Hey! (Wow...Gary Gygax...heh)
I read something really interesting in the last Paizo produced Dragon magazine (before the rights reverted to WotC). They quoted you from a 1984 interview with Polyhedron saying:
"By the way, action takes place on Yarth, a place somewhat similar to Oerth, the setting of Greyhawk et al. It has fewer magical properties than Oerth, but more than Earth. It is not impossible that additional works will be contracted for in months to come, action being set on Yarth or perhaps another alternate world, Aerth. On Earth, magic is virtually non-existant. On Uerth, dwomers are weak, chancy things. Yarth has a sprinking of things magical, and Oerth is pure magic."
Now I am well aware of what has been done with the various D&D settings by other people, but I'm curious to go to the man himself:
What's the structure of YOUR Greyhawk cosmology? How do the different settings (Earth, Yarth, Oerth, etc.) connect?
Any more info you have on what Yarth, Aerth, and Uerth are like in contrast to Greyhawk and Earth?
I read what you wrote and was really intrigued.