Where did the idea of Coruscant being in the center of the galaxy first appear?

Question

I'm re-reading Heir to the Empire (1991) in its 2011 annotated edition, which has footnotes from author Timothy Zahn about ideas he'd come up with for the book, and whether or not they fit with later canon. One example is a mention of Tatooine being a "quarter of the way across the galaxy" from Coruscant:

A quarter of the way across the galaxy,¹⁵ Han Solo sipped at his mug and surveyed the semiorganized chaos flowing all around him. Didn't we, he quoted to himself, just leave this party?

¹⁵ As no official map of the Star Wars galaxy existed at this time (and wouldn't for many years to come), any positional relationship between Coruscant and Tatooine was pure guesswork. —TZ

_{(Heir to the Empire: The 20th Anniversary Edition, Chapter 2)}

Since then, maps of the Star Wars galaxy have all placed Coruscant near the center and Tatooine in the Outer Rim. It seems like Tatooine's location is based on Luke's comment from A New Hope about Tatooine being the "place that's farthest from" the "bright center to the universe", but where did the idea of Coruscant being at the center come from? Was this from the West End Games sourcebooks, or was it introduced after the Thrawn trilogy?

Answer 1

The division of the known galaxy between the "galactic core" full of rich, powerful, high-tech planets (including the Imperial Capital) and the "outer rim" of lawless frontier worlds, with various stages of development in between, seems to date to the first edition of the West End Games Star Wars: The Roleplaying Game (1987). As in later RPG books, this broad categorization lets them fit in a lot of breadth of different planets to play with, without getting too bogged down in details and taking up the whole book.

I don't have the original book on hand, so I don't know if these terms were originally intended to be relative to the galaxy or to the Empire, but they suggest the former (especially the use of "galactic core" instead of the "core worlds" that is more common in later works). The RPG itself never included a map of the galaxy, only of individual sectors and trade routes of note to a particular campaign.

The Star Wars: Behind the Magic CD extras (1998) included the first definitive map of the galaxy, with Coruscant in its familiar location due north of the core and Tatooine in the Outer Rim (although interestingly, not that far out in comparison; there are some much more remote planets featured).

In short, the division of core, colonies, and rim seems to have been set down by the early West End RPGs and though later maps have elaborated on it a lot, the basic idea is still the same.