The Galilean moons are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei on January 7, 1610, and they were identified as satellites of Jupiter in March of that year. They were the first ever objects to be discovered orbiting an object other than Earth. They are some of the largest objects in the solar system, other than the sun and planets, of course. Indeed, Ganymede is the largest moon in the solar system, and is even larger than the planet Mercury, despite having half the mass. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 orbital resonance with each other. These Galilean moons are all spherical in shape, in contrast to the rest of Jupiter’s moons, which are more irregularly shaped, due to their weaker forces of self-gravitation.

Galileo discovered these moons after he had made some improvements to his telescope that allowed him to see celestial objects even better than before. His discoveries showed the importance of telescopes in observing such objects, especially compared to the naked eye, as the Galilean moons would not have ever been discovered without telescopes. In addition, the discovery was also important because they felt a crippling blow to the idea of the Ptolemaic world system, the geocentric theory that posited that everything revolved around earth.

Originally, Galileo named his discover the Cosmica Sidera, or “Cosimo’s Stars”, but their current names were eventually chosen by the astronomer Simon Marius. Marius had managed to discover the moons at virtually the same time that Galileo had, and a third astronomer, Johannes Kepler, had suggested the names. Until 1892, it was believed that these moons were the only ones of Jupiter, until the discovery of the moon Amalthea.

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