Doing a Google Search for Magrathea results in thousands of web
references. I didn't find any Middle Ages-related references in a quick
glance-through, but there are many (!) references to Douglas Adams and THE
HITCHHIKERS' GUIDE TO THE GALAXY, so "Magrathea" apparently figures in his
writings. I've never been able to get interested in Adams' work, so I
certainly don't know more than that.
Seems to me that there's a novel or story entitled DOPPELGANGER PLANET
which has it that there's a Terra-double which rotates exactly on the other
side of Sol, always out of our sight-- hmmm, and somehow apparently always
out of sight of any probes we send out, also.
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To the Taliban: "Give us Osama bin Laden, or we'll take your
women and send them to college."