The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though only around half as massive.
Introduction
Galilean moons
Galilean moons | Introduction
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Galilean moons | Discovery
As a result of improvements 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 Galilei to observe in either December 1609 or January 1610 what came to be known as the Galilean moons. On January 7, 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. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.
Galilean moons | Discovery
The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610, and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth. They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with radii larger than any of the dwarf planets. Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury, though only around half as massive.
Galilean moons | Introduction
Galileo was able to develop a method of 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. The main problem with the 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. The method was used by Giovanni Domenico Cassini and Jean Picard to re-map France.
Galilean moons | Determination of longitude
On 7 January 1610, Galileo observed with his telescope what he described at the time as "three fixed stars, totally invisible by their smallness", all close to Jupiter, and lying on a straight line through it. Observations on subsequent nights showed that the positions of these "stars" relative to Jupiter were changing in a way that would have been inexplicable if they had really been fixed stars. On 10 January, Galileo noted that one of them had disappeared, an observation which he attributed to its being hidden behind Jupiter. Within a few days, he concluded that they were orbiting Jupiter: he had discovered three of Jupiter's four largest moons.
Galileo Galilei | Jupiter's moons
He discovered the fourth on 13 January. Galileo named the group of four the "Medicean stars", in honour of his future patron, Cosimo II de' Medici, Grand Duke of Tuscany, and Cosimo's three brothers. Later astronomers, however, renamed them "Galilean satellites" in honour of their discoverer. These satellites were independently discovered by Simon Marius on 8 January 1610 and are now called Io, Europa, Ganymede, and Callisto, the names given by Marius in his "Mundus Iovialis" published in 1614. Galileo's observations of the satellites of Jupiter caused a revolution in astronomy: a planet with smaller planets orbiting it did not conform to the principles of Aristotelian cosmology, which held that all heavenly bodies should circle the Earth, and many astronomers and philosophers initially refused to believe that Galileo could have discovered such a thing.
Galileo Galilei | Jupiter's moons
Europa (Jupiter II), the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 3121.6 kilometers in diameter, which is slightly smaller than the Moon. The name comes from a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete, though the name did not become widely used until the mid-20th century. It has a smooth and bright surface, with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick. The smooth surface includes a layer of ice, while the bottom of the ice is theorized to be liquid water.
Galilean moons | Europa
Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621). At Cosimo'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: The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter): "Io", "Europa", "Ganymede" and "Callisto", in his "Mundus Jovialis", published in 1614.
Galilean moons | Name
Chinese historian Xi Zezong claimed that the earliest record of a Jovian moon (Ganymede or Callisto) was a note by Chinese astronomer Gan De of an observation around 364 BC regarding a "reddish star". However, the first certain observations of Jupiter's satellites were those of Galileo Galilei in 1609. By January 1610, he had sighted the four massive Galilean moons with his 20× magnification telescope, and he published his results in March 1610. Simon Marius had independently discovered the moons one day after Galileo, although he did not publish his book on the subject until 1614. Even so, the names Marius assigned are used today: Ganymede; Callisto; Io; and Europa.
Moons of Jupiter | Discovery
Each generation of moons would have spiraled into Jupiter, due to drag from the disk, with new moons then forming from the new debris captured from the Solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned out to the point that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io.
Galilean moons | Origin and evolution
The "Galileo" spacecraft was the first to enter orbit around Jupiter, arriving in 1995 and studying it until 2003. During this period, "Galileo" gathered a large amount of information about the Jovian system, making close approaches to all of the Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath the surfaces of Europa, Ganymede, and Callisto. It also discovered a magnetic field around Ganymede. In 2016, the "Juno" spacecraft imaged the Galilean moons from above their orbital plane as it approached Jupiter orbit insertion, creating a time-lapse movie of their motion.
Moons of Jupiter | Exploration
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 much smaller remaining moons have irregular forms because of their weaker self-gravitation. The Galilean moons were observed in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than ever. Galileo's observations 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 Ptolemaic world system, a geocentric theory in which everything orbits around Earth.
Galilean moons | Introduction
The moons discovered by Galileo—Io, Europa, Ganymede, and Callisto—are among the largest satellites in the Solar System. The orbits of three of them (Io, Europa, and Ganymede) form a pattern known as a Laplace resonance; for every four orbits that Io makes around Jupiter, Europa makes exactly two orbits and Ganymede makes exactly one. This resonance causes the gravitational effects of the three large moons to distort their orbits into elliptical shapes, because each moon receives an extra tug from its neighbors at the same point in every orbit it makes. The tidal force from Jupiter, on the other hand, works to circularize their orbits.
Jupiter | Galilean moons
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.
Galilean moons | Comparative structure
In 1610, Italian polymath Galileo Galilei discovered the four largest moons of Jupiter (now known as the Galilean moons) using a telescope; thought to be the first telescopic observation of moons other than Earth's. One day after Galileo, Simon Marius independently discovered moons around Jupiter, though he did not publish his discovery in a book until 1614. It was Marius's names for the four major moons, however, that stuck—Io, Europa, Ganymede and Callisto. These findings were also the first discovery of celestial motion not apparently centered on Earth. The discovery was a major point in favor of Copernicus' heliocentric theory of the motions of the planets; Galileo's outspoken support of the Copernican theory placed him under the threat of the Inquisition.
Jupiter | Ground-based telescope research
Galileo's next astronomical discovery would prove to be a surprising one. While observing Jupiter over the course of several days, he noticed four stars close to Jupiter whose positions were changing in a way that would be impossible if they were fixed stars. After much observation, he concluded these four stars were orbiting the planet Jupiter and were in fact moons, not stars. This was a radical discovery because, according to Aristotelian cosmology, all heavenly bodies revolve around the Earth and a planet with moons obviously contradicted that popular belief. While contradicting Aristotelian belief, it supported Copernican cosmology which stated that Earth is a planet like all others.
