1 quoted line
David Stockhoff- Maybe he skidded past the exact point where the light reaches Urth, because he was going so fast.
Yeah, that is my impression of what happened also. A bit of temporal inertia.
I guess Jason Walker is wondering why he had to stop at that point in history on Urth. If he was outside space and time, why would access to an electromagnetic energy source have any impact? I suppose we must guess that access to that power source is somehow tied to the timeframe and physical presence on Urth. There may be no way to plausibly explain it with our knowledge of science.
However, I think the best answer might be auctorial rather than scientific. If Severian was able to travel in unlimited fashion in time, he'd come out on a molten blob of planetary rock and be burnt up or destroyed in the Big Bang or something. That would be a crappy end to the story and we can't have that. Best to make his time travel ability restricted only to places and times where human beings can and do exist.
Makes me wonder about how long Severian was dead and buried in the stone temple before he resucitated and spoke to B, F and O. I think Apu Punchau was still a body, not a mummy or skeleton so I guess it couldn't have been very long. But if Apu Punchau was dead long enough to be reduced to dust would Severian still be resurrected when the New Sun winked into existence and was close enough? I guess so...but again, Severian seeing a pile of dust wouldn't have made as good a story, I think.