The Urth and its Sun One of the most difficult things to decipher about the series is the proper relationship of "Urth" to Earth as we know it. Through the series is the theme that the sun is dimming and will eventually go out entirely. In a dream, Severian seems to see the sky as it was in his past: |
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Behind the altar rose a wonderful mosaic of blue, but it was blank, as if a fragment of sky without cloud or star had been torn away and spread upon the curving wall. I walked toward it down the aisle, and as I did so I wa struck by how much lighter is was than the true sky, whose blue is nearly black even on the brightest day. |
In Severian's time, the sun has already dimmed considerably, and cooled the Urth. This is strange, because astrophysics tells us that Earth's sun will eventually turn into a red giant a swallow Earth prior to dimming. There is ample indication given that Urth is indeed a future version of Earth. For example, |
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The picture he was cleaning showed an armored figure standing in a desolate landscape. It had no weapon, but held a staff bearing a strange, stiff banner. The visor of this figure's helmet was entirely of gold, without eye slits or ventilation; in its polished surface the deathly desert could be seen in reflection, and nothing more. ... "...There's your blue Urth coming over his shoulder again, fresh as the Autarch's fish.'" "Is that the moon? I have been told it's more fertile." "Now it is, yes. This was done before they got it irrigated. See that gray-brown? In those times, that's what you'd see if you looked up at her. Not green like she is now. Didn't seem so big either, because it wasn't so close in - that's what old Branwallader used to say. Now there's trees enough on it to hide Nilammon, as the saw goes." |
The picture appears to depict a scene from an Apollo-era moon landing. 
It's unclear exactly why the Urth is dimming in the Book of the New Sun, although there are hints at a kind of cyclicism induced by powers beyond our comprehension. It is also possible that Urth's technology may have been sufficient at the one time to survive being incinerated by the sun prior to its decline. |