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.
Nevertheless, Galileo accepted the Copernican theory. A Chinese historian of astronomy, Xi Zezong , has claimed that a "small reddish star" observed near Jupiter in BCE by Chinese astronomer Gan De may have been Ganymede , predating Galileo's discovery by around two millennia. In , Galileo had been employed as a mathematics tutor for Cosimo de' Medici. Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter's moons to gain it. 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.
The secretary replied that the latter name would be best. On March 12, , 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 March 19, 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.
Scarcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time. Behold, therefore, four stars reserved for your illustrious name Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others. The discovery was announced in the Sidereus Nuncius "Starry Messenger" , published in Venice in March , less than two months after the first observations.
The names that eventually prevailed were chosen by Simon Marius , who discovered the moons independently at the same time as Galileo: 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 numbered names Jupiter x were used until the midth century when other inner moons were discovered, and Marius' names became widely used. Galileo was able to develop a method of determining longitude based on the timing of the orbits of the Galilean moons.
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 Cassini and Picard to re-map France. 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. 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 midth century. With over active volcanos, Io is the most geologically active object in the Solar System.
Although not proven, recent data from the Galileo orbiter indicate that Io might have its own magnetic field. Europa Jupiter II , the second of the four Galilean moons, is the second closest to Jupiter and the smallest at 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 midth century.
It has a smooth and bright surface,  with a layer of water surrounding the mantle of the planet, thought to be kilometers thick. So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there. The prominent markings that criss-cross the moon seem to be mainly albedo features , which emphasize low topography. There are few craters on Europa because its surface is tectonically active and young.
Galileo (satellite navigation)
Also, volcanic water eruptions splitting the surface of Europa, and even geysers have been considered as a cause. The color of the markings, reddish-brown, is theorized to be caused by sulfur, but scientists cannot confirm that, because no data collection devices have been sent to Europa. It has a tenuous atmosphere composed primarily of oxygen. It is the only satellite in the Solar System known to possess a magnetosphere , likely created through convection within the liquid iron core.
The surface 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 its icy crust forming over them. The satellite has a thin oxygen atmosphere that includes O, O 2 , and possibly O 3 ozone , and some atomic hydrogen. Callisto Jupiter IV is the fourth and last Galilean moon, and is the second largest of the four, and at 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. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide  and probably molecular oxygen.
However, this is less likely than on nearby Europa.
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.
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.
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. 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. Ganymede's larger mass means that it would have migrated inward at a faster rate than Europa or Io. 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. The main difficulty in observing the moons from Earth is their proximity to Jupiter, since they are obscured by its brightness. Ganymede and Callisto, at their maximum separation, are the likeliest targets for potential naked-eye observation.
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Tupan Patera on Io. Surface features of the four members at different levels of zoom in each row. Jupiter and Galilean moons circa , imaged by New Horizons during flyby.
Galilean Genesis: an alternative to inflation | Perimeter Institute
University of Chicago Press. Translated and prefaced by Albert Van Helden. Journal for the History of Astronomy. Information References 19 Citations Files Plots. An Alternative to inflation - Creminelli, Paolo et al.