Callisto
Jupiter is the giant of the Solar System and has something like 50 or more satellites. A satellite is something that orbits around a body in space.
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Of these satellites, or moons, there are about 16 large ones.
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The four largest are known as the Galilean moons because they were first seen and described by the great Italian scientist Galileo in 1610.
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They were later named Io, Europa, Ganymede and Callisto.
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These moons were not known before 1610 because they cannot be seen with the naked eye.
Galileo was the first person to design a telescope and was using this to examine the night sky.