The subastral firmament is the barrier which protects the Mortal Realms from the direct influence of the other Inner Realms. It maintains the Mortalrealm's shape and structure, and is directly responsible for the change in day length throughout the seasons.
Appearance
The subastral firmament is invisible to the naked eye. It's shape is mostly spherical, but deviates as it approaches the edge of the world, straightening out along the world edge and giving the Mortal Realms their distinct coin shape. This gives the subastral firmament it's distinct oblong shape.
Interactions with Other Phenomena
The subastral firmament prevents the passage of any matter and even blocks metaphysical energies such as force energy. It also blocks the transfer of heat and other energy, but doesn't completely block the passage of light. It doesn't block all magic; teleportation and conjuration spells function as if the fermament weren't there, as does scrying and other non-physical magic.
Intangable creatures cannot directly pass through the subastral firmament, but passage beyond it can be achieved by traveling through the Ethereal Plane.
Effect on the Seasons and Length of Day
The subastral firmament dims light that passes through it, which effects how much of the Realm of Fire's light reaches the mortal realm, which has an effect on the seasons and alters the length of daylight in a given day. This light dimming effect becomes more prominent near the equator and in particular where the sphrical shape gives wy to a more cylindrical shape. This dimming follows a fixed pattern, and functions independently for each hemisphere. The dimming effect of a given hemisphere is strongest on it's respective winter solstice and weakest on it's summer solstice.