Galilean moons

The Galilean moons , or Galilean satellites, are the four largest <a href="/facts/Moons_of_Jupiter/Rq3GMt28">moons of Jupiter</a>. They are, in descending-size order, <a href="/facts/Ganymede_(moon)/AmKsF10c">Ganymede</a>, <a href="/facts/Callisto_(moon)/et0eIutu">Callisto</a>, <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a>, and <a href="/facts/Europa_(moon)/2A9nleK3">Europa</a>. They are the most <a href="/facts/Apparent_magnitude/7pCTF1Me">readily visible</a> <a href="/facts/Solar_System_object/R2sAlEFF">Solar System objects</a> after <a href="/facts/Saturn/D54LcGJ9">Saturn</a>, the dimmest of the <a href="/facts/Classical_planet/wBq07pNt">classical planets</a>; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common <a href="/facts/Binoculars/DPHVmKIL">binoculars</a>, even under <a href="/facts/Night_sky/IM1PakDh">night sky</a> <a href="/facts/Bortle_scale/IFXbNMQ2">conditions</a> of high <a href="/facts/Light_pollution/AD03VyzX">light pollution</a>. The invention of the <a href="/facts/Telescope/BDwYahhR">telescope</a> allowed astronomers to discover the moons in 1610. Through this, they became the first <a href="/facts/Timeline_of_discovery_of_Solar_System_planets_and_their_moons/1IHNRArs">Solar System objects discovered</a> since humans have started tracking the classical planets, and the first objects to be found to orbit any <a href="/facts/Planet/rbPjE6Bx">planet</a> beyond Earth.
They are <a href="/facts/Planetary-mass_moon/LSfyrXPB">planetary-mass moons</a> and among the <a href="/facts/List_of_Solar_System_objects_by_size/Aob1rVgh">largest objects</a> in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>. All four, along with <a href="/facts/Titan_(moon)/lg0l9PCH">Titan</a>, <a href="/facts/Triton_(moon)/mxxRUygc">Triton</a>, and Earth's <a href="/facts/Moon/zKHqEMPb">Moon</a>, are larger than any of the Solar System's <a href="/facts/Dwarf_planet/IkQoCdwD">dwarf planets</a>. The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet <a href="/facts/Mercury_(planet)/lVa1GIyX">Mercury</a> in size (though not mass). Callisto is only slightly smaller than Mercury in size; the smaller ones, Io and Europa, are about the size of the Moon. The three inner moons — Io, Europa, and Ganymede — are in a 4:2:1 <a href="/facts/Orbital_resonance/eQLMfBWu">orbital resonance</a> with each other. While the Galilean moons are spherical, all of <a href="/facts/Jupiter/95m7XjMT">Jupiter</a>'s remaining moons have irregular forms because they are too small for their <a href="/facts/Self-gravitation/JMNuzEYJ">self-gravitation</a> to pull them into spheres.
The Galilean moons are named after <a href="/facts/Galileo_Galilei/98VT2z3v">Galileo Galilei</a>, who observed them in either December 1609 or January 1610, and recognized them as satellites of Jupiter in March 1610; they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter <a href="/facts/Amalthea_(moon)/XaNHMQLD">Amalthea</a> in 1892. Galileo initially named his discovery the Cosmica Sidera ("<a href="/facts/Cosimo_II_de%2527_Medici%2c_Grand_Duke_of_Tuscany/MMe9GyRn">Cosimo</a>'s stars") or Medicean Stars, but the names that eventually prevailed were chosen by <a href="/facts/Simon_Marius/yb54huCF">Simon Marius</a>. Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, after mythological characters that <a href="/facts/Zeus/9wTwkEDt">Zeus</a> seduced or <a href="/facts/List_of_rape_victims_from_ancient_history_and_mythology/gpSLd9Hx">abducted</a>, which were suggested by <a href="/facts/Johannes_Kepler/wk6GTN3C">Johannes Kepler</a> in his Mundus Jovialis, published in 1614. Their 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. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted (among educated Europeans) <a href="/facts/Geocentric_model/yIc6na3w">Ptolemaic world system</a>, a geocentric theory in which everything orbits around Earth.

Galilean moons open-in-new

Galilean moons