Copernican Revolution | Galileo Galilei
His observations were confirmed by the observatory of Christopher Clavius and he received a hero's welcome when he visited Rome in 1611. Galileo continued to observe the satellites over the next eighteen months, and by mid-1611, he had obtained remarkably accurate estimates for their periods—a feat which Johannes Kepler had believed impossible.
Galileo Galilei | Jupiter's moons
Amalthea was discovered on 9 September 1892 by Edward Emerson Barnard using the 36 inch (91 cm) refractor telescope at Lick Observatory. It was the last planetary satellite to be discovered by direct visual observation (as opposed to photographically) and was the first new satellite of Jupiter since Galileo Galilei's discovery of the Galilean satellites in 1610.
Amalthea (moon) | Discovery
Some models predict that there may have been several generations of Galilean satellites in Jupiter's early history. Each generation of moons to have formed would have spiraled into Jupiter and been destroyed, due to tidal interactions with Jupiter's proto-satellite disk, with new moons forming from the remaining debris. By the time the present generation formed, the gas in the proto-satellite disk had thinned out to the point that it no longer greatly interfered with the moons' orbits. Other models suggest that Galilean satellites formed in a proto-satellite disk, in which formation timescales were comparable to or shorter than orbital migration timescales.
Galilean moons | Members
The exploration of Io, Jupiter's innermost Galilean and third-largest moon, began with its discovery in 1610 and continues today with Earth-based observations and visits by spacecraft to the Jupiter system. Italian astronomer Galileo Galilei was the first to record an observation of Io on January 8, 1610, though Simon Marius may have also observed Io at around the same time. During the 17th century, observations of Io and the other Galilean satellites helped with the measurement of longitude by map makers and surveyors, with validation of Kepler's Third Law of planetary motion, and with measurement of the speed of light.
Exploration of Io | Introduction
Galileo claimed the right to name the moons he'd discovered. He considered "Cosmian Stars" and settled on "Medicean Stars". The French astronomer Nicolas-Claude Fabri de Peiresc suggested individual names from the Medici family for the moons, but his proposal was not taken up. Simon Marius, who had originally claimed to have found the Galilean satellites, tried to name the moons the "Saturn of Jupiter", the "Jupiter of Jupiter" (this was Ganymede), the "Venus of Jupiter", and the "Mercury of Jupiter", another nomenclature that never caught on. From a suggestion by Johannes Kepler, Marius once again tried to name the moons: This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century.
Ganymede (moon) | Name
On 25 August 1609, Galileo Galilei demonstrated his first telescope to a group of Venetian merchants, and in early January 1610, Galileo observed four dim objects near Jupiter, which he mistook for stars. However, after a few days of observation, Galileo realized that these "stars" were in fact orbiting Jupiter. These four objects (later named the Galilean moons in honor of their discoverer) were the first celestial bodies observed to orbit something other than the Earth or Sun. Galileo continued to observe these moons over the next eighteen months, and by the middle of 1611 he had obtained remarkably accurate estimates for their periods.
Mass | Planetary motion
697 Galilea is a minor planet orbiting the Sun. It was named in honor of Galileo Galilei, as it was discovered just after the 300th anniversary of his discovery of the Galilean moons.
697 Galilea | Introduction
He desired to return to Florence, and in hopes of gaining patronage there, he dedicated "Sidereus Nuncius" to his former pupil, now the Grand Duke of Tuscany, Cosimo II de' Medici. In addition, he named his discovered four moons of Jupiter the "Medicean Stars," in honor of the four royal Medici brothers. This helped him receive the position of Chief Mathematician and Philosopher to the Medici at the University of Pisa. Ultimately, his effort at naming the moons failed, for they are now referred to as the "Galilean moons".
Sidereus Nuncius | Medicean Stars (Moons of Jupiter)
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.
Galilean moons | Comparative structure
Callisto has long been considered the most suitable place for a human base for future exploration of the Jupiter system since it is furthest from the intense radiation of Jupiter.
Galilean moons | Callisto
It was Laplace who found the first answers explaining the linked orbits of the Galilean moons (see below). Before Newton, there was also consideration of ratios and proportions in orbital motions, in what was called "the music of the spheres", or "musica universalis".
Orbital resonance | History
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 February 13, 1610, Galileo wrote to the Grand Duke's secretary: "God graced me with being able, through such a singular sign, to reveal to my Lord my devotion and the desire I have that his glorious name live as equal among the stars, and since it is up to me, the first discoverer, to name these new planets, I wish, in imitation of the great sages who placed the most excellent heroes of that age among the stars, to inscribe these with the name of the Most Serene Grand Duke." Galileo asked whether he should name the moons the "Cosmian Stars", after Cosimo alone, or the "Medician Stars", which would honor all four brothers in the Medici clan.
Galilean moons | Dedication to the Medicis
There are 79 known moons of Jupiter. The most massive of the moons are the four Galilean moons, which were independently discovered in 1610 by Galileo Galilei and Simon Marius and were the first objects found to orbit a body that was neither Earth nor the Sun. Much more recently, beginning in 1892, dozens of far smaller Jovian moons have been detected and have received the names of lovers or daughters of the Roman god Jupiter or his Greek equivalent Zeus. The Galilean moons are by far the largest and most massive objects to orbit Jupiter, with the remaining 75 known moons and the rings together composing just 0.003% of the total orbiting mass.
Moons of Jupiter | Introduction
The eccentricity of their orbits causes regular flexing of the three moons' shapes, with Jupiter's gravity stretching them out as they approach it and allowing them to spring back to more spherical shapes as they swing away. This tidal flexing heats the moons' interiors by friction. This is seen most dramatically in the extraordinary volcanic activity of innermost Io (which is subject to the strongest tidal forces), and to a lesser degree in the geological youth of Europa's surface (indicating recent resurfacing of the moon's exterior).
