Galileo discovered four satellites revolving around Jupiter, printing more than 60 observations of their positions from night to night.
The satellites of Jupiter proved that multiple centers of revolution exist in the solar system. These satellites removed an anomaly in the Copernican system, for why should the Earth have its own Moon unlike any of the other planets? The satellites also proved, contrary to the argument of the Aristotelian physicists, that a moving planet would not leave its Moon behind. Jupiter was a miniature Copernican system set in the sky as a confirmation of the theory of the leading 16th century Catholic astronomer.
Then Galileo had another idea: the satellites of Jupiter were his ticket to return to Tuscany. He hastily printed a new name to paste over the name already appearing on the previously printed pages, in order to dedicate the work to Cosimo, and call them the Medicean stars. Jupiter stood for the Grand Duke, and the four satellites stood for the Grand Duke’s four sons. What could be better, other than the fact that the eldest son, Cosimo, had just become the next Grand Duke?
In the dedication, Galileo noted that Jupiter appeared in a royal position at Cosimo’s birth, and waxed eloquent in courtly praise:
Serenissimo Grand Duke, “scarcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens, which, like tongues [longer lived than poets] will speak of and celebrate your most excellent virtues for all time.”
No wonder the King of Spain, the Holy Roman Emperor, the French ambassador to Italy and others invited Galileo to join their courts.
“We have moreover an excellent and splendid argument for taking away the scruples of those who, while tolerating with equanimity the revolution of planets around the Sun in the Copernican system, are so disturbed by the attendance of one Moon around the Earth while the two together complete the annual orb around the Sun that they conclude that this constitution of the universe is impossible. For ... our vision offers us four stars wandering around Jupiter like the Moon around the Earth.” Galileo, Sidereus Nuncius, trans. Albert Van Helden (University of Chicago, 1989).