Which astronomer proposed the names Io, Europa, Ganymede, and Callisto?

Simon Marius proposed the names Io, Europa, Ganymede, and Callisto for Jupiter’s four largest moons.

Galileo Galilei observed the four moons in 1610 and initially called them the Medicean Stars, honoring the Medici family. German astronomer Simon Marius observed them independently around the same time and published his account later, in 1614, in Mundus Iovialis. He proposed names drawn from figures associated with Jupiter in Greek mythology; Johannes Kepler had suggested the naming theme.

A common mix-up is crediting Galileo with both the discovery and the names. Galileo is credited with the discovery in mainstream histories, while the moons are collectively called the Galilean moons in his honor. Their standard numerical designations—Jupiter I through IV—run outward from the planet: Io, Europa, Ganymede, and Callisto.

The four satellites are scientifically diverse. Io is intensely volcanic, Europa has an ice-covered ocean of major astrobiological interest, Ganymede is the Solar System’s largest moon, and Callisto is heavily cratered and ancient.