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 radii 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.
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.
Galileo initially named his discovery the Cosmica Sidera ("Cosimo's stars"), but the names that eventually prevailed were chosen by Simon Marius. Marius discovered the moons independently at the same time as Galileo, and gave them their present names, which were suggested by Johannes Kepler, in his Mundus Jovialis, published in 1614....LESS
TRENTON, N.J. (AP) — A 9-year-old New Jersey boy who described himself as a "Guardian of the Galaxy" is hoping to add the real-life NASA title "Planetary Protection Officer" to his resume. NASA received an application for the position from fourth-grader Jack Davis, who asked to apply for the job. In a letter the agency posted online , Jack acknowledged his youth, but said that will make it easier for him to learn how to think like an alien. He said he has seen all the space and alien movies he can see, and he is great at video games.