Abstract

Though the details of who was first to see the four major satellites of Jupiter are obscured by the mists of time, it seems that Simon Mayr nearly simultaneously and independently discovered them and noted the discovery only 1 day after Galileo similarly discovered and noted it. The twin discoveries were confused by the use of different calendars by Marius and by Galileo, the former using the Julian calendar then still in use in Protestant regions and the latter using the new Gregorian calendar that was adopted in Catholic regions. Galileo was particularly sensitive to his priority, and the use of 1609 by Marius in the title of his Mundus Iovialis of 1614 raised particular ire, though adding the required 10 days for the conversion from O.S. to N.S. brought Marius’s discovery into early 1610. In the long run, we now use the names that Marius gave—Io, Europa, Ganymede, and Callisto—to what are called the Galilean satellites.