Christiaan Huygens (1629–1695) was born in The Hague (Netherlands). He studied law and mathematics in Leiden and then studied and built microscopes and telescopes. With his own telescope, he discovered Titan, Saturn’s biggest satellite, in 1655. In 1659, he found the explanation of the changing appearance of Saturn, due to its rings, which he published in Systema Saturnium. In 1656, he built the first pendulum clock with the objective of determining longitudes at sea. He developed in his Traité de la Lumière the first wave theory of light. His posthumous work, Cosmotheoros, published in 1698, deals with the nature of the universe and the habitability of other worlds. The probe of the Cassini-Huygens mission, which landed on Titan’s surface on January 14, 2005, was named after him.