Christiaan Huygens was a Dutch mathematician, astronomer, and physicist who made significant contributions to the fields of mathematics, optics, and mechanics. He is best known for his work on the wave theory of light and for the invention of the pendulum clock. Huygens also discovered Saturn's largest moon, Titan, and made important advancements in the study of celestial bodies. His work laid the foundation for many developments in the fields of physics and astronomy.