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. They are by far the largest of the many moons of Jupiter. 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. ... A false-color image of the Great Red Spot of Jupiter from Voyager 1. ... Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ... Apparent magnitude: 5. ... This article or section does not adequately cite its references or sources. ... There is also an asteroid named 204 Kallisto. ... Atmospheric characteristics Atmospheric pressure 70 kPa Hydrogen ~86% Helium ~14% Methane 0. ... Galileo Galilei (15 February 1564 – 8 January 1642) was an Italian physicist, mathematician, astronomer, and philosopher who is closely associated with the scientific revolution. ...

Discovery

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 one of Jupiter's moons in 362 BC, nearly 2 millennia earlier^[1]. is the 7th day of the year in the Gregorian calendar. ... // Events January 7 - Galileo Galilei discovers the Galilean moons of Jupiter. ... This article or section is not written in the formal tone expected of an encyclopedia article. ... Centuries: 5th century BC - 4th century BC - 3rd century BC Decades: 410s BC 400s BC 390s BC 380s BC 370s BC 360s BC 350s BC 340s BC 330s BC 320s BC 310s BC 367 BC 366 BC 365 BC 364 BC 363 BC 362 BC 361 BC 360 BC 359...

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. Heliocentric Solar System Heliocentrism (lower panel) in comparison to the geocentric model (upper panel) In astronomy, heliocentrism is the idea that the sun is at the center of the Universe and/or the Solar System. ... “Copernicus” redirects here. ... This article is about Earth as a planet. ...

Name

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. Cosimo II de Medici, Grand Duke of Tuscany (12 May 1590 – 28 February 1621) ruled as Grand Duke of Tuscany from 1609 to 1621. ... Bold text{| align=right cellpadding=3 id=toc style=margin-left: 15px; |- | align=center colspan=2 | Years: 1587 1588 1589 - 1590 - 1591 1592 1593 |-vdsf gno[gldw[pvkijxaiamknn csogfhbvdowkhbfkqhjkhrjkhwgfhbjkpnkfokfgok3pkpk9pjhkt9erktyujkip9kijker9thhrkg9hkitr9gtkih9t0ykltk[u0jo0iey9uhyit90ertyhige9rity9riyh9ujirtyuhjnh-4e9tyigh9thiuy0h8tyh34tu8uy8u8u8u8rtu5y8ru8thu0tru0ut0rhutuh0trhu0hseogtrhr8uyhju8t89er9te9r8fy8shit ass dick bitch fuck | align=center colspan=2 | Decades: 1560s 1570s 1580s - 1590s - 1600s 1610s 1620s |- | align=center | Centuries... 1621 was a common year starting on Friday of the Gregorian calendar (or a common year starting on Monday of the 10-day slower Julian calendar). ... Tuscany (Italian: ) is one of the 20 Regions of Italy. ... // 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. ... The Medician Stars are four moons of Jupiter, which were discovered by Galileo Galilei, mostly likely in December of 1609 or January of 1610. ... Carlo de Medici (March 19, 1595–June 17, 1666) was the son of Ferdinando I de Medici, Grand Duke of Tuscany and Chrétienne de Lorraine. ... 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. ... Venice (Italian: Venezia, Venetian: Venezsia, Latin: Venetia) is a city in northern Italy, the capital of region Veneto, and has a population of 271,251 (census estimate January 1, 2004). ... // Events January 7 - Galileo Galilei discovers the Galilean moons of Jupiter. ...

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 Committees', and Jacques Ozanam called them 'Gardes' or 'Satellites' (from the Latin satelles, satellitis, meaning "escorts"). 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 (Latin), also called Johann Hewelke, Johannes Höwelcke or Johannes Hewel (in German), or Jan Heweliusz (in Polish), (born January 28, 1611 – died January 28, 1687), was a councillor and mayor in Danzig (Gdańsk). ... Jacques Ozanam (1640 - April 3, 1717) was a French mathematician. ...

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 Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614. Simon Marius Simon Marius (January 10, 1573 – December 26, 1624) was a German astronomer. ... Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ... Apparent magnitude: 5. ... This article or section does not adequately cite its references or sources. ... There is also an asteroid named 204 Kallisto. ... Events April 5 - In Virginia, Native American Pocahontas marries English colonist John Rolfe. ...

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:

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

Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ... 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... Apparent magnitude: 5. ... 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. ... This article or section does not adequately cite its references or sources. ... 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. ... There is also an asteroid named 204 Kallisto. ... 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...

Visibility

All four Galilean moons are bright enough that they could, if they were farther away from Jupiter, be sighted without a telescope. They have apparent magnitudes between 4.5 and 5.2 when Jupiter is in opposition with the Sun, and about one unit of magnitude higher when Jupiter is in conjunction. The main difficulty in observing them is due to the fact that they are located very close to Jupiter, and are masked by its brightness. Their maximum angular separations from Jupiter are between 2 and 8 minutes of arc, close to the limit of human visual acuity. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation. The easiest way to observe them is to cover Jupiter with an object, e.g. a tree limb or a power line that is perpendicular to the plane of moons' orbits. Opposition is a term used in positional astronomy and astrology to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place (usually the Earth). ... This article does not cite any references or sources. ...

