Portrait of Christiaan Huygens (1629-95), Dutch astronomer and physicist. Huygens was well- educated, studying at Leiden University. He made his name as an astronomer, discovering the rings of Saturn, its moon Titan, the Orion Nebula and many of the markings on Mars. His greatest achievement was in the measurement of time. In 1656, Huygens made the first pendulum clock. This gave a vast improvement over the existing mechanical clocks, and may be said to be the beginning of truly accurate time measurement. Huygens also provided much of the groundwork for the law of conservation of energy, the wave theory of light and the mathematics of probability.