Follow us Add us as a preferred source on Google Newsletter Subscribe to our newsletter On October 25, 1671, Italian astronomer Giovanni Cassini discovered a new moon on Saturn! Iapetus was the second moon anyone had found orbiting Saturn, and it was the first of four moons Cassini would discover on the ringed planet. Dutch astronomer Christiaan Huygens had discovered Titan, the largest of Saturn’s moons, about six years earlier. After Cassini found Iapetus, it also found the moons Rhea, Tethys, and Dione. When Cassini first saw Iapetus, the moon looked like a small speck on the west side of Saturn. He calculated its orbit and thought he might see it again on the east side of Saturn 39 days later, but it was nowhere to be seen. When it reappeared again on the west side, Cassini realized that Iapetus must be darker on one side than the other, making it more difficult to see when the dark side was facing Earth. More than 300 years later, NASA sent a spacecraft called Cassini to Saturn, and Cassini took some pretty impressive photos of Iapetus. Noticing that Iapetus looked a bit like a yin-yang, it has since been nicknamed the “yin-yang moon.” This image taken by the Cassini spacecraft shows what the two-color surface of the moon Iapetus looks like without false colors. (Image credit: NASA/JPL-Caltech/Space Science Institute) How It Mattered The discovery of Iapetus, with its bright and dark yin-yang hemispheres, has contributed to our understanding of how moons form. “Using computer simulations, the researchers concluded…Saturn’s inner moons and their magnificent rings are all relatively young. The more distant moons Titan and Iapetus would not have formed at the same time.” “Our best guess is that Saturn had a similar collection of moons before, but their orbits were disturbed by a special type of orbital resonance that involves Saturn’s motion around the sun. Eventually, the orbits of neighboring moons crossed and these objects collided. From this debris formed the current set of moons and rings. “Want more space history? Check out our full archive of On This Day In Space stories and watch our On This Day In Space videos on YouTube. On this day in space! October 24, 1946: V-2 rocket takes first photographs of Earth from space
