Jupiter's Moons
<p>The <b>Galilean moons</b> are the four largest <a href="https://en.wikipedia.org/wiki/Moons_of_Jupiter" title="Moons of Jupiter">moons of Jupiter</a>—<a href="https://en.wikipedia.org/wiki/Io%28moon%29" title="Io (moon)">Io</a>, <a href="https://en.wikipedia.org/wiki/Europa%28moon%29" title="Europa (moon)">Europa</a>, <a href="https://en.wikipedia.org/wiki/Ganymede%28moon%29" title="Ganymede (moon)">Ganymede</a>, and <a href="https://en.wikipedia.org/wiki/Callisto%28moon%29" title="Callisto (moon)">Callisto</a>. </p>
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They were discovered by <a href="https://en.wikipedia.org/wiki/Galileo_Galilei" title="Galileo Galilei">Galileo Galilei</a> around January 1610 and were the first group of objects found to orbit another planet. </p>
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Their names derive from the lovers of <a href="https://en.wikipedia.org/wiki/Zeus" title="Zeus">Zeus</a>. They are among <a href="https://en.wikipedia.org/wiki/List_of_moons_by_diameter" class="mw-redirect" title="List of moons by diameter">the largest objects</a> in the <a href="https://en.wikipedia.org/wiki/Solar_System" title="Solar System">Solar System</a> with the exception of the <a href="https://en.wikipedia.org/wiki/Sun" title="Sun">Sun</a> and the eight <a href="https://en.wikipedia.org/wiki/Planet" title="Planet">planets</a>, with radii larger than any of the <a href="https://en.wikipedia.org/wiki/Dwarf_planet" title="Dwarf planet">dwarf planets</a>. Ganymede is the largest moon in the Solar System, and is even bigger than the planet <a href="https://en.wikipedia.org/wiki/Mercury_%28planet%29" title="Mercury (planet)">Mercury</a>. The three inner moons—Io, Europa, and Ganymede—are in a 4:2:1 <a href="https://en.wikipedia.org/wiki/Orbital_resonance" title="Orbital resonance">orbital resonance</a> with each other.
The Galilean moons were discovered in either 1609 or 1610 when Galileo made improvements to his <a href="https://en.wikipedia.org/wiki/Telescope" title="Telescope">telescope</a>, which enabled him to observe celestial bodies more distinctly than ever.<sup id="cite_ref-Galileo89_1-0" class="reference"><a href="https://en.wikipedia.org/wiki/Galilean_moons#cite_note-Galileo89-1">[1]</a></sup> 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 Earth dealt a serious blow to the then-accepted <a href="https://en.wikipedia.org/wiki/Geocentric_model" title="Geocentric model">Ptolemaic world system</a>, or the geocentric theory in which everything orbits around Earth.
Galileo initially named his discovery the <b>Cosmica Sidera</b> ("<a href="https://en.wikipedia.org/wiki/Cosimo_II_de%27_Medici,_Grand_Duke_of_Tuscany" title="Cosimo II de' Medici, Grand Duke of Tuscany">Cosimo</a>'s stars"), but the names that eventually prevailed were chosen by <a href="https://en.wikipedia.org/wiki/Simon_Marius" title="Simon Marius">Simon Marius</a>. Marius discovered the moons independently at the same time as Galileo, and gave them their present names, which were suggested by <a href="https://en.wikipedia.org/wiki/Johannes_Kepler" title="Johannes Kepler">Johannes Kepler</a>, in his <i>Mundus Jovialis</i>, published in 1614.<sup id="cite_ref-2" class="reference"><a href="https://en.wikipedia.org/wiki/Galilean_moons#cite_note-2">[2]</a></sup></p>