Jupiter | Galilean moons
The four largest moons of Jupiter were in poor positions for observation; the necessary path of the gravity-assist maneuver meant that "New Horizons" passed millions of kilometers from any of the Galilean moons. Still, its instruments were intended for small, dim targets, so they were scientifically useful on large, distant moons. Emphasis was put on Jupiter's innermost Galilean moon, Io, whose active volcanoes shoot out tons of material into Jupiter's magnetosphere, and further. Out of eleven observed eruptions, three were seen for the first time. That of Tvashtar reached an altitude of up to . The event gave scientists an unprecedented look into the structure and motion of the rising plume and its subsequent fall back to the surface.
New Horizons | Jovian moons
Jupiter's regular satellites are believed to have formed from a circumplanetary disk, a ring of accreting gas and solid debris analogous to a protoplanetary disk. They may be the remnants of a score of Galilean-mass satellites that formed early in Jupiter's history. Simulations suggest that, while the disk had a relatively high mass at any given moment, over time a substantial fraction (several tenths of a percent) of the mass of Jupiter captured from the Solar nebula was processed through it. However, the disk mass of only 2% that of Jupiter is required to explain the existing satellites. Thus there may have been several generations of Galilean-mass satellites in Jupiter's early history.
Galilean moons | Origin and evolution
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.
Galilean moons | Comparative structure
On January 15, he observed the motions of three of these satellites, including Io, and came to the conclusions that these objects were not background stars, but were in fact, "three stars in the heavens moving about Jupiter, as Venus and Mercury round the Sun." These were the first moons of a planet other than the Earth to be discovered. The discoveries of Io and the other Galilean satellites of Jupiter were published in Galileo's "Sidereus Nuncius" in March 1610. While the Jovian moons he discovered would later be known as the Galilean satellites, after himself, he proposed the name "Medicea Sidera" (Medicean Stars) after his new patrons, the de'Medici family of his native Florence.
Exploration of Io | Discovery: 1610
The Chinese historian Xi Zezong has claimed that Gan De, an ancient Chinese astronomer, discovered one of Jupiter's moons in 362 with the unaided eye. If accurate, this would predate Galileo's discovery by nearly two millennia. In his 2nd century work the "Almagest", the Hellenistic astronomer Claudius Ptolemaeus constructed a geocentric planetary model based on deferents and epicycles to explain Jupiter's motion relative to Earth, giving its orbital period around Earth as 4332.38 days, or 11.86 years.
Jupiter | Pre-telescopic research
Io is anhydrous and likely has an interior of rock and metal. Europa is thought to contain 8% ice and water by mass with the remainder rock. These moons are, in increasing order of distance from Jupiter:
Galilean moons | Members
The Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto) were named by Simon Marius soon after their discovery in 1610. However, by the late 19th century these names had fallen out of favor, and for a long time it was most common to refer to them in the astronomical literature simply as "Jupiter I", "Jupiter II", etc., or as "the first satellite of Jupiter", "Jupiter's second satellite", etc. By the first decade of the 20th century, the names Io, Europa, Ganymede, and Callisto had once again recovered popularity, but the later-discovered moons, numbered, usually in Roman numerals V (5) through XII (12), remained unnamed.
Naming of moons | Jupiter
The secretary replied that the latter name would be best.
Galilean moons | Dedication to the Medicis
Surrounding Jupiter is a faint planetary ring system and a powerful magnetosphere. Jupiter has 79 known moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a diameter greater than that of the planet Mercury. "Pioneer 10 was" the first spacecraft to visit Jupiter, making its closest approach to the planet on December 4, 1973; "Pioneer 10" identified plasma in Jupiter's magnetic field and also found that Jupiter's magnetic tail was nearly 800 million kilometers long, covering the entire distance to Saturn. Jupiter has been explored on a number of occasions by robotic spacecraft, beginning with the "Pioneer" and "Voyager" flyby missions from 1973 to 1979, and later by the "Galileo" orbiter, which arrived at Jupiter in 1995.
Jupiter | Introduction
The Galilean moons of Jupiter (Io, Europa, Ganymede, and Callisto) were named by Simon Marius soon after their discovery in 1610. However, these names fell out of favor until the 20th century. The astronomical literature instead simply referred to "Jupiter I", "Jupiter II", etc., or "the first satellite of Jupiter", "Jupiter's second satellite", and so on. The names Io, Europa, Ganymede, and Callisto became popular in the mid-20th century, whereas the rest of the moons remained unnamed and were usually numbered in Roman numerals V (5) to XII (12). Jupiter V was discovered in 1892 and given the name "Amalthea" by a popular though unofficial convention, a name first used by French astronomer Camille Flammarion.
Moons of Jupiter | Naming
Callisto (Jupiter IV) is the fourth and last Galilean moon, and is the second-largest of the four, and at 4820.6 kilometers in diameter, it is the third largest moon in the Solar System, and barely smaller than Mercury, though only a third of the latter's mass. It is named after the Greek mythological nymph Callisto, a lover of Zeus who was a daughter of the Arkadian King Lykaon and a hunting companion of the goddess Artemis. The moon does not form part of the orbital resonance that affects three inner Galilean satellites and thus does not experience appreciable tidal heating.
Galilean moons | Callisto
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 "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.
