They are the most readily visible Solar System objects after Saturn , the dimmest of the classical planets ; though their closeness to bright Jupiter makes naked-eye observation very difficult, they are readily seen with common binoculars , even under night sky conditions of high light pollution.
The invention of the telescope enabled the discovery of the moons in Through this, they became the first Solar System objects discovered since humans have started tracking the classical planets, and the first objects to be found to orbit any planet beyond Earth.
They are planetary-mass moons and among the largest objects in the Solar System. The largest, Ganymede, is the largest moon in the Solar System and surpasses the planet Mercury 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 orbital resonance with each other. While the Galilean moons are spherical, all of Jupiter 's remaining moons have irregular forms because they are too small for their self-gravitation to pull them into spheres.
The Galilean moons are named after Galileo Galilei , who observed them in either December or January , and recognized them as satellites of Jupiter in March ; [ 2 ] they remained the only known moons of Jupiter until the discovery of the fifth largest moon of Jupiter Amalthea in Marius discovered the moons independently at nearly the same time as Galileo, 8 January , and gave them their present individual names, after mythological characters that Zeus seduced or abducted , which were suggested by Johannes Kepler in his Mundus Jovialis , published in The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted among educated Europeans Ptolemaic world system , a geocentric theory in which everything orbits around Earth.
