In 1655, Christiaan Huygens, a Dutch astronomer, observed Saturn through a telescope and discovered Saturn’s largest moon, Titan, although it wouldn’t be named this for another 200 years. Titan’s diameter is about 5,150 km (3,200 mi), or 1.5 times as large as than the diameter of Earth’s Moon — 3,500 km (2,200 mi). Titan is the second-largest moon in the Solar System — Jupiter’s Ganymede is the largest. Both Titan and Ganymede are larger than the planets Mercury and Pluto. Titan and Ganymede are defined as moons because they orbit planets, while Mercury and Pluto are defined as planets because they orbit the Sun. Later in the 1600s, the Italian–French astronomer Jean-Dominique Cassini discovered four more moons of Saturn: Iapetus (1671), Rhea (1672), and Tethys and Dione (1684). Not surprisingly, these moons were Saturn’s next largest, with diameters ranging from 1,400 km (870 mi) for Tethys to 1,500 km (960 mi) for Rhea. Cassini noted that when Iapetus was on one side of Saturn, it could be easily seen; however, when it was on the other side of its orbit, it was invisible. He correctly deduced that Iapetus was keeping the same side always toward Saturn, and that one side of the moon (its leading hemisphere) was much darker than the other side (its trailing hemisphere). In 1789, William Herschel in England discovered moons of Saturn that would later be named Mimas and Enceladus. These moons have even smaller diameters: about 500 km (310 mi) for Enceladus and 400 km (250 mi) for Mimas. In 1848, astronomers William Bond and George Bond (father and son) of Harvard College discovered Hyperion, with a diameter of 290 km (180 mi). The very same night, William Lassell of England also discovered it with his telescope. In 1898, William Pickering, also of Harvard, discovered Phoebe, with a diameter of 220 km (140 mi). Phoebe was the first moon discovered using photography, rather than by looking directly through a telescope’s eyepiece. The innermost four moons (Pan, Atlas, Prometheus, and Pandora), which are intertwined with Saturn’s A and F rings, were not discovered until Voyager 1 flew past Saturn in 1980. Pan, in fact, eluded discovery until even after Voyager. It was not until 1991 that astronomer Mark Showalter searched through Voyager images of the narrow, clear Encke Gap in Saturn’s A ring and found Pan. The rest of Saturn’s currently known moons were discovered by observers on Earth during the 1966 and 1980 ring-plane crossings, when Saturn’s thin rings were seen edge-on from Earth. With the rings temporarily not visible from Earth, faint objects near the planet are easier to see. During the 1966 ring-plane crossing, Audoin Dollfus discovered Janus, and John Fountain and Steve Larson discovered its companion, Epimetheus. Telesto, Calypso, and Helene were discovered by three different groups of astronomers during the 1980 ringplane crossing. Ring-plane crossings occur about every 15 years. Like Earth’s North Pole, Saturn’s north pole is tilted with respect to the plane of its orbit, and this causes our view of the rings to change as Saturn travels in its 30-year orbit around the Sun. Ring-plane crossings occur near Saturn’s equinoxes when the planet’s tilt is neither toward nor away from the Sun. In 1995, astronomers using the Hubble Space Telescope announced that they had discovered two — perhaps even four — previously unknown moons. Amanda Bosh and Andrew Rivkin spotted what appeared to be new moons of Saturn in photographs they made during the ring-plane crossing. Two of the “newly discovered moons” in the photos turned out to be the previously known moons Atlas and Prometheus, but they were at different positions than predicted by previous estimates of their orbits. Careful analysis of the remaining two moons showed that they were at the distance of the F ring, and appeared to change shape as they orbited Saturn. These objects are now believed not to be moons, but rather “clumps” of ring material within the F ring. Bosh was not disappointed that the “moons” turned out not to be moons after all. Astronomers were still excited, because this was the first time the F ring clumps had been seen from Earth.