Galilean moons | Discovery
Galilaei is the name of two craters named after the astronomer Galileo Galilei:
Galilaei | Introduction
Although Simon Marius is not credited with the sole discovery of the Galilean satellites, his names for the moons were adopted. In his 1614 publication "Mundus Iovialis anno M.DC.IX Detectus Ope Perspicilli Belgici", he proposed several alternative names for the innermost of the large moons of Jupiter, including "The Mercury of Jupiter" and "The First of the Jovian Planets". Based on a suggestion from Johannes Kepler in October 1613, he also devised a naming scheme whereby each moon was named for a lover of the Greek mythological Zeus or his Roman equivalent, Jupiter. He named the innermost large moon of Jupiter after the Greek mythological figure Io: Marius's names were not widely adopted until centuries later (mid-20th century).
Io (moon) | Nomenclature
By occluding Jupiter itself behind a high tree limb perpendicular to the satellites' orbital plane to prevent the planet's glare from obscuring them, one or more of the Galilean moons might be spotted in favorable conditions. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Shi and Gan together made fairly accurate observations of the five major planets.
Gan De | Observations
He named the innermost large moon of Jupiter after the Greek mythological figure Io.
Exploration of Io | Discovery: 1610
Gan De made some of the first detailed observations of Jupiter in recorded history. He described the planet as "very large and bright". In one of his observations on Jupiter, he reported a "small reddish star" next to Jupiter. The historian Xi Zezong has claimed that this was a naked-eye observation of Ganymede in the summer of 364 BC = 9637 HE, 11610 - 9637 = 1973 years before Galileo Galilei's celebrated discovery of the same in 11610 HE (all four of the brightest moons are technically visible to the unaided eye, but in practice are normally hidden by the glare of Jupiter).
Gan De | Observations
Io is the closest of the Galilean satellites to Jupiter. It is almost the same size as the Moon. Because of its position, it is subject to constant tidal flexing and heavy radiation from Jupiter's magnetic field. Since 1979, it has also been known for its abundant volcanism.
Jupiter's moons in fiction | Io
GIF animations depicting the Galilean moon orbits and the resonance of Io, Europa, and Ganymede
Galilean moons | Orbit animations
All four Galilean moons are bright enough to be viewed from Earth without a telescope, if only they could appear farther away from Jupiter. (They are, however, easily distinguished with even low-powered binoculars.) They have apparent magnitudes between 4.6 and 5.6 when Jupiter is in opposition with the Sun, and are about one unit of magnitude dimmer when Jupiter is in conjunction. The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness. The maximum angular separations of the moons are between 2 and 10 arcminutes from Jupiter, which is close to the limit of human visual acuity.
Galilean moons | Visibility
The "Voyager" missions vastly improved our understanding of the Galilean moons, and also discovered Jupiter's rings. They also took the first close-up images of the planet's atmosphere, revealing the Great Red Spot as a complex storm moving in a counter-clockwise direction. Other smaller storms and eddies were found throughout the banded clouds (see animation on the right). Two new, small satellites, Adrastea and Metis, were discovered orbiting just outside the ring, making them the first of Jupiter's moons to be identified by a spacecraft. A third new satellite, Thebe, was discovered between the orbits of Amalthea and Io. The discovery of volcanic activity on the moon Io was the greatest unexpected finding of the mission, as it was the first time an active volcano was observed on a celestial body other than Earth.
Exploration of Jupiter | "Voyager" program (1979)
During this observation, the three stars in a line to the west of Jupiter were (from east to west): Io, Europa, and Ganymede. This was the first time that Io and Europa were observed and recorded as distinct points of light so this date, January 8, 1610 is used as the discovery date for the two moons by the International Astronomical Union. Galileo continued to observe the Jupiter system for the next month and a half. On January 13, Galileo observed all four of what would later be known as the Galilean moons of Jupiter for the first time in a single observation, though he had observed all four at various times in the preceding days.
Exploration of Io | Discovery: 1610
So Marius' first recorded observation from December 29, 1609 equates to Galileo's second observation of the Jupiter system on January 8, 1610. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Given that Galileo published his work before Marius and that his first recorded observation came one day before Marius', Galileo is credited with the discovery. Despite this, it is one of Marius' naming schemes for the moons of Jupiter that is regularly used today. Based on a suggestion from Johannes Kepler in October 1613, he proposed that each moon was given its own name based on the lovers of the Greek mythological Zeus or his Roman equivalent, Jupiter.
Exploration of Io | Discovery: 1610
In 1998 Gangale adapted the Darian calendar for use on the four Galilean moons of Jupiter discovered by Galileo in 1610: Io, Europa, Ganymede, and Callisto. In 2003 he created a variant of the calendar for Titan.
Darian calendar | Other Darian calendars
"Pioneer 11"'s closer encounter with Jupiter allowed the spacecraft to discover Jupiter's intense radiation belts similar to Earth's Van Allen Belts. One of the peaks in charged particle flux was found near the orbit of Io. Radio tracking during the encounters of both Pioneers with Io provided an improved estimate of the moon's mass. This was accomplished by analyzing slight changes in trajectory of the two probes due to the influence of Io's gravity and calculating the mass necessary to produce the deviations. When this estimate was combined with the best available information on Io's size, Io was found to have the highest density of the four Galilean satellites and that the densities of the four Galilean satellites trended downward with increasing distance from Jupiter.
Exploration of Io | "Pioneer" era: 1973–1979
No additional satellites were discovered until E. E. Barnard observed Amalthea in 1892. With the aid of telescopic photography, further discoveries followed quickly over the course of the 20th century. Himalia was discovered in 1904, Elara in 1905, Pasiphae in 1908, Sinope in 1914, Lysithea and Carme in 1938, Ananke in 1951, and Leda in 1974. By the time that the Voyager space probes reached Jupiter, around 1979, 13 moons had been discovered, not including Themisto, which had been observed in 1975, but was lost until 2000 due to insufficient initial observation data. The Voyager spacecraft discovered an additional three inner moons in 1979: Metis; Adrastea; and Thebe.
