21st March

On this day in 1684, Giovanni Cassini discovered two moons of Saturn: Tethys and Dione. He had previously discovered two other satellites of Saturn: Iapetus (in 1671) and Rhea (in 1672). Christiaan Huygens was the first to discover a moon of Saturn, when he viewed Titan (the largest and easiest to see) in 1655.