<P> The Galilean moons are the four largest moons of Jupiter--Io, Europa, Ganymede, and Callisto . They were first seen by Galileo Galilei in January 1610, and recognized by him as satellites of Jupiter in March 1610 . They are the first objects found to orbit another planet . Their names derive from the lovers of Zeus . They are among the largest objects in the Solar System with the exception of the Sun and the eight planets, with a radius larger than any of the dwarf planets . Ganymede is the largest moon in the Solar System, and is even bigger than the planet Mercury . The three inner moons--Io, Europa, and Ganymede--are in a 4: 2: 1 orbital resonance with each other . Because of their much smaller size, and therefore weaker self - gravitation, all of Jupiter's remaining moons have irregular forms rather than a spherical shape . </P> <P> The Galilean moons were discovered in either 1609 or 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than ever . 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 Ptolemaic world system, or the geocentric theory in which everything orbits around Earth . </P>

What are the four biggest moons of jupiter
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