Galilean moons
On the 7th of January 1610, Galileo Galilei wrote a letter containing the first mention of Jupiter's moons. He saw only three of them at that time and believed they were 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. By January 15, Galileo concluded that the stars were actually bodies orbiting Jupiter. He continued to observe these celestial orbs until the 2nd of March 1610.
Galileo initially called his discovery the Cosmica Sidera or Cosimo's stars in honor of Cosimo de' Medici. Cosimo became Grand Duke Cosimo II of Tuscany in 1609. Galileo sought patronage from his now-wealthy former student and his powerful family. His secretary suggested changing the name to Medicea Sidera or the Medician stars to honor all four Medici brothers. The discovery was announced in the Sidereus Nuncius published in Venice in March 1610 less than two months after the first observations.
Simon Marius claimed another observation and reported observing the moons in November 1609 with a first written record for the 29th of December 1609. However because he did not publish these findings until after Galileo there is uncertainty around his records. Later it turned out that he used the old style Julian calendar while Galilei used the modern Gregorian one. So in fact Marius observed the moons first on the 8th of January 1610 one day after Galilei. Johannes Kepler suggested names after mythological characters Zeus seduced or abducted in his Mundus Jovialis published in 1614.
Io has over 400 active volcanos making it 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 Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. It has an extremely thin atmosphere made up mostly of sulfur dioxide. If a surface data vessel were to land on Io it would have to be extremely tough to survive the radiation and magnetic fields originating from Jupiter.
Europa has a smooth and bright surface with a layer of water surrounding the mantle thought to be 100 kilometers thick. The apparent youth and smoothness of the surface led to the hypothesis that a water ocean exists beneath it. Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity. Europa is primarily made of silicate rock and likely has an iron core. It has a tenuous atmosphere composed primarily of oxygen.
Ganymede is the largest natural satellite in the Solar System at 5262.4 kilometers in diameter. It is larger than the planet Mercury although only at about half of its mass since Ganymede is an icy world. It is the only satellite in the Solar System known to possess a magnetosphere likely created through convection within the liquid iron core. A salt-water ocean is believed to exist nearly 200 km below Ganymede's surface sandwiched between layers of ice.
Callisto is the fourth and last Galilean moon and is the second-largest of the four at 4820.6 kilometers in diameter. It is one of the most heavily cratered satellites in the Solar System and one major feature is a basin around 3000 km wide called Valhalla. Callisto is surrounded by an extremely thin atmosphere composed of carbon dioxide and probably molecular oxygen. Investigation revealed that Callisto may possibly have a subsurface ocean of liquid water at depths less than 300 kilometres.
The three inner moons Io Europa and Ganymede are in a 4:2:1 orbital resonance with each other. Fluctuations in their orbits indicate that their mean density decreases with distance from Jupiter. The rotation of the three inner moons 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 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.
of the celatone.
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. A map of France presented in 1684 showed the outline of a previous map compared to the new survey using the moons of Jupiter as timing reference. The King of France reportedly quipped that the astronomers had taken more territory from him than his enemies.
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. Large prizes were offered for its solution at various times by Spain The Netherlands and The United Kingdom.
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 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.
A Chinese historian of astronomy
Xi Zezong has claimed that a small reddish star observed near Jupiter in 364 BCE by Chinese astronomer Gan De may have been Ganymede. If true this might predate Galileo's discovery by around two millennia. 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 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. Jupiter is about 750 times brighter than Ganymede and about 2000 times brighter than Callisto.
Common questions
When did Galileo Galilei first observe Jupiter's moons?
Galileo Galilei wrote the first letter mentioning Jupiter's moons on the 7th of January 1610. He observed all four moons for the first time on the 13th of January 1610.
Who named the Galilean moons and when was the discovery published?
Simon Marius claimed to have observed the moons in November 1609 with a written record dated the 29th of December 1609. The discovery was announced in Sidereus Nuncius published in Venice in March 1610 less than two months after the initial observations.
Which Galilean moon is the most geologically active object in the Solar System?
Io has over 400 active volcanos making it the most geologically active object in the Solar System. Its surface contains more than 100 mountains some of which are taller than Earth's Mount Everest.
What makes Ganymede unique among natural satellites in the Solar System?
Ganymede is the largest natural satellite in the Solar System at 5262.4 kilometers in diameter. It is the only satellite known to possess a magnetosphere likely created through convection within its liquid iron core.
How did Chinese astronomers observe Jupiter's moons before Galileo?
Chinese astronomical records report that Gan De detected what might have been a moon of Jupiter in 365 BC with the naked eye. A historian claims a small reddish star observed near Jupiter in 364 BCE by Gan De may have been Ganymede.