Saturn is the sixth planet of our solar system. The four moons of Saturn such as Iapetus, Rhea, Tethys and Dione were discovered by French-Italian astromer Giovanni Domenico Cassini. Iapetus the third-largest moon of Saturn was discovered in 1671. Rhea Saturn’s second-largest moon was discovered in 1672. Tethys and Dione were discovered in 1684.