Callisto, or Jupiter IV, is the second-largest moon of Jupiter, after Ganymede. It is the third-largest moon in the Solar System after Ganymede and Saturn's largest moon Titan, and the largest object in the Solar System that may not be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At 4821 km in diameter. Callisto is named after one of Zeus's many lovers in Greek Mythology. Callisto was a nymph who was associated with the goddess of the hunt, Artemis.