Galilean Moons

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This composite includes the four largest moons of Jupiter which are known as the Galilean satellites. The Galilean satellites were first seen by the Italian astronomer Galileo Galilei in 1610. Shown from left to right in order of increasing distance from Jupiter, Io is closest, followed by Europa, Ganymede, and Callisto.

The Galilean Moons are the four largest moons of Jupiter. They are much different from the other 77 moons, in terms of appearance, composition, and size. They were named so because they were discovered by Icons-flag-it.png Italian astronomer Galileo Galilei, but also discovered and named by Icons-flag-de.png German astronomer Simon Marius, using names suggested by Johannes Kepler's Mundus Jovialis, published in 1614.[1] However, early astronomer Xi Zezong claimed that a "small reddish star" observed close to Jupiter in 362 BC by early astronomer Gan De may have been Ganymede, predating Galileo's discovery by around almost 2000 years.[2]

The Galilean Moons are Io, Europa, Ganymede, and Callisto.

Ganymede is larger than the planet Mercury. All of the Galilean moons, except for Europa, are larger than the Earth's moon.


  1. Pasachoff, Jay M. (2015). "Simon Marius's Mundus Iovialis: 400th Anniversary in Galileo's Shadow". Journal for the History of Astronomy 46 (2): 218–234. Bibcode 2015AAS...22521505P. doi:10.1177/0021828615585493.
  2. Zezong, Xi, "The Discovery of Jupiter's Satellite Made by Gan De 2000 years Before Galileo", Chinese Physics 2 (3) (1982): 664–67.