The Galilean moons (), or Galilean satellites, are the four largest moons of Jupiter. They are, in descending-size order, Ganymede, Callisto, Io, and Europa. They are the most readily visible Solar System objects after Saturn, the dimmest of the classical planets; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars, even under night sky conditions of high light pollution. The invention of the telescope allowed astronomers to discover the moons in 1610. Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.
They are planetary-mass moons and among the largest objects in the Solar System. All four, along with Titan, Triton, and Earth's Moon, are larger than any of the Solar System's dwarf planets. The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet Mercury in size (though not mass). Callisto is only slightly smaller than Mercury in size; the smaller ones, Io and Europa, are about the size of the Moon. The three inner moons — Io, Europa, and Ganymede — are in a 4:2:1 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter's remaining moons have irregular forms because they are too small for their self-gravitation to pull them into spheres.
The Galilean moons are named after Galileo Galilei, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in 1892. Galileo initially named his discovery the Cosmica Sidera ("Cosimo's stars") or Medicean Stars, but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, after mythological characters that Zeus seduced or abducted, which were suggested by Johannes Kepler in his Mundus Jovialis, published in 1614. Their discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted (among educated Europeans) Ptolemaic world system, a geocentric theory in which everything orbits around Earth.
History
Pre-discovery
Chinese astronomical records report that in 365 BC, Gan De observed a small reddish star beside Jupiter, which might have been a moon of Jupiter, possibly Ganymede, with the naked eye. However, Gan De reported the color of the companion as reddish, which is puzzling since Ganymede is too faint for its color to be perceived with the naked eye. Shi Shen and Gan De together made fairly accurate observations of the five major planets. If the companion is indeed Ganymede, this might predate Galileo's discovery by around two millennia.
Discovery
As a result of improvements that Galileo Galilei made to the telescope, with a magnifying capability of 20×, he was able to see celestial bodies more distinctly than was previously possible. This allowed Galileo to observe in January 1610 what came to be known as the Galilean moons.
On 7 January 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. It turned out to be Ganymede, Callisto, and the combined light from Io and Europa. The next night he noticed that they had moved. On January 13, he saw all four at once for the first time, but had seen each of the moons before this date at least once. By January 15, Galileo concluded that the stars were actually bodies orbiting Jupiter. He continued to observe these celestial orbs to 2 March 1610.
Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye. More importantly, the discovery of celestial bodies orbiting something other than Earth dealt a blow to the then-accepted Ptolemaic world system, which held that Earth was at the center of the universe and all other celestial bodies revolved around it. Galileo's 13 March 1610, Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe. Nevertheless, Galileo accepted the Copernican theory.
Simon Marius claimed another observation, and he later reported observing the moons in November 1609, with a first written record for 29 December 1609. However, because he did not publish these findings until after Galileo, there is a degree of uncertainty around his records. Later, it turned out that he had used the old style Julian calendar, while Galilei used the modern style Gregorian one. So, in fact, Marius observed the moons first on 8 January 1610, one day after Galilei.
Names
In 1605, Galileo had been employed as a mathematics tutor for Cosimo de' Medici. In 1609, Cosimo became Grand Duke Cosimo II of Tuscany. Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter's moons to gain it. On 13 February 1610, Galileo wrote to the Grand Duke's secretary:
Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo alone. Cosimo's secretary suggested to change 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.
On 12 March 1610, Galileo wrote his dedicatory letter to the Duke of Tuscany, and the next day sent a copy to the Grand Duke, hoping to obtain the Grand Duke's support as quickly as possible. On 19 March, he sent the telescope he had used to first view Jupiter's moons to the Grand Duke, along with an official copy of Sidereus Nuncius (The Starry Messenger) that, following the secretary's advice, named the four moons the Medician Stars. In his dedicatory introduction, Galileo wrote:
Other names put forward include: I. Principharus (for the "prince" of Tuscany), II. Victripharus (after Vittoria della Rovere), III. Cosmipharus (after Cosimo de' Medici) and IV. Fernipharus (after Duke Ferdinando de' Medici) – by Giovanni Battista Hodierna, a disciple of Galileo and author of the first ephemerides (Medicaeorum Ephemerides, 1656); Circulatores Jovis, or Jovis Comites – by Johannes Hevelius; Gardes, or Satellites (from the Latin satelles, satellitis, meaning "escorts") – by Jacques Ozanam.
