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THE GALILEAN MOONS The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance. They are among the most massive objects in the Solar System outside the Sun and the eight planets, with a radius larger than any of the dwarf planets. The four moons were discovered sometime between 1609 and 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than had ever been possible before.[1] Galileo’s 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. More importantly, the incontrovertible discovery of celestial bodies orbiting something other than the Earth dealt a serious blow to the then-accepted Ptolemaic world system, or the geocentric theory in which everything orbits around the Earth. .
THE Discovery OF THE GALILEAN MOONS • As a result of improvements Galileo Galilei made to the telescope, with a magnifying capability of 20×,[2] he was able to see celestial bodies more distinctly than was ever possible before. This allowed Galilei to discover sometime between December 1609 and January 1610 what came to be known as the Galilean moons.[1][3] Nevertheless, a Chinese historian of astronomy, Xi Zezong, claimed that the Chinese astronomer Gan De observed one of Jupiter's moons in 362 BC, nearly 2 millennia earlier than Galileo.[4] • An apparatus from the mid-18th century for demonstrating the orbits of Jupiter's satellites • 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.[1]
THE Dedication to the Medicis • 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.[1] 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.[1]
THE FIRST GALILEAN MOON • Io is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 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 mid-20th century
THE SECOND GALILEAN MOON • Europa, 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 Earth's Moon. The name, Europa was after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete, but did not become widely used until the mid-20th century
