Lorebook, Spelljammer boxset 1989, "Foreword" by Jeff Grubb: "But if space functions normally, how (for example) do the constellations of Krynn move around without messing up other planets? Zeb Cook pulled out some medieval woodcuts showing a traveler passing through the spheres of the world to discover the sun and planets on tracks, and with that the idea of crystal shells was born. Each fantasy solar system could have its own placement and rules, while being a part of a larger whole."
