Merseia

Originally published on Poul Anderson Appreciation, Tues 24 April 2012.

Astronomical:

in the system of the Sol-like star Korych;
four moons - Neihevin, Seith, Lythyr and Wythna;
lethally close (about one parsec) to the supernova, Valendary.

When visited by a ship of the Grand Survey:

an industrial revolution starting by the Wilwidh Ocean in the northern hemisphere;
scientific method invented;
heliocentric astronomy;
post-Newtonian, pre-Maxwellian physics;
chemistry beginning;
a well-developed taxonomy;
speculations about evolution;
steam trains;
political power fragmented among the Vachs;
scientists, engineers and teachers each under the patronage of a Hand of a Vach.

When visited, two hundred years later, by the trader team:

the Vachs have confederated;
in the social dislocation following the industrial revolution, the baronial tradition survives as the Gethfennu, organized crime;
the most powerful Vach, Dathyr, is based in Castle Afon in the city of Ardaig;
other Vachs - Hallen, Ynvory, Rueth, Isthyr, landless Urdiolch;
the Republic of Lafdigu in the southern hemisphere;
interplanetary travel;
inconclusive space battles;
a Gethfennu colony on the planet Ronraud;
Star Believers regarding galactics as divine;
Demonists regarding them as demonic;
Adzel addresses a Believer gathering of clients, commoners and city proletariat. 

When visited, several centuries later, by Ensign Dominic Flandry:

the capital planet of an interstellar empire, the Roidhunate;
unified under the Roidhun, who is of Vach Urdiolch;
Brechdan Ironrede, current Hand of the Vach Ynvory, is Protector of the Roidhun's Grand Council;
twin capitals, ancient Ardaig on the bay of the River Oiss on the Wilwidh Ocean and modern antipodal Tridaig;
Castle Afon, the Roidhun's official primary residence, and the newly built Admiralty House are in Ardaig.

When Flandry is an Admiral:

his opposite number, Tachwyr the Dark, has become Hand of the Vach Dathyr and Protector of the Grand Council.

Sources: "Day of Burning"; Ensign Flandry; The Game of Empire.