Remember...

Series characters can reminisce either about earlier episodes or about adventures remembered by them but not by us. A Nomad recalls:

"'The flying city on Aesgil IV, and the war between the birds and the centauroids.'"
-Poul Anderson, The Peregrine (New York, 1979), Chapter XIX, p. 164.

Maybe we can read about this flying city in a previous episode? No, all that we find earlier in the Psychotechnic History is the founders of the Nomads remembering that:

"We had ridden centauroids who conversed with us as they went to the aerial city of their winged enemies -"
-Poul Anderson, "Gypsy" IN Anderson, Starship (New York, 1982), pp. 12-34 AT p. 29.

This is a slightly different description:

an "aerial" city might hover rather than fly;
"winged" antagonist might not be birds.

I am grateful for these two differently worded accounts.

Thus, Aesgil IV was visited at least twice, first by the lost Traveler, then later by a Nomad ship, the Peregrine. Aesgil IV must be the fourth planet of a fictional star called "Aesgil."

We are intrigued by this war between centaurs and sky-dwelling fliers. I thought that there might have been a corresponding Greek myth. Certainly centaurs fought. No matter how much Anderson wrote, some of his characters were going to recall adventures that we have not been able to read about. Nevertheless, we thank the Muses and the author himself for the extent of Anderson's works.

A Flying City In An Anderson Future History

The Traveler before she became the first Nomad ship: 

"We had ridden centauroids who conversed with us as they went to the aerial city of their winged enemies -"

-Poul Anderson, "Gypsy" IN Anderson, The Complete Psychotechnic League, Volume 2 (Riverdale, NY, February 2018), pp. 255-270 AT p. 267.

The Nomad ship, the Peregrine:

"'The flying city on Aesgil IV, and the war between the birds and the centauroids.'"

-The Peregrine, CHAPTER XIX, p. 164.

At least two ships had visited this planet. Each of these brief accounts gives us slightly different information. Aesgil IV is either the fourth planet of a star called Aesgil or the fourth moon of a gas giant planet of that name.

There are a few flying cities elsewhere in sf. We would have liked it if Anderson had written a story about this one. However, his works were bound to include more enigmatic references than complete stories. A film or a graphic adaptation could at least present some visuals of the city and the centauroids. 

Intelligent centauroids are one possibility. Such species also appear in the Technic History.

Untold Stories?

An author might create more background material for a work of science fiction than appears in the text. For example, a planetary ecology requires more globally interacting geographical areas and local environments than the hero is likely to encounter while traversing the planetary surface. Thus, the Author's Note to Poul Anderson's Virgin Planet discloses details about an uninhabited continent that remains unvisited during the novel.

Robert Heinlein's Future History Time Chart presents data, like an Antarctican Revolution, that do not appear in any of the stories and even lists in brackets six "stories-to-be-told" that never did get told. Might there be some such untold stories in Anderson's Psychotechnic History? Of course, none are listed in the Chronology. However, Anderson tells us that he:

compiled a Heinleinian chart of story titles;
occasionally wrote a story corresponding to one of the titles;
eventually did not complete the series but merely gave it up.

So there might have been more titles on his original time chart? The opening story mentions but does not feature Valti, the founder of psychotechnics. The Traveler crew had many adventures before colonizing Harbor. One involved riding and talking with centauroids en route to attack their winged enemies' aerial city. This adventure is mentioned again but not recounted. We learn that the centauroids, birds and flying city were on the planet Aesgil IV. But we would like to read that adventure.

Memories Of Memories

We can remember either the ambience of a dream or the enjoyableness of a book while forgetting every single detail.

In Poul Anderson's Psychotechnic History, did we read an account of centaurs attacking a flying city? No, but we did read two memories of this. See Aesgil IV.

In HG Wells' The Time Machine, did we read an account of an organism like a tentacled football flopping on a beach on the dying Earth in the Further Vision? Yes, but the Time Traveler momentarily thinks that he has imagined it, thus making us think that we have misremembered it when we reread the book. See A Few More Details In The Time Machine.

Memories are elusive and some authors play tricks with them.

Contexts

(Neptune: a gas giant planet and an alternative to either Jupiter or Saturn.)

Alternatively, Aesgil might be a gas giant and I-IV its satellites. That seems more likely.

Right now, this blog is focused on a few installments of a single future history series. However, our wider contexts remain:

the complete works of Poul Anderson;
all future history series;
in fact, all science fiction and imaginative literature;
the history, cosmology and philosophical issues that form the background of Anderson's works.

And here is one of those issues. Recently (here), we referred to the evolution of intelligence. Intelligence emerges when consciousness advances from sensation and perception to abstraction. An intelligent animal not only feels hot, and not only perceives sources of heat, but also applies the concepts of "hot" and "heat." I suggest that this advance from perception to abstraction is a qualitative transformation inside material brains and not an interaction between those brains and any specially created immaterial entities. We are part of "One Universe." In fact, we are its self-consciousness.