Grammatically speaking, the antecedent of this could possibly be "the western spiral arm" or "the Galaxy."

However, the phrase a distance of roughly ninety-two million miles makes sense only if this refers to the yellow sun, in relationship to the "little blue-green planet." Note that the Galaxy is approximately 6x10^17 miles in diameter (600 quadrillion in American terminology), compared with a mere 9x10^7 miles between the Earth and the Sun!

By the way, there's obviously no astronomical logic for using compass directions like "north" or "western" when talking about the Galaxy. However, a repeating theme of HHGG is that familiar human institutions like bureaucracy, and loud rock music, and fashionable restaurants where it's very difficult to get a table, are found throughout the Universe. So in that context, it seems perfectly reasonable that extraterrestrial civilizations would refer to "the unfashionable end of the western arm of the Galaxy."