My understanding is basically that a "divine year" refers to the lifespan of a universe, from the big bang to however it ends, and upon that end a new universe and divine year begins. Maybe that's a little simplistic, in which case please correct me.
What I really want to understand is how that relates to Severian and everything that happens in BotNS. When the new sun arrives, it literally replaces (merges with?) the old sun, flooding Urth and creating a clean slate, which is then settled by humans who arrive from elsewhere (maybe on a generation ship like the Whorl?). But the arrival of the new sun doesn't mark the transition into a new divine year right? Like it only affects one solar system, and the inhabitants of the new Ushas are humans who are descended from those of Urth. So is it supposed to be read as a microcosm of the divine year, applying only to one world? Or are these concepts entirely separate? I know about the whole "first Severian" thing and the idea that the Yesodians are beings from a previous universe (previous divine year?) who survived in another dimension. Is Yesod itself a previous divine year, or just another plane that they escaped to?
The evidence for the nature of Yesod is somewhat contradictory. The khabbalic term "Yesod" might suggest that this is a heavenly realm; a realm above all universes where things are managed.
But as the Ship passes from Severian's universe of Briah to Yesod, the text suggests the portal is both the end and the beginning of a universe. Moreover, in Yesod, Tzadkiel tells Severian that he, himself had been an acolyte of Severian's in a previous iteration. These bits suggest Yesod is a universe (under construction?) which exists in a universal year in the future from Urth and Briah.
From what I've understood it, the New Sun, in relation to a divine year, is about shaping humanity to become their, meaning the hierogrammates, creators. IIRC, it was mentioned in Urth. Somewhat similar to the contingency argument. The hierogrammates can't just exist without their creators, and thus the created create their own creators, and the cycle continues. That said, I might be wrong or misremembering.
So the Hierogrammates were created by a previous universe's "humans", and are now working to make sure that the current universe's humans create Hierogrammates? Does this mean that the previous universe's "humans" were guided by the Hierogrammates of an event older universe? I guess this is the same thing that's going on with the idea of there being an original Severian, who somehow works to ensure that the next universe's Severian succeeds (who in turn helps the next Severian after him?)
How does the Hierogrammates' plan ensure that mankind creates them? Is it just that mankind has started to regress technologically and thus need a boost so they can reach the point where they're advanced enough to uplift some aliens into Hierogrammates?
I can't really say anything about intricate details, as I'm no expert. From my understanding, the hierogrammates shape humanity to ensure the condition of their existence. Your comparison to original Severian is pretty apt. There was the original "humanity", who created them, and they in turn shaped the humanity (of different divine years) to ensure the condition for their continual creation.
Regarding mankind's regression, that is, again from my understanding, somewhat caused by the hierogrammates. Urth's sun is dying artificially fast, due to a black hole in its heart. It's the hierogrammates shaping humanity, and in a way punishing humanity.
Right yeah, I remember that. So the Hierogrammates are pushing mankind to arrive at the point where they can create Hierogrammates of their own, by sorta throwing them into the crucible.
That is correct.
In an interview, "the Grand Gnab" is how Gene Wolfe referred to the end of a universe.
As I understand it, Yesod is something like a higher plane, not a previous universe. The New Sun just marks the very first step in what apparently will take until the near end of the current divine year to complete.
At the end of Citadel, Severian is told by Malrubius that he is not sure what happens to the humans who survived (if they did) the end of their universe. But the beings they had created (like Tzadkiel):
Shaping our shapers
I think that's right. At any rate, it's what I think too.
I think it is that the whole universal day (as in lifetime of the universe) takes place, and the changes made in one iteration are all the Hierogrammates care about. Maybe there is more to it than just the role Severian plays—maybe he's just the opening act, and there are a million more important events later. Or, maybe it's literally that one difference, and rest of the lifetime of the universe plays, for however many billions of years it takes until this cycle is complete. That seems like a massive thing to do for us, but it wouldn't matter to the inhabitants of Yesod, who are outside our time frame.
Side note: when I give my computer some computationally intensive task, like playing a video game or rendering a 3D model, I always think about rooms full of guys with slide rules calculating all the linear algebra necessary to do that by hand, and how it would take millions of person years to do what the computer does in an hour. But, it's nothing to me, because I'm not in there living it out. Maybe that's analogous to how the Hierogrammates view it. higher dimensional privilege!
Check out media death cult synopsis. Does a good job about the divine year
I watched that the other day actually, and made a start on their Q&A video with the Alzabo Soup guys and Driussi. Great video... But it's kinda why I asked this question. He at one point connected the start of a divine year to Severian's actions in bringing the new sun, treating it as if they're the same thing, which is what confused me.
Quote from Malrubius to Severian:
This is from the end of Citadel but Urth of the New Sun stays true to it. Severian travels for a while in Tzadkiel's Ship to reach the point which is both the beginning and end of his circular universe. The voyage to Yesod involved reaching this beginning/end of the universe. But the actual damage to the old sun and the repair of it that Severian conciliates and facilitates all happen somewhere in the middle of that circle.