Editing Spelljammer (section)

===Gravity and air===
All bodies of a sufficiently large size have [[gravity]]. This gravity usually (but not always) exerts a force equal to the standard gravitational attraction on the surface of an Earth-sized planetary body. Gravity in the ''Spelljammer'' universe is also an exceptionally convenient force, and almost always works such that "down" orients itself in a manner most humanoids would find sensible.

All bodies of any size carry with them an envelope of air whenever they leave the surface of a planet or other stellar object. Unlike real-world astrophysics, this air envelope is not dispersed by the vacuum of space. These bubbles of air provide breathable atmosphere for varying lengths of time, but 3 months is considered "standard".