What name did Galileo give the four largest moons of Jupiter?

Galileo called the four largest moons of Jupiter the Medicean stars. He chose the name to honor the powerful Medici family of Florence, especially his former pupil Cosimo II de’ Medici.

Galileo discovered the moons during his January 1610 observations of Jupiter and announced them in Sidereus Nuncius, published in March that year. He initially proposed a name honoring Cosimo alone, but the final plural title, Medicea Sidera in Latin, recognized the four Medici brothers: Cosimo, Francesco, Carlo, and Lorenzo. The dedication was also a strategic appeal for patronage.

The name did not remain the standard one. Astronomer Simon Marius later proposed the individual names Io, Europa, Ganymede, and Callisto, drawn from figures associated with Jupiter or Zeus in classical mythology. Those names eventually became standard, while the collective term Galilean moons commemorates their discoverer.

“Galilean moons” is therefore the modern name, not the name Galileo originally gave them. “Cosmian stars” is a related near-miss: Galileo briefly considered a name based on Cosimo, but Medicean stars was the formal designation used in his publication.