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This week I’m back with my equatorium, trying to pin down precisely how well it models Ptolemy’s planetary theories (and, in turn, finding out the strengths and weaknesses of those theories in predicting the locations of the planets in the night sky).  Today it’s the turn of the Winged Messenger, Mercury. In my last “medieval craftsmanship” post I described the procedure for using the Equatorie of the Planetis to find the longitude of most planets.  I mentioned that the proce