Moons of Jupiter | Discovery
In Galileo's 1604 observation of Kepler's Supernova and conclusion that it was a group of distant stars, Galileo disproved the Aristotelian notion of the immutability of the heavens. Using his refracting telescope, Galileo observed in late 1609 that the surface of the Moon is not smooth. Early the next year, he observed the four largest moons of Jupiter. Later in 1610, he observed the phases of Venus—a proof of heliocentrism—as well as Saturn, though he thought the planet's rings were two other planets. In 1612, he observed Neptune and noted its motion, but did not identify it as a planet.
Galileo Galilei | Astronomy
Io (), or Jupiter I, is the innermost and third-largest of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon in the Solar System, has the highest density of all of them, and has the lowest amount of water (by atomic ratio) of any known astronomical object in the Solar System. It was discovered in 1610 by Galileo Galilei and was named after the mythological character Io, a priestess of Hera who became one of Zeus's lovers. With over 400 active volcanoes, Io is the most geologically active object in the Solar System. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior as it is pulled between Jupiter and the other Galilean moons—Europa, Ganymede and Callisto.
Io (moon) | Introduction
The star furthest to the east from Jupiter turned out to be Callisto while the star to the west of Jupiter was Ganymede. The third star, the closest one to the east of Jupiter, was a combination of the light from Io and Europa as Galileo's telescope, while having a high magnification for a telescope from his time, was too low-powered to separate the two moons into distinct points of light. Galileo observed Jupiter the next evening, January 8, 1610, this time seeing three stars to the west of Jupiter, suggesting that Jupiter had moved to the west of the three stars.
Exploration of Io | Discovery: 1610
Although not proven, recent data from the Galileo orbiter indicate that Io might have its own magnetic field. Io has an extremely thin atmosphere made up mostly of sulfur dioxide (SO). 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.
Galilean moons | Io
Galileo had discovered the moons of Jupiter in 1609. Scheiner argued that what appeared to be spots on the Sun were in fact clusters of small moons, thereby trying to deploy one of Galileo's own discoveries as an argument for the Aristotelian model. In his "Letters on Sunspots" Galileo showed how sunspots were nothing like the moons of Jupiter, and the comparison was false. Scheiner claimed that the sunspots, with their irregular movements, were like the moons of Jupiter whose positions were similarly hard to predict. To counter this argument, Galileo published tables of predictions for the future position of the moons of Jupiter, so that astronomers could easily distinguish between the regular, predictable movements they followed with the ephemeral and irregular sunspots.
Letters on Sunspots | Moons of Jupiter
The first spacecraft to visit Jupiter were "Pioneer 10" in 1973, and "Pioneer 11" a year later, taking low-resolution images of the four Galilean moons and returning data on their atmospheres and radiation belts. The "Voyager 1" and "Voyager 2" probes visited Jupiter in 1979, discovering the volcanic activity on Io and the presence of water ice on the surface of Europa. The "Cassini" probe to Saturn flew by Jupiter in 2000 and collected data on interactions of the Galilean moons with Jupiter's extended atmosphere. The "New Horizons" spacecraft flew by Jupiter in 2007 and made improved measurements of its satellites' orbital parameters.
Moons of Jupiter | Exploration
Callisto was discovered by Galileo in January 1610, along with the three other large Jovian moons—Ganymede, Io, and Europa.
Callisto (moon) | Discovery
The "Galileo" spacecraft arrived at Jupiter in 1995 after a six-year journey from Earth to follow up on the discoveries of the two "Voyager" probes and ground-based observations taken in the intervening years. Io's location within one of Jupiter's most intense radiation belts precluded a prolonged close flyby, but "Galileo" did pass close by shortly before entering orbit for its two-year, primary mission studying the Jovian system. Although no images were taken during the close flyby on 7 December 1995, the encounter did yield significant results, such as the discovery of a large iron core, similar to that found in the rocky planets of the inner Solar System.
Io (moon) | "Galileo"
Each generation of moons might have spiraled into Jupiter, because of drag from the disk, with new moons then forming from the new debris captured from the solar nebula. By the time the present (possibly fifth) generation formed, the disk had thinned so that it no longer greatly interfered with the moons' orbits. The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance with each other, which still exists for Io, Europa, and Ganymede. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io. The outer, irregular moons are thought to have originated from captured asteroids, whereas the protolunar disk was still massive enough to absorb much of their momentum and thus capture them into orbit.
Moons of Jupiter | Origin and evolution
Amalthea is a moon of Jupiter. It has the third closest orbit around Jupiter among known moons and was the fifth moon of Jupiter to be discovered, so it is also known as . Edward Emerson Barnard discovered the moon on 9 September 1892 and named it after Amalthea of Greek mythology. It was the last natural satellite to be discovered by direct visual observation; all later moons were discovered by photographic or digital imaging. Amalthea is in a close orbit around Jupiter and is within the outer edge of the Amalthea Gossamer Ring which is formed from dust ejected from its surface.
Amalthea (moon) | Introduction
In his book "Mundus Iovialis" ("The World of Jupiter"), published in 1614, Simon Marius, the court astronomer to the Margraves of Brandenburg-Ansbach, claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. According to Marius, he began observing the Jupiter system in late November 1609. He continued to observe the moons of Jupiter through December 1609, but did not record his observations until December 29, 1609 when he came to the conclusion "that these stars moved round Jupiter, just as the five solar planets, Mercury, Venus, Mars, Jupiter, and Saturn revolve round the Sun." However, Marius' observations were dated based on the Julian calendar, which was 10 days behind the Gregorian calendar used by Galileo.