The names that eventually prevailed were chosen by Simon Marius, who had discovered the moons independently at about the same time as Galileo. At the suggestion of Johannes Kepler, he named them after four lovers of the god Zeus (the Greek equivalent of Jupiter) 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 numbered names (Jupiter x) were used until the mid-20th century when other inner moons were discovered, and Marius's names became widely used.
Determination of longitude
Galileo's discovery had practical applications. Safe navigation required accurately determining a ship's position at sea. While latitude could be measured well enough by local astronomical observations, determining longitude required knowledge of the time of each observation synchronized to the time at a reference longitude. The longitude problem was so important that large prizes were offered for its solution at various times by Spain, The Netherlands, and The United Kingdom.
Galileo proposed determining longitude based on the timing of the orbits of the Galilean moons. The times of the eclipses of the moons could be precisely calculated in advance and compared with local observations on land or on ship to determine the local time and hence longitude. Galileo applied in 1616 for the Spanish prize of 6,000 gold ducats with a lifetime pension of 2,000 a year, and almost two decades later for the Dutch prize, but by then he was under house arrest for possible heresy.
The main problem with the Jovian moon technique was that it was difficult to observe the Galilean moons through a telescope on a moving ship, a problem that Galileo tried to solve with the invention of the celatone. Others suggested improvements, but without success.
Land mapping surveys had the same problem determining longitude, though with less severe observational conditions. The method proved practical and was used by Giovanni Domenico Cassini and Jean Picard to re-map France.
Comparative structure
+ Jovian radiation Moon rem/day Io 3600 Europa 540Ganymede 8 Callisto 0.01 Earth (Max) 0.07 Earth (Avg) 0.0007
Fluctuations in the orbits of the moons indicate that their mean density decreases with distance from Jupiter. Callisto, the outermost and least dense of the four, has a density intermediate between ice and rock whereas Io, the innermost and densest moon, has a density intermediate between rock and iron. Callisto has an ancient, heavily cratered and unaltered ice surface and the way it rotates indicates that its density is equally distributed, suggesting that it has no rocky or metallic core but consists of a homogeneous mix of rock and ice. This may well have been the original structure of all the moons. The rotation of the three inner moons, in contrast, indicates differentiation of their interiors with denser matter at the core and lighter matter above. They also reveal significant alteration of the surface. Ganymede reveals past tectonic movement of the ice surface which required partial melting of subsurface layers. Europa reveals more dynamic and recent movement of this nature, suggesting a thinner ice crust. Finally, Io, the innermost moon, has a sulfur surface, active volcanism and no sign of ice. All this evidence suggests that the nearer a moon is to Jupiter the hotter its interior. The current model is that the moons experience tidal heating as a result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet. In all but Callisto this will have melted the interior ice, allowing rock and iron to sink to the interior and water to cover the surface. In Ganymede a thick and solid ice crust then formed. In warmer Europa a thinner more easily broken crust formed. In Io the heating is so extreme that all the rock has melted and water has long ago boiled out into space.
Members + NameImageModel of interior Diameter(km) Mass(kg) Density(g/cm3) Semi-major axis(km) Orbital period (days) (relative to Io) Inclination(°) Eccentricity Io Jupiter I 1.769 (1) 0.050 0.0041 Europa Jupiter II 3.014 3.551 (2.0) 0.471 0.0094 Ganymede Jupiter III 1.936 7.155 (4.0) 0.204 0.0011 Callisto Jupiter IV 1.834 16.689 (9.4) 0.205 0.0074 Io
Io (Jupiter I) is the innermost of the four Galilean moons of Jupiter; with a diameter of 3642 kilometers, it is the fourth-largest moon in the Solar System, and is only marginally larger than Earth's moon. It was named after Io, a priestess of Hera who became one of the lovers of Zeus. It was referred to as "Jupiter I", or "The first satellite of Jupiter" until the mid-20th century.