the cutesy way some OSR/DIY D&D folks name shit, I decided to name my Earth-Luna System setting, "Orbital Space Refuse." The inspirations for it are Spelljammer, Mystara and the Voyages of the Princess Ark, Aristotelian physics, Eberron, and early modern alchemy and ceremonial magic (fuck that special "k").
The Ptolemaic conception of the cosmos, concentric crystal spheres, from which Spelljammer gets its crystal spheres, is itself inspired by the Aristotelian concentric crystal spheres. The Aristotelian cosmos is geocentric based on Aristotle's hierarchy of the elements, earth, water, air, fire, aether. In brief, this is why rocks sink, air is above water, and heat rises. Ignoring the justifications given (sorta) in Eberron for why fire and air elemental airships fly and move, I used this hierarchy of elements to explain why a bound fire elemental and provide life and motive force for a ship in the air. Since this all arose from my desire to have a spelljamming ship with a ring of fire around it I wanted some kind of reasoning for why one could operate in space, which in Orbital Space Refuse is mostly like our space, as well solving some logistical questions I had, are elementals alive and do they consume air.
Enter the sublunary sphere. There's a bunch of shit going on that I don't feel arsed to figure out as I don't know much Latin, but we have the element of earth beneath water, which is beneath air, which is beneath fire, which is beneath aether. Earth through air are the Earth itself while the sphere of fire is the separation between the Earth and Luna, or space. Backing up a little bit, these are philosophical as the intervening space is the insulating vacuum we all know and love. This is an in game explanation of an effect that is observable. By extensions, this means that even though the airplanes of the Flying City are prop-driven they will operate in the vacuum of space, within the space between the Earth and Luna, because they somehow convert elemental fire, fire spells, into motive force via the props. This applies around and on Luna, L1, L3, L4, and L5, as Luna's orbit marks the boundary of the spheres of fire and aether, and the listed Lagrange points either occupy that orbit or are within it.
I've extended this idea to other planets, with the twist that while the hierarchy is the same, elementals from different planets are foreigners, which requires some kind of negotiations for elementally powered craft to operate. This doesn't apply to anything powered by an aether elemental, as aether has dominance over the other elements. Unless I expand this beyond the Solar System and enclose systems within crystal spheres ala Spelljammer.
