What Are The Galilean Moons?

Who discovered them?

Galileo Galilei in 1610. According to NASA, Jupiter's four largest moons were the first moons discovered beyond Earth. They are called the Galilean satellites after Italian astronomer Galileo Galilei, who is credited with their discovery in 1610. German astronomer Simon Marius observed them around the same time but published later. Galileo first spotted three of them on January 7, 1610, with a fourth appearing the following night. He used a primitive telescope of his own design that magnified only about 20 times.

How did they change science?

They disproved geocentrism. The Galilean moons are important in the history of science because their discovery disproved the theory that all objects in the cosmos orbited the Earth. Before 1610, the prevailing view was that Earth was the center of the universe, with everything else (including Sun, Moon, planets, and stars) orbiting around it. Galileo's observations clearly showed four objects orbiting Jupiter, not Earth. This provided strong support for the Copernican model in which planets orbit the Sun. The discovery contributed to the scientific revolution.

How were they named?

After mythological figures associated with Jupiter. In 1614, German astronomer Johannes Kepler suggested naming the satellites after mythological figures associated with Jupiter (the king of the gods), specifically lovers and companions of his Roman counterpart: Io, Europa, Ganymede, and Callisto. Marius proposed these names too. Galileo had originally called them the Medicean Stars to honor his patrons, but Kepler's mythological names eventually became standard. The moons are listed by their distance from Jupiter, from innermost (Io) to outermost (Callisto).

What makes each unique?

Distinct geology and history. Io is the most volcanically active body in the solar system, with hundreds of active volcanoes. Europa is covered by a smooth, cracked ice shell hiding a global ocean of liquid water beneath. Ganymede is the largest moon in the solar system (bigger than Mercury) and the only moon with its own magnetic field. Callisto is the most heavily cratered body in the solar system, with an ancient surface preserved from the early solar system. Each will be visited by future missions.