A Dutch astronomer and mathematician, one of the first real representatives of the modern scientific spirit, both as an experimenter and as a theorist, and author of the first complete treatise on the calculation of probabilities. In astronomy Huygens invented a combination of lenses which eliminated chromatic aberration, and doubled the length of astronomical telescopes, which in addition increased their magnifying power. This enabled him to notably discover the rotation of Mars and its period. He was amongst the first to point out that the stars were other suns, much further away from ‘ours’. He estimated, with remarkable precision for the period, the distance between the Earth and the Sun. In mechanics, he established the theory of the pendulum, which he used to regulate the movement of clocks. We also owe to him the use of a hairspring for watches, the conception of centrifugal force and the definition of the moment of inertia.