Galilean Moons

Callisto, Ganymede, Europa, and Io

From Astronomy Online

 

 

The Galilean moons are the four largest moons orbiting Jupiter (though they are not the closest). They were first discovered by Galileo Galilei (and independently, by Simon Marius) in 1609 and 1610 after Galileo turned his telescope towards Jupiter. He noticed these bright dots near Jupiter and as he followed Jupiter in the sky over the course of weeks, he observed those dots staying with Jupiter as it moved in the sky. He correctly concluded that these dots, which he first called stars, were gravitationally bound to Jupiter. He was the first to discover satellites orbiting another body other than the Earth. As we learned in the post about Galileo, there could be objects that did not orbit the Earth. This put another nail into the coffin, successfully debunking the theory that everything orbited the Earth, the Geocentric model of the Solar System.

 

When Galileo discovered these four moons, he wanted to name them for his sponsor, Cosimo de Medici, calling them Cosmian stars, or the Medici family, the Medician stars. He labeled them Jupiter I, Jupiter II, Jupiter III, and Jupiter IV based on increasing distance from Jupiter. This convention held up until the mid 20th century until moons closer to Jupiter were discovered. The four names we use today (Io, Europa, Ganymede, and Callisto), were actually proposed by Johannes Kepler and adopted by Simon Marius who had also observed the moons. Galileo did not like the names and kept his convention, never using the names we use today.

 

All four moons are bigger than any dwarf planet, including Pluto. In fact, Ganymede has a larger diameter, though a smaller mass, than Mercury. The three inner Galilean moons also experience a 4:2:1 resonance. For every four orbits around Jupiter completed by Io, Europa completes two orbits and Ganymede completes one. Callisto has an orbital period of 9.4 times that of Io, so it doesn't really fit the pattern.

 

All the moons are actually visible with amateur telescopes and binoculars, as long as none are obscured by Jupiter's face. The next four posts will go into more detail about these fascinating moons.