The Flammarion Engraving

In the foreword to the Lorebook of the Void, one of the two volumes included in the original Spelljammer boxed set, Jeff Grubb talks about a bit about its creative origins. One paragraph of that foreword has long stuck with me.

When I first read this, I thought it was very cool, because, even at the time, I thought D&D could do with a little more genuinely medieval influence on its fantasy. Even if, in the end, the cosmos presented by Spelljammer bore only the most superficial resemblance to the conceptions of medieval thinkers, it was still (in my youthful eyes anyway) a step in the right direction. 
I spent some time in the early '90s trying to find these "medieval woodcuts" to no avail. This was before Internet search engines were very good, so I wasn't completely surprised that I might not find a good example of what he might have been referring to. However, some years later I did come across one image that looked like it might have been the kind of thing Grubb had seen. This is the so-called "Flammarion engraving," which first appeared in the 1888 book, L'atmosphère : météorologie populaire, by Camille Flammarion. Apparently, its artist is unknown, but it became very popular in the 1960s as an illustration of psychedelic experiences, the opening of the human mind to new realities. Regardless of its original intent, it's a very striking and evocative image, so I can understand why it is that Grubb might have been inspired by it, if indeed this is the "medieval woodcut" that David Cook showed him all those years ago.
Looking around online, I discovered a blog post by Grubb from more than a decade ago in which he talks a bit about the creation of Spelljammer. It's a very interesting post, filled with plenty of details I didn't know. Among those details is Grubb's admission that, yes, the above image was indeed the one that inspired him, though he connects it to Daniel Boorstin's 1983 book, The Discoverers, rather than Flammarion. I'm glad to know that my guess was correct. Anyway, read the whole blog post if you'd like to know more about the prehistory of Spelljammer.