Giovanni Cassini Discovered Saturn Moon Iapetus on October 25, 1671

Italian astronomer Giovanni Cassini discovered Saturn's moon Iapetus on October 25, 1671. It was the second moon ever found orbiting Saturn and the first of four that Cassini would eventually discover at the ringed planet. Dutch astronomer Christiaan Huygens had discovered Saturn's largest moon, Titan, about six years prior. Following his discovery of Iapetus, Cassini also found the moons Rhea, Tethys and Dione.

When Cassini first spotted Iapetus, it appeared as a tiny speck on the west side of Saturn. He calculated its orbit and expected to see it on the east side 39 days later, but the moon was nowhere to be seen. When it reappeared on the west side again, Cassini deduced that one side of the moon was darker than the other, which made it harder to see when the dark side faced Earth.

Centuries later, NASA's Cassini spacecraft visited Saturn and captured detailed images of the body. Because the spacecraft's photos revealed a surface that resembled a yin-yang, Iapetus has since been nicknamed the "yin-yang moon."The discovery of Iapetus and its contrasting bright and dark hemispheres has added to our understanding of how moons form.

Using computer simulations, researchers concluded that the inner moons of Saturn and its rings are relatively young, and that the more distant moons Titan and Iapetus would not have been formed at the same time.

Researchers' best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special kind of orbital resonance involving Saturn's motion around the sun.

Eventually, the orbits of neighboring moons crossed and these objects collided, and the present set of moons and rings formed from this rubble.