Jupiter's 4 Galilean moons, in a composite image comparing their sizes and the size of Jupiter (Great Red Spot visible). From the top, they are Io, Europa, Ganymede, Callisto

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 small telescope or binoculars. In fact, if the observing conditions are sufficient, it is possible to see Callisto, the outermost, with the unaided eye. Image File history File links Download high resolution version (1028x1469, 97 KB)Courtesy NASA/JPL-Caltech. ... Image File history File links Download high resolution version (1028x1469, 97 KB)Courtesy NASA/JPL-Caltech. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... The Great Red Spot as seen from Voyager 1 The Great Red Spot is an anticyclonic storm on the planet Jupiter, 22° south of the equator, which has lasted at least 340 years. ... Atmospheric characteristics Atmospheric pressure trace Sulfur dioxide 90% Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter. ... Atmospheric characteristics Atmospheric pressure 1 µPa Oxygen 100% Europa (ew-roe-pə, IPA: (help· info); Greek Ευρώπη) is a moon of the planet Jupiter, smallest of the four Galilean moons. ... Atmospheric characteristics Atmospheric pressure trace Oxygen 100% Ganymede (IPA: , gan-i-meed, Greek Γανυμήδης) is Jupiters largest moon, and indeed the largest moon in the entire solar system; it is larger in diameter than Mercury but only about half its mass. ... Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (kə-lis-toe, IPA: ; Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Galileo Galilei Galileo Galilei (Pisa, February 15, 1564 – Arcetri, January 8, 1642), was an Italian physicist, astronomer, and philosopher who is closely associated with the scientific revolution. ... 50 cm refracting telescope at Nice Observatory. ... Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (kə-lis-toe, IPA: ; Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. ...

They were first observed by Galileo on January 7, 1610. It is now claimed that Gan De, a Chinese astronomer, may have discovered the moons 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 revolves around Earth. January 7 is the seventh day of the year in the Gregorian calendar. ... // Events January 7 - Galileo Galilei discovers the Galilean moons of Jupiter. ... Gan De (c. ... Heliocentric Solar System In astronomy, heliocentrism is the theory that the Sun is at the center of the Universe and/or the Solar System. ... Nicolaus Copernicus Nicolaus Copernicus (Polish: Mikołaj Kopernik; February 19, 1473 – May 24, 1543) was a Polish astrologer, astronomer, mathematician, administrator and economist. ... Earth is the third planet from the Sun. ...

Galileo first called his discovery the Cosmica Sidera, in honour of Cosimo II de' Medici (1590–1621), grand-duke of Tuscany from 1609, whose patronage Galileo wanted to secure. At the grand-duke's suggestion, Galileo changed the name to Medicea Sidera ("Medici stars"), because the Medici were four 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. Cosimo II de Medici, Grand Duke of Tuscany (12 May 1590 – 28 February 1621) ruled as Grand Duke of Tuscany from 1609 to 1621. ... Events March 14 - Battle of Ivry - Henry IV of France again defeats the forces of the Catholic League under the Duc de Mayenne. ... Events February 9 - Gregory XV is elected pope. ... Tuscany (Italian Toscana) is a region in central Italy, bordering on Latium to the south, Umbria and Marche to the east, Emilia-Romagna and Liguria to the north, and the Tyrrhenian Sea to the west. ... // Events 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. ... Sidereus 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. ... Location within Italy Venice (Italian: Venezia, Venexia in the local dialect), the city of canals, is the capital of the region of Veneto and of the province of Venice, 45°26′N 12°19′E, population 271,663 (census estimate January 1, 2004). ... // Events January 7 - Galileo Galilei discovers the Galilean moons of Jupiter. ...

Amongst the other names that were put forward, there is 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 them the Circulatores Jovis or Jovis Comites, and Jacques Ozanam called them Gardes or Satellites (from the Latin satelles, satellitis: escort). It would be the names proposed by Simon Marius (Simon Mayer), who claimed 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. Giovanni Batista Hodierna (1597-1660) was an astronomer at the court of the Duke of Montechiaro. ... // Events Mehmed Köprülü becomes Grand Vizier of the Ottoman Empire. ... Johannes Hevelius Johannes Hevelius or Johann Hewelke or Johannes Hewel (German) or Jan Heweliusz (Polish) (January 28, 1611–January 28, 1687) was a Polish astronomer; called the founder of lunar topography. ... Jacques Ozanam (1640 - April 3, 1717) was a French mathematician. ... Simon Marius Simon Marius (January 10, 1573 – December 26, 1624) was a German astronomer. ... Atmospheric characteristics Atmospheric pressure trace Sulfur dioxide 90% Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter. ... Atmospheric characteristics Atmospheric pressure 1 µPa Oxygen 100% Europa (ew-roe-pÉ™, IPA: (help· info); Greek Ευρώπη) is a moon of the planet Jupiter, smallest of the four Galilean moons. ... Atmospheric characteristics Atmospheric pressure trace Oxygen 100% Ganymede (IPA: , gan-i-meed, Greek Γανυμήδης) is Jupiters largest moon, and indeed the largest moon in the entire solar system; it is larger in diameter than Mercury but only about half its mass. ... Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (kÉ™-lis-toe, IPA: ; Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. ... Events April 5 - In Virginia, Native American Pocahontas marries English colonist John Rolfe. ...

