The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610.


The four moons were discovered sometime between 1609 and 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than had ever been possible before.

On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons.

At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.

The name:

Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621). At Cosimo's suggestion, Galileo changed the name to Medicea Sidera ("the Medician stars"), honouring all four Medici brothers (Cosimo, Francesco, Carlo, and Lorenzo).

The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.

The Satellites:

The satellites discovered by Galileo with his small telescope form a small, "mini" solar system around Jupiter. They each have special characteristics related to their formation process, but have the following traits in common:

  1. all orbit Jupiter
  2. they all are tidally locked to Jupiter
  3. they all have radii larger than our Moon
  4. the inner moons have densities higher than outer moons (implies that Jupiter was much warmer in the past, such that the moons formed near Jupiter have less of the volatile elements such as CO2 and H2O)


Io is the innermost world, closest to Jupiter and can be classified as one of the most unusual moons in our solar system. Its unique properties include:


Europa is the next world out from Io and is considered a strong candidate to find primitive life. Its characteristics are:


Ganymede is much less dense than Europa or Io. Its interesting characteristics are:


Callisto is the outermost of the 4 primary satellites:

Summary of Galilean Moons: