I think it telling, and somewhat a fault of SE's storytelling on this, that there is not a plain, straightforward explanation for how an Astrologian does what it does. Most of the other classes have this.

Strip away the flavor and the ideals and the concepts of a Class/Job, and you get a basic, simple answer to the question: "Where does *insert class* get their aether from and what do they do with it?"

There does not seem to be a straighr explanation for the AST, only high-minded speculation that I may just not be smart enough to grasp.