Exploration of Io | Discovery: 1610
After announcing the discovery of Jupiter's moons in his 1610 treatise "Sidereus Nuncius", Galileo began the systematic study of their periods in 1611, developing a micrometer for the purpose. Galileo immediately realized that eclipses of the moons could provide a precise method to determine longitude. Galileo compiled tables of the periods that he offered, with his telescopes, first to the King of Spain (1611, 1612, 1616, and 1627-1628), then to the States General of Holland (1637-1641). To convince his Spanish interlocutors that Jupiter and its moons could be observed on unstable ground, such as a ship's deck, Galileo designed a special helmet carrying a small telescope on a hinged mount.
Jovilabe | Introduction
On 30 November, 1609, Galileo aimed his telescope at the Moon. While not being the first person to observe the Moon through a telescope (English mathematician Thomas Harriot had done it four months before but only saw a "strange spottednesse"), Galileo was the first to deduce the cause of the uneven waning as light occlusion from lunar mountains and craters. In his study, he also made topographical charts, estimating the heights of the mountains. The Moon was not what was long thought to have been a translucent and perfect sphere, as Aristotle claimed, and hardly the first "planet", an "eternal pearl to magnificently ascend into the heavenly empyrian", as put forth by Dante.
Galileo Galilei | Moon
The celatone was a device invented by Galileo Galilei to observe Jupiter's moons with the purpose of finding longitude on Earth. It took the form of a piece of headgear with a telescope taking the place of an eyehole.
Celatone | Introduction
Despite observations made in the 1990s by the "Galileo" spacecraft, very little is known about the moon's physical characteristics other than its size and the fact that it is tidally locked to Jupiter.
Adrastea (moon) | Introduction
Io (Jupiter I) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3642 kilometers, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus. Nevertheless, it was simply referred to as "Jupiter I", or "The first satellite of Jupiter", until the mid-20th century. With over 400 active volcanos, Io is the most geologically active object in the Solar System. Its surface is dotted with more than 100 mountains, some of which are taller than Earth's Mount Everest. Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core.
Galilean moons | Io
Galileo is sometimes credited with the discovery of the lunar libration in latitude in 1632, although Thomas Harriot or William Gilbert might have done it before. A friend of Galileo's, the painter Cigoli, included a realistic depiction of the Moon in one of his paintings, though probably used his own telescope to make the observation.
Galileo Galilei | Moon
"Ulysses" further studied Jupiter's magnetosphere in 1992 and then again in 2000. The "Cassini" probe approached the planet in 2000 and took very detailed images of its atmosphere. The "New Horizons" spacecraft passed by Jupiter in 2007 and made improved measurements of its and its satellites' parameters. The "Galileo" spacecraft was the first to have entered orbit around Jupiter, arriving in 1995 and studying the planet until 2003. During this period "Galileo" gathered a large amount of information about the Jovian system, making close approaches to all of the four large Galilean moons and finding evidence for thin atmospheres on three of them, as well as the possibility of liquid water beneath their surfaces.
Exploration of Jupiter | Introduction
Precovery images have been found back to 26 June 2009. It reached perihelion (closest approach to the Sun) around 1952 and has moved beyond 70 AU in September 2014. It is possibly a dwarf planet.
2010 GB174 | Introduction
Despite the lack of close-up imaging and mechanical problems that greatly restricted the amount of data returned, several significant discoveries were made during "Galileo" primary mission. "Galileo" observed the effects of a major eruption at Pillan Patera and confirmed that volcanic eruptions are composed of silicate magmas with magnesium-rich mafic and ultramafic compositions. Distant imaging of Io was acquired for almost every orbit during the primary mission, revealing large numbers of active volcanoes (both thermal emission from cooling magma on the surface and volcanic plumes), numerous mountains with widely varying morphologies, and several surface changes that had taken place both between the "Voyager" and "Galileo" eras and between "Galileo" orbits.
Io (moon) | "Galileo"
In Weinbaum's Solar System, Jupiter radiates enough heat to create Earthlike environments on the Galilean moons. Io, the innermost Galilean satellite, has a tropical climate, so that two human settlements are located at the poles, Junopolis in the north and Herapolis in the south. Extending partway around the equator are the Idiots' Hills, whose peaks extend beyond Io's "dense but shallow" atmosphere. Weinbaum was ignorant that Io is tidally locked, and therefore showed Jupiter rise and set during the story. Two intelligent races are native to Io: the loonies, a humanoid race of only moderate intelligence with large balloonlike heads upon long, slim necks, and the rodentine, warlike slinkers.
The Mad Moon | Weinbaum's Io
Galileo was an American automatic space probe that studied the planet Jupiter and its moons, as well as several other Solar System bodies. Named after the Italian astronomer Galileo Galilei, it consisted of an orbiter and an entry probe. It was delivered into Earth orbit on August 18, 1990 by . "Galileo" arrived at Jupiter on December 7, 1995, after gravitational assist flybys of Venus and Earth, and became the first spacecraft to orbit Jupiter. It launched the first probe into Jupiter, directly measuring its atmosphere. Despite suffering major antenna problems, "Galileo" achieved the first asteroid flyby, of 951 Gaspra, and discovered the first asteroid moon, Dactyl, around 243 Ida.
Galileo (spacecraft) | Introduction
Himalia , or Jupiter VI, is the largest irregular satellite of Jupiter, with a diameter of at least . It is the fifth largest Jovian satellite, after the four Galilean moons. It was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904 and is named after the nymph Himalia, who bore three sons of Zeus (the Greek equivalent of Jupiter). It is one of the largest planetary moons in the Solar System not imaged in detail, and the largest within the orbit of Neptune.
Himalia (moon) | Introduction
The trend in the densities of the four Galilean satellites found by "Pioneer 10" suggested that the satellites formed as part of a collapsing nebula, like a miniature version of what took place in the Solar System as a whole. The initial hot Jupiter prevented the condensation of water at the orbits of Io and Europa, leading those bodies to have higher densities than the outer two moons. Spectroscopic measurements of the light reflected from Io and its surrounding space were made with increasing spectral resolution during the 1970s, providing new insights into its surface composition. Other observations suggested that Io had a surface dominated by evaporites composed of sodium salts and sulfur.
