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The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei. By far the largest of the many moons of Jupiter, they are visible even in a low-power telescope. In fact, if the observing conditions are perfect, it is just possible to see Callisto, the outermost, with the unaided eye. They were first observed by Galileo on January 7, 1610.

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

Galileo first called his discovery the Cosmica Sidera, in honour of Cosimo II dé Medici ( 1590- 1621), grand-duke of TuscanyTuscany ( Italian Toscana is a region in central Italy, bordering on Latium to the south, Umbria to the east, Emilia-Romagna and Liguria to the north, and the Tyrrhenian Sea to the west. It is often regarded as among the most beautiful parts of Italy. from 1609Events April 4 King of Spain signs an edit of expulsion of all moriscos from Spain April 9 Spain recognizes Dutch independence May 23 Official ratification of the Second Charter of Virginia. July 6 Bohemia is granted freedom of religion. August 25 Galileo, whose patronage Galileo wanted to secure. At the grand-duke's suggestion, Galileo changed the name to Medicea Sidera ("Médici stars"), because the Médici were four brothers. The discovery was announced in the Sidereus NunciusSidereus Nuncius (usually translated into English as Sidereal Messenger although Starry Messenger and Sidereal Message are also seen) is a short treatise published in Latin by Galileo Galilei in March 1610. It was the first scientific treatise based on ob ("Starry Messenger"), published in VeniceVenice ( Italian Venezia German Venedig , the city of canals, is the capital of the region of Veneto, population 271,073 (2001). The city stretches across numerous small islands in a marshy lagoon along the Adriatic Sea in northeast Italy. The saltwater l in March 1610, less than two months after the first observations.

Amongst the other names that were put forward, there is Principharus, Victipharus, Cosmipharus and Ferdinandipharus, for each of the four Médici brothers, proposed by Hodierna , a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656Events Masuria is devastated during the Deluge when it was raided by Tartars and Poles End of the war started in 1648 between Poland, Ducal Prussia, Russia and Transylvania. The only year the British coin Fifty Shillings was minted. Births October 29 ( Ju).

It would be the names proposed by Simon Marius (Simon Mayer), who pretended to have discovered the moons at the same time as Galileo, that would eventually prevail: Io, Europa, Ganymede and Callisto, published in his Mundus Jovialis in 1614. Galileo steadfastly refused to use Marius' 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:

See also: Jupiter's natural satellites