Callisto (moon) Callisto (moon) </math>.</ref> Callisto (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 not to be properly differentiated. Callisto was discovered in 1610 by Galileo Galilei. At in diameter, Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass. It is the fourth Galilean moon of Jupiter by distance, with an orbital radius of about . It is not in an orbital resonance like the three other
Formation and evolution of the Solar System The first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had developed for millennia (Aristarchus of Samos had suggested it as early as 250 BC), but was not widely accepted until the end of the 17th century. The first recorded use of the term "Solar System" dates from 1704. The current standard theory for Solar System formation, the nebular hypothesis, has fallen into and out of favour since its formulation by Emanuel Swedenborg,