Exploration of Io | "Pioneer" era: 1973–1979
In 1892, the discovery of Amalthea, whose orbit lay closer to Jupiter than those of the Galilean moons, pushed Europa to the third position. The "Voyager" probes discovered three more inner satellites in 1979, so Europa is now counted as Jupiter's sixth satellite, though it is still referred to as . The adjectival form has stabilized as "Europan".
Europa (moon) | Discovery and naming
so the three-halves power of the distances of Jupiter's satellites from Jupiter, to be in the ratio of their periodic times.) On page 370 (near end of paragraph 4 on right side of page), during a discussion of the number of moons of Jupiter (De Numero Satellitum Jovis), he cites as a source: "Deniq; Vuendelinus in Epistola ad me anni 1647." (Finally; Wendelin in a letter to me in the year 1647.) This might suggest that Wendelin made his discovery in the 1640s. However, Pierre Costabel states that Peiresc (1580-1637) made observations of Jupiter's moons in 1610, a time when Wendelin was living near Peiresc, and that by 1624, Wendelin was using Peiresc's data to construct tables of the motions of Jupiter's moons, so that navigators could determine their longitude Johannes Kepler recognized at least as early as 1622 that Jupiter's moons obey his third law.
Godefroy Wendelin | Moons of Jupiter
The Roman numbering system for satellites arose with the very first discovery of natural satellites other than Earth's Moon: Galileo referred to the Galilean moons as I through IV (counting from Jupiter outward), refusing to adopt the names proposed by his rival Simon Marius. Similar numbering schemes naturally arose with the discovery of multiple moons around Saturn, Uranus, and Mars. The numbers initially designated the moons in orbital sequence, and were re-numbered after each new discovery; for instance, before the discovery of Mimas and Enceladus in 1789, Tethys was Saturn I, Dione Saturn II, etc., but after the new moons were discovered, Mimas became Saturn I, Enceladus Saturn II, Tethys Saturn III and Dione Saturn IV.
Naming of moons | Roman numeral designations
It may belong to the Pasiphae group, though its orbit is not known well enough to confirm the assignment. Its orbit has a semi-major axis ~ and inclination ~160°. The limits of Jupiter's gravitational influence are defined by its Hill sphere, whose radius is . Retrograde moons with semi-major axes up to 67% of the Hill radius are believed to be stable. Consequently, it is possible that even more distant moons of Jupiter may be discovered. This moon has not been seen since its discovery in 2003 and is currently considered lost.
S/2003 J 2 | Introduction
Himalia was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904. Himalia is Jupiter's most easily observed small satellite; though Amalthea is brighter, its proximity to the planet's brilliant disk makes it a far more difficult object to view.
Himalia (moon) | Discovery
Chinese astronomical records report that in 365 BC, Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye. However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Shi Shen and Gan De together made fairly accurate observations of the five major planets. On January 7, 1610, Galileo Galilei observed what he thought were three stars near Jupiter, including what turned out to be Ganymede, Callisto, and one body that turned out to be the combined light from Io and Europa; the next night he noticed that they had moved.
Ganymede (moon) | History
In the middle of the 19th century, however, the numeration became fixed, and later discoveries failed to conform with the orbital sequence scheme. Amalthea, discovered in 1892, was labelled "Jupiter V" although it orbits more closely to Jupiter than does Io (Jupiter I). The unstated convention then became, at the close of the 19th century, that the numbers more or less reflected the order of discovery, except for prior historical exceptions (see Timeline of discovery of Solar System planets and their natural satellites); though if a large number of satellites were discovered in a short span of time, the group could be numbered in orbital sequence, or according to other principles than strictly by order of discovery.
Naming of moons | Roman numeral designations
In his "Mundus Jovialis", published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Regardless, Marius's first recorded observation came from 29 December 1609 in the Julian calendar, which equates to 8 January 1610 in the Gregorian calendar, which Galileo used. Given that Galileo published his work before Marius, Galileo is credited with the discovery. For the next two and a half centuries, Io remained an unresolved, 5th-magnitude point of light in astronomers' telescopes. During the 17th century, Io and the other Galilean satellites served a variety of purposes, including early methods to determine longitude, validating Kepler's third law of planetary motion, and determining the time required for light to travel between Jupiter and Earth.
Io (moon) | Observational history
Philibert Jacques Melotte (29 January 1880 – 30 March 1961) was a British astronomer whose parents emigrated from Belgium. In 1908 he discovered a moon of Jupiter, today known as Pasiphaë. It was simply designated "Jupiter VIII" and was not given its present name until 1975. The outer main-belt asteroid 676 Melitta, the only asteroid he discovered, is named after the Attic form of the Greek "Melissa", the bee, but its resemblance to the discoverer's name is not fortuitous. The conspicuous star cluster in the Coma Berenices constellation is commonly designated Mel 111 since it appeared in Melotte's 1915 catalogue of star clusters, but not in Charles Messier's famous catalogue of deep sky objects or in the New General Catalogue since it was not proved to be a true cluster until 1938 by the astronomer R J Trumpler.
Philibert Jacques Melotte | Introduction
Galilei is a surname, and may refer to: Two craters have been named for Galileo Galilei, with a different spelling:
Galilei | Introduction
The equinoxial charts in question indicate the author as "J. Laurent" "(see image)" and this is also indicated in a library catalog. A notation (see image) at the top of a hand-drawn star chart in the Observatoire de Marseille - Patrimoines archives notes the discovery of the asteroid as follows: "Némausa (51) dec. [ = découverte ] à Nîmes par J. Laurent (22 janvier 1858)", which means "51 Nemausa disc. [ = discovered ] at Nîmes by J. Laurent (22 January 1858)".
