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Galilean moons were first observed by Galileo.

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Galilean moons are often called Galilean satellites. Galileo was the person who witnessed the moons of Jupiter. These are now termed Galilean moons. Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. They are by far the largest of the many moons of Jupiter.

Visibility of Galilean moons.

Galilean moons.
Galileo discovered Jupiter's 4 moons. Jupiter (Great Red Spot visible). From the top, they are Io, Europa, Ganymede, Callisto.

The Galilean moons are visible from Earth with a small telescope or binoculars. In fact, if the observing conditions are sufficient, it is possible to see Ganymede with the naked eye. At their closest distance to Earth, the moons have magnitudes of 4.6 (Ganymede) to 5.6 (Callisto). Io at its apsis is separated from Jupiter by about two arc minutes. It is theoretically possible that dedicated and well-trained observers could see the moons with the naked eye, but there is no evidence that this has ever been achieved.

Discovery of Galilean moons.

The Galilean moons were first observed by Galileo on January 7, 1610. A Chinese historian of astronomy, Xi Zezong, has claimed that Gan De, a Chinese astronomer, may have seen Ganymede in 362 BC, nearly 2 millennia earlier.

Galileo observed the moons' motion over several days and realized that they were in orbit around Jupiter. This discovery supported the heliocentric theory of Nicolaus Copernicus and showed that not everything in the heavens revolves around Earth.

Name of Galilean moons.

Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars", in honour of Cosimo II de' Medici (1590-1621), grand-duke of Tuscany from 1609, whose patronage he wanted to secure. At the grand-duke's suggestion, Galileo changed the name to Medicea Sidera ("the Medician stars"), honouring all four Medici brothers (Cosimo, Francesco, Carlo, and Lorenzo). The discovery was announced in the Sidereus Nuncius ("Starry Messenger"), published in Venice in March 1610, less than two months after the first observations.

Other names put forward include 'Principharus, Victipharus, Cosmipharus and Ferdinandipharus', for each of the four Medici brothers, proposed by Giovanni Batista Hodierna, a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656). Johannes Hevelius called the moons the 'Circulatores Jovis' or 'Jovis Comites', and Jacques Ozanam called them 'Gardes' or 'Satellites' (from the Latin satelles, satellitis, meaning "escorts").

The names that eventually prevailed were chosen by Simon Marius, who claimed to have discovered the moons at the same time as Galileo: he named them after lovers of the god Jupiter: Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.

Galileo steadfastly refused to use Marius's names and invented as a result the numbering scheme that is still used nowadays, in parallel with proper moon names. The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede and Callisto respectively. Galileo used this system in his notebooks but never actually published it.

The Galilean moons are, in order from closest to Jupiter to farthest away:

axis (km)
period (days)
Io Io. Io interior. 3643 8.931022 421,800 1.77
Galilean moon Europa. Europa. Europa interior. 3122 4.81022 671,100 3.55
Galilean moon Ganymede. Ganymede. Ganymede interior. 5262 1.481023 1,070,400 7.16
Galilean moon Callisto. Callisto. Callisto interior. 4821 1.081023 1,882,700 16.69

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