Christiaan Huygens discovered Saturn's moon Titan

On March 25, 1655, Dutch astronomer Christiaan Huygens spotted a faint point of light tracking alongside Saturn. He had built the telescope himself, grinding its lenses to sharpen the blurry views of the era, and he was hunting Saturn’s puzzling rings. Instead he found Titan, the first moon of Saturn ever seen and the sixth known satellite of any planet.

Huygens simply called it Luna Saturni, Latin for “Saturn’s moon.” The name Titan came two centuries later. The discovery helped confirm that the universe was far more crowded with worlds than anyone had imagined, and it cemented Huygens as one of the great observers of his age.

A search for rings ended in the discovery of a world larger than the planet Mercury.

Titan turned out to be extraordinary: the second-largest moon in the solar system and the only one wrapped in a thick atmosphere. In 2005, the European probe Huygens landed there, the most distant touchdown humans have ever achieved.