Planets And Timelines

Poul Anderson, "The Queen of Air and Darkness" IN Anderson, The Queen Of Air And Darkness and other stories (New York, December 1973), pp. 3-49.

Poul Anderson's "The Queen of Air and Darkness" is definitely the ninth and last instalment in his Rustum future history series whereas the same author's "Time Lag" is much more tentatively and speculatively - although I might be alone in suggesting this - the concluding instalment of a considerably shorter and diffuser "Directorate" future history series. What these two stories have in common is that each is set in a future period when enough time has elapsed that human beings, travelling in slower-than-light spaceships, have colonized several extra-solar planets and there is also some travel between the colonies although there is no longer any contact with Earth. In "Time Lag," at least in the volume of space where the action occurs, Earth is simply too far, a thousand parsecs, away whereas, in "The Queen...":

"Earth has long ago sunk into alien concerns." (p. 10)

That fits with what we are told in the Rustum stories. 

One of the reasons why people had left Earth was to preserve:

"...constitutional government..." (p. 6)

We remember the Constitutionalists on Rustum.

Eric Sherrinford says that birds big enough to carry a small child a short distance:

"'...do exist on Beowulf - and on Rustum, I've read.'" (p. 15)

This confirms that Roland and Rustum exist in the same timeline although, having said that, a fictional planet can, like Earth, Mars etc, exist in alternative timelines:

in James Blish's "A Case of Conscience," later adapted and expanded as A Case Of Conscience, Book One, the planet Lithia exists in 2049;

in Blish's "Seeding Program," near future events on Earth differ from those recounted in "A Case of Conscience" and Lithia is not mentioned;

in Blish's "Watershed," set millennia after "Seeding Program," Lithia exists;

in A Case Of Conscience, Book Two, Lithia is destroyed in 2050.

I think that this makes sense. Lithia plays a major role in A Case... and a minor role in "Watershed" and, in each case, its fate, whether imminent destruction or longer term survival, is appropriate to that narrative.

In any case, Rustum and Roland exist in a single timeline.