On Aether polarization: I think the answer you are looking for is simply that the current lore lacks a clear exploration and clarification of where does astral and umbral comes from. Maybe the MSQ or some quests might explore it further at some point? I personally suspect it has something to do with said aether proximity with stars and constellations while umbral is the opposite, the difference here is that light and dark aren't an element like in some other FF titles. Which means you can get light fire or dark fire. I want to insist that it's not lore but my own suspicion through various hints ingame or the way it's often worded.
Other people have said above that aether polarities are similar to electricity, but I feel like a better analogy would actually be heat. For one electricity's polarity is binary (or at least discontinuous) and here we have a very organic scale of infinite degrees between pure Umbral and pure Astral. Secondly, heat tends to agitate atoms while lack of heat makes them still. I feel like it's better suited as an analogy to aether: umbral aether is very still and lingering while astral is rather active and swirling. This is also why some elements are more attuned naturally to one or the other polarity: fire, wind and ligthning are restless, elements full of life, while ice, earth and water are often the opposite. The former will burn or sweep everything they touch while the latter will probably more seep, sap, and slowly gnaw at things. So. Heat? The closer to stars (cf, astral), the "hotter", the more stirred it will be. All this in very rough layman's terms of course. It also doesn't mean closer in distance. Just more magically exposed to either stars, or at the contrary, exposed to the umbral depths (light vs dark).
Edit: you might ask, how does the Coco brothers THM school of magic fits into this? Well, I'm pretty positive that you would probably be able to invert the method and use astral ice and umbral fire and whatnot, though that sounds very clunky and counter intuitive to use, requiring a lot more effort probably considering those elements' affinities. Hell, maybe you can even use other elements instead, like astral wind and umbral earth and whatnot? In any case, the umbral aspect in that specific school of magic seems to increase tenfolds the personal aether regen while the astral one sacrifices it in favor of an increase magic potency.
On Eorzean Castra: I'm sure they named them from cardinal directions probably because they all were born in the same jurisdiction/region, so one becomes the castrum of the south of the Eorzea region, and so on.
Other people have said above that aether polarities are similar to electricity, but I feel like a better analogy would actually be heat. For one electricity's polarity is binary (or at least discontinuous) and here we have a very organic scale of infinite degrees between pure Umbral and pure Astral. Secondly, heat tends to agitate atoms while lack of heat makes them still. I feel like it's better suited as an analogy to aether: umbral aether is very still and lingering while astral is rather active and swirling. This is also why some elements are more attuned naturally to one or the other polarity: fire, wind and ligthning are restless, elements full of life, while ice, earth and water are often the opposite. The former will burn or sweep everything they touch while the latter will probably more seep, sap, and slowly gnaw at things. So. Heat? The closer to stars (cf, astral), the "hotter", the more stirred it will be. All this in very rough layman's terms of course. It also doesn't mean closer in distance. Just more magically exposed to either stars, or at the contrary, exposed to the umbral depths (light vs dark).
Edit: you might ask, how does the Coco brothers THM school of magic fits into this? Well, I'm pretty positive that you would probably be able to invert the method and use astral ice and umbral fire and whatnot, though that sounds very clunky and counter intuitive to use, requiring a lot more effort probably considering those elements' affinities. Hell, maybe you can even use other elements instead, like astral wind and umbral earth and whatnot? In any case, the umbral aspect in that specific school of magic seems to increase tenfolds the personal aether regen while the astral one sacrifices it in favor of an increase magic potency.
On Eorzean Castra: I'm sure they named them from cardinal directions probably because they all were born in the same jurisdiction/region, so one becomes the castrum of the south of the Eorzea region, and so on.
Balmung:Â Suen Shyu