Joseph Jean Pierre Laurent | First name
They also had spectrometers with a combined spectral range from the far-ultraviolet to the mid-infrared, useful for examining Io's surface and atmospheric composition and to search for thermal emission sources on its surface. "Voyager 1" was first of the two probes to encounter the Jupiter system in March 1979. On approach to Jupiter in late February and early March 1979, "Voyager" imaging scientists noticed that Io appeared distinct from the other Galilean satellites. Its surface was orange in color and marked by dark spots, which were initially interpreted as the sites of impact craters. Among the most intriguing features was a heart-shaped, dark ring across that would later turn out to be the plume deposit of the volcano Pele.
Exploration of Io | "Voyager" era: 1979–1995
Speculation about the existence of the moons of Mars had begun when the moons of Jupiter were discovered. When Galileo Galilei, as a hidden report about him having observed two bumps on the sides of Saturn (later discovered to be its rings), used the anagram "smaismrmilmepoetaleumibunenugttauiras" for "Altissimum planetam tergeminum observavi" ("I have observed the most distant planet to have a triple form"), Johannes Kepler had misinterpreted it to mean "Salve umbistineum geminatum Martia proles" (Hello, furious twins, sons of Mars). Perhaps inspired by Kepler (and quoting Kepler's third law of planetary motion), Jonathan Swift's satire "Gulliver's Travels" (1726) refers to two moons in Part 3, Chapter 3 (the "Voyage to Laputa"), in which Laputa's astronomers are described as having discovered two satellites of Mars orbiting at distances of 3 and 5 Martian diameters with periods of 10 and 21.5 hours.
Moons of Mars | Early speculation
From these estimates of the orbital periods of Io and the other Galilean moons, astronomers hoped to generate ephemeris tables predicting the positions of each moon with respect to Jupiter, as well as when each moon would transit the face of Jupiter or be eclipsed by it. One benefit of such predictions, particularly those of satellite eclipses by Jupiter since they were subject to less observer error, would be determining an observer's longitude on Earth with respect to the prime meridian. By observing an eclipse of a Jovian satellite, an observer could determine the current time at the prime meridian by looking up the eclipse in an ephemeris table.
Exploration of Io | Io as a tool: 1610–1809
In his [book] "The World of Jupiter" ["Mundus Jovialis", 1614], [Simon] Mayr [1573-1624] presents these distances, from Jupiter, of the four [moons] of Jupiter: 3, 5, 8, 13 (or 14 [according to] Galileo) … Mayr presents their time periods: 1 day 18 1/2 hours, 3 days 13 1/3 hours, 7 days 3 hours, 16 days 18 hours: for all [of these data] the proportion is greater than double, thus greater than [the proportion] of the distances 3, 5, 8, 13 or 14, although less than [the proportion] of the squares, which double the proportions of the distances, namely 9, 25, 64, 169 or 196, just as [a factor of] 3/2 is also greater than 1 but less than 2.
Godefroy Wendelin | Moons of Jupiter
Dactyl was found on 17 February 1994 by "Galileo" mission member Ann Harch, while examining delayed image downloads from the spacecraft. "Galileo" recorded 47 images of Dactyl over an observation period of 5.5 hours in August 1993. The spacecraft was from Ida and from Dactyl when the first image of the moon was captured, 14 minutes before "Galileo" made its closest approach. Dactyl was initially designated 1993 (243) 1. It was named by the International Astronomical Union in 1994, for the mythological dactyls who inhabited Mount Ida on the island of Crete.
243 Ida | Discovery
In the last part of "Sidereus Nuncius", Galileo reported his discovery of four objects that appeared to form a straight line of stars near Jupiter. On the first night he detected a line of three little stars close to Jupiter parallel to the ecliptic; the following nights brought different arrangements and another star into his view, totalling four stars around Jupiter. Throughout the text, Galileo gave illustrations of the relative positions of Jupiter and its apparent companion stars as they appeared nightly from late January through early March 1610. That they changed their positions relative to Jupiter from night to night and yet always appeared in the same straight line near it, persuaded Galileo that they were orbiting Jupiter.
Sidereus Nuncius | Medicean Stars (Moons of Jupiter)
was discovered by Sahlmann et al. (2013) using the ESO telescopes at the La Silla Paranal Observatory. It is part of an ultracool binary system.
DENIS-P J082303.1−491201 b | Discovery
Swedish astronomer Pehr Wilhelm Wargentin (1717–83) used Rømer's method in the preparation of his ephemerides of Jupiter's moons (1746), as did Giovanni Domenico Maraldi working in Paris. The remaining irregularities in the orbits of the Galilean moons would not be satisfactorily explained until the work of Joseph Louis Lagrange (1736–1813) and Pierre-Simon Laplace (1749–1827) on orbital resonance. In 1809, again making use of observations of Io, but this time with the benefit of more than a century of increasingly precise observations, the astronomer Jean Baptiste Joseph Delambre (1749–1822) reported the time for light to travel from the Sun to the Earth as 8 minutes 12 seconds.
Rømer's determination of the speed of light | Later measurements
Adrastea was discovered by David C. Jewitt and G. Edward Danielson in "Voyager 2" probe photographs taken on July 8, 1979, and received the designation . Although it appeared only as a dot, it was the first moon to be discovered by an interplanetary spacecraft. Soon after its discovery, two other of the inner moons of Jupiter (Thebe and Metis) were observed in the images taken a few months earlier by "Voyager 1". The "Galileo" spacecraft was able to determine the moon's shape in 1998, but the images remain poor. In 1983, Adrastea was officially named after the Greek nymph Adrastea, the daughter of Zeus and his lover Ananke.
Adrastea (moon) | Discovery and observations