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Peter Apian's famous book, the Astronomicum Cæsareum of 1540, was dedicated to the Hapsburg rulers Emperor Charles V and King Ferdinand II - hence the title 'Astronomy of the Cæsars'. On the 14th June 1599, Tycho added his signature to the copy from which the facsimile edition (depicted here) was made, noting that he was in Wittenberg on his way from Denmark to Prague and, since he was entering the service of another Hapsburg ruler, that the journey was being made 'a Cæsare vocatûs'.