Which Saturn moon was discovered by Giovanni Cassini in 1671? Iapetus was discovered by Giovanni Domenico Cassini in October 1671.
Iapetus was the first Saturnian moon Cassini discovered, following Christiaan Huygens’s discovery of Titan in 1655. Cassini initially observed Iapetus on Saturn’s western side but could not see it on the eastern side because its two hemispheres differ dramatically in brightness.
Cassini eventually recognized that Iapetus was tidally locked and that its bright hemisphere faced forward in one part of its orbit while its dark hemisphere faced Earth in another. That unusual contrast made the moon difficult to track with early telescopes. Iapetus is now known for its two-tone surface and a huge equatorial ridge extending roughly three-quarters of the way around it.
A common mix-up is with Rhea, Dione, and Tethys, which Cassini also discovered in the late seventeenth century. Iapetus is the outermost of Saturn’s large moons and is designated Saturn VIII.