Cassini’s 1671 Discovery of Iapetus: The Origin of Saturn’s Yin-Yang Moon
On October 25, 1671, Italian astronomer Giovanni Cassini discovered Iapetus, the second known moon of Saturn. Initially appearing as a faint speck, its elusive orbit led Cassini to deduce that one hemisphere is significantly darker than the other. This two-toned appearance, later confirmed by NASA’s Cassini spacecraft, earned Iapetus the nickname 'yin-yang moon' and has since informed models of how Saturn’s moons and rings formed from ancient collisions.