The Galilean moons (or Galilean satellites) are the four largest moons of Jupiter—Io, Europa, Ganymede, and Callisto. They were first seen by Galileo Galilei in December 1609 or January 1610 and recognized by him as satellites of Jupiter in March 1610. They were the first objects found to orbit a planet other than the Earth. The discovery of celestial bodies orbiting something other than Earth dealt a serious blow to the then-accepted Ptolemaic world system, a geocentric theory in which everything orbits around Earth. These were the only four moons of Jupiter to be known, until the discovery of the "fifth moon of Jupiter" in 1892.

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