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:

Name	Image	Interior structure	Diameter (km)	Mass (kg)	Semi-major axis (km)	Orbital period
Io			3643	8.93×1022	421,800	1.77 days
Europa			3122	4.8×1022	671,100	3.55 days
Ganymede			5262	1.48×1023	1,070,400	7.16 days
Callisto			4821	1.08×1023	1,882,700	16.69 days

The Galilean moons may have been known to the ancients: Babylonian Marduk (Jupiter) was said to be accompanied by four dogs (Jensen, Die Kosmologie der Babylonier, p. 131) and Egyptian Horus (Jupiter) had four sons (Mercer, Horus, the Royal God of Egypt, 1942). This is a conjecture, and not widely accepted as probable. At their closest distance to Earth, the moons have a magnitude 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 manage to see the moons with the naked eye, but whether this was actually achieved by the Chaldeans remains a matter of speculation. Atmospheric characteristics Atmospheric pressure trace Sulfur dioxide 90% Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter. ... Wikipedia does not have an article with this exact name. ... Image File history File links Download high resolution version (2666x2000, 566 KB) Interior of Io original description: Cutaway view of the possible internal structure of Io The surface of the satellite is a mosaic of images obtained in 1979 by NASAs Voyager spacecraft The interior characteristics are inferred from... Atmospheric characteristics Atmospheric pressure 1 µPa Oxygen 100% Europa (ew-roe-pə, IPA: (help· info); Greek Ευρώπη) is a moon of the planet Jupiter, smallest of the four Galilean moons. ... Image File history File links Download high resolution version (913x913, 462 KB) [[Melanie is a cool moon my aunt found this moon and she is still living nnnnnnaaaaaaasssssssass[[Media:--~~~~Example. ... Image File history File links Download high resolution version (2666x2000, 528 KB) Interior of Europa original description: Cutaway view of the possible internal structure of Europa The surface of the satellite is a mosaic of images obtained in 1979 by NASAs Voyager spacecraft. ... Atmospheric characteristics Atmospheric pressure trace Oxygen 100% Ganymede (IPA: , gan-i-meed, Greek Γανυμήδης) is Jupiters largest moon, and indeed the largest moon in the entire solar system; it is larger in diameter than Mercury but only about half its mass. ... Wikipedia does not have an article with this exact name. ... Image File history File links Download high resolution version (2667x2000, 609 KB) Interior of Ganymede original description: Voyager images are used to create a global view of Ganymede. ... Atmospheric characteristics Atmospheric pressure trace Carbon dioxide 100% Callisto (kə-lis-toe, IPA: ; Greek Καλλιστώ) is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. ... Wikipedia does not have an article with this exact name. ... Image File history File links Download high resolution version (2220x1700, 479 KB) Interior of Callisto original description: This artists concept, a cutaway view of Jupiters moon Callisto, is based on recent data from NASAs Galileo spacecraft which indicates a salty ocean may lie beneath Callistos icy... Marduk [märdook] (Sumerian spelling in Akkadian AMAR.UTU solar calf; Biblical Merodach) was the name of a late generation god from ancient Mesopotamia and patron deity of the city of Babylon, who, when Babylon permanently became the political center of the Euphrates valley in the time of Hammurabi... Horus is an ancient god of Egyptian mythology, whose cult survived so long that he evolved dramatically over time and gained many names. ... Chaldean can refer to an ancient people of lower Mesopotamia and their culture, or a contemporary Christian people living mostly in Iraq, Turkey, Syria, Lebanon, Iran, Michigan, as well as a relativley widespread diaspora concentrated in the western world. ...

Jupiter (satellites)edit
Inner satellites | Galilean moons: Io, Europa, Ganymede, and Callisto | Themisto
Himalia group | Carpo | S/2003 J 12 | Ananke group | Carme group | Pasiphaë group | S/2003 J 2
See also: Pronunciation key | Rings of Jupiter