Giovanni Cassini Discovered Saturn's Moon Iapetus in 1671

Astronomer Giovanni Cassini discovered Saturn’s moon Iapetus on October 25, 1671. Christiaan Huygens had discovered Titan about six years earlier, making Iapetus the second moon identified orbiting Saturn.

Key details

• Iapetus has contrasting bright and dark regions, which affect how visible it appears from Earth as it moves in its orbit.
• NASA’s Cassini spacecraft photographed Iapetus and documented its two-tone surface.
• Cassini later discovered Saturn’s moons Rhea, Tethys, and Dione.

Cassini discovered Iapetus on October 25, 1671.

Why it matters

Iapetus’s changing visibility and sharply contrasting surface make it a distinctive target in the study of Saturn’s moons. NASA’s Cassini spacecraft added close-up observations to the record that began with Cassini’s 17th-century discovery.

What happens next

No further timeline was announced.