Io

Main article: Io (moon)

Of the four Galilean moons, Io is the closest to Jupiter and the second smallest. Io is the fourth largest moon in the solar system, is the most volcanically active body in the solar system, has had the largest recorded volcanic eruptions, and has the highest density of all the moons in the solar system. Io is estimated to be 3642.6 kilometers in diameter, and, because of its smaller size, only has 18.3% Earth’s gravity. Io got its name from Greek mythology, and although Simon Marius suggested the name soon after its discovery, Io was simply referred to as “Jupiter I,” or “The first satellite of Jupiter,” until mid-20th century. Io does not seem to have many craters in pictures because it has many active volcanoes, 9 of which were observed by Voyager I. Although not proven, recent data from the Galileo orbiter indicates that Io might have its own magnetic field. Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO₂). If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter. Atmosphere Surface pressure: trace Composition: 90% sulfur dioxide Io (eye-oe, IPA: , Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, is the fourth largest moon in the Solar System. ... Sulfur dioxide (or Sulphur dioxide) has the chemical formula SO2. ... Venera 7 lander Color image taken from the surface of Venus by the Soviet Venera 13 lander The Venera (Russian: Венера; formerly, sometimes referred to as Venusik in the West) series of probes was developed by the USSR to gather data from Venus. ...

Europa

Main article: Europa (moon)

Europa, the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter. Europa is the smoothest object in the solar system. There is a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick. The top of the water is ice and under the ice, the water is liquid. If any life exists in the water, it would be similar to Earth’s deep-sea creatures. Several things could cause the marks on the surface of the moon. Some theories are that Jupiter’s gravity is causing these markings, as one side of Europa is constantly facing Jupiter; volcanic water eruptions splitting the surface of Europa; and even geysers have been considered as a cause. The colour of the markings, reddish-brown, is thought to be caused by sulfur, but scientists can't be sure, as no data collection devices have been sent to Europa. These markings crossing the moon emphasize a flatter, smoother surface. Some have even considered the reddish-brown markings to be bacteria under the ice on Europa. Any bacteria on Europa would be similar to the extremophile bacteria found on Earth because of the intense conditions on this moon. The name Jupiter II was used for a long time instead of Europa. Again, Europa’s name was from Greek mythology, was suggested by Simon Marius, and wasn't used widely until mid-20th century. Apparent magnitude: 5. ...

Ganymede

Main article: Ganymede (moon)

Ganymede, the third Galilean moon and the seventh known satellite from Jupiter, is less shrouded in mystery than Europa. Ganymede is the largest moon in the entire solar system at 5262.4 kilometers in diameter. Ganymede is the only moon in the solar system known to have a magnetic field. Ganymede is composed of silicate rock and water ice, with an ice crust floating over a warmer ice mantle. The metallic core of Ganymede suggests a greater heat at some time in Ganymede's past than had previously been proposed. The surface of Ganymede is a mix of two types of terrain – highly cratered dark regions and younger, but still ancient, regions with a large array of grooves and ridges. Ganymede has a high number of craters, but many are gone or barely visible due to Ganymede’s icy crust forming over them. A small oxygen atmosphere is present on Ganymede. Ganymede was simply called Jupiter III (three) until mid 20th-century, when Simon Marius’s name began to be used. This article or section does not adequately cite its references or sources. ...

Callisto

Main article: Callisto (moon)

Callisto is the fourth and last Galilean moon, and is the second largest at 4820.6 kilometers in diameter. Callisto is also the least dense of the Galilean moons. Callisto is one of the most heavily cratered satellites in the solar system, and has one major feature, a basin around 3000 km wide called Valhalla, which probably dates back to the formation of Callisto’s crust. This moon’s surface lies above a layer of ice, which is 150 kilometers thick, and a layer of water, which is ten kilometers thick. Callisto’s crust is approximately four billion years old. Callisto has a small atmosphere of carbon dioxide. Callisto too was called Jupiter IV (four) until mid 20th-century, when Marius’s name came into operation. There is also an asteroid named 204 Kallisto. ... Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms, and is in a gaseous state in the atmosphere of the Earth. ...

Notes

See also

• Jupiter's natural satellites
• Jupiter's moons in fiction

Jupiters outer moons and their highly inclined orbits. ... Galilean moons of Jupiter Jupiters extensive system of natural satellites – in particular the four large Galilean moons (Io, Europa, Ganymede and Callisto) – has been a common science fiction setting. ...

External links

• Animation of Galileo's observation, march 1613