Simon Marius’s Mundus Iovialis: 400th Anniversary in Galileo’s Shadow

Simon Marius, Court Astronomer in Ansbach in Germany, independently discovered the moons of Jupiter one day after Galileo’s widely accepted discovery on 7 January 1610. Because Marius was using the Julian calendar (so-called O.S., Old Style), his discovery was made in 1609, though adding the 10 days of difference to transform, to the Gregorian calendar (so-called N.S., New Style) that Galileo was using, his notes of his discovery give 8 January 1610 (N.S.). Further, though Galileo famously published his Sidereus Nuncius in March 1610, Marius did not publish his discovery of four moons circling Jupiter until 1611, in a locally circulated almanac. He then published this work in a major book, Mundus Iovialis, though not until 1614. Galileo, who was forceful in asserting his priority, accused Marius of plagiarism in Il Saggiatore (1623), and Marius’s reputation was ruined for hundreds of years. Only in the early 1900s did a jury in the Netherlands assess the discovery claims and vindicate Marius, though Marius deserves more credit and recognition with the general public than he currently has. Still, the current names we use for the four major moons of Jupiter—Io, Europa, Ganymede, and Callisto—come from Marius’s book, of which approximately 30 copies from 1610 survive. Marius’s 1614 frontispiece, and his earlier almanac, show the four satellites in orbits, in contrast to Galileo’s use of asterisks and the letter O, so arguably Marius provided the first images of the orbits of what we call the Galilean satellites and what Galileo himself called first the Cosmean stars (Cosmica Sidera) and finally, in print, the Medicean stars (Medicea Sidera).