I've always been curious about this. From the example of Dalamud and its current absence, it's clear that what the Astrologians believe to be true may not actually be the case... but they still manage to construct an entire discipline, complete with tangible effects and outcomes, out of this belief. When questioned how, I recall the in-game answer being "we have no idea, but it all works out anyway".
They base their calculations on mistaken premises, but even after the truth is revealed, those calculations still work. Something is going on here.
