Galilean moons

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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.

Galileo initially named his discovery the Cosmica Sidera ("Cosimo's stars"), but names that eventually prevailed were chosen by Simon Marius. Marius claimed to have discovered the moons at the same time as Galileo, and gave them their present names in his Mundus Jovialis, published in 1614.




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]

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