On Dec. 23, 1672, Italian astronomer Giovanni Cassini discovered Rhea, Saturn’s second-largest moon.
Rhea is made up of rock and ice and is covered in craters. Rhea is also the only moon discovered to have an oxygen atmosphere. Rhea was the second moon that Cassini discovered. He discovered Saturn's moon Iapetus about a year earlier. Rhea wasn’t formally named until 1847. The British astronomer John Herschel suggested that Saturn’s moons should be named after the Titans. In Greek mythology, the Titans were the brothers and sisters of Cronus, who the Romans referred to as "Saturn." Cassini originally named the moons he discovered after the Sidera Lodoicea, meaning the "Stars of Louis," after King Louis XIV. Because Cassini discovered four of Saturn’s moons, NASA named a spacecraft after him.
