Oct. 25, 1671: Iapetus is discovered

Giovanni Cassini discovered Saturn's moon Iapetus on Oct. 25, 1671, using a 34 foot Campani refractor at the Paris Observatory. It was the second Saturn moon found, after Titan, and Cassini noticed it appeared only on the west side of the planet, which let him deduce that one face is much darker and that the moon is tidally locked. That split is why Iapetus swings in brightness as it orbits.