In 1655 Dutch astronomer Christiaan Huygens set up a refractor telescope of his own construction and aimed it at Saturn. He thought the planet was encircled by a single solid ring and planned to observe its tilt, which astronomers knew changed over several years. Instead he saw something unexpected in his viewfinder: a giant moon, now called Titan. Saturn became the third planet, after Earth and Jupiter, known to have a satellite. Even if Saturn’s rings were rare, moons in our solar system were apparently commonplace.