The movement of thought and culture in Europe now known as the Enlightenment reached its peak in the eighteenth century. Having its roots in the humanism of the Renaissance and the Scientific Revolution of the seventeenth century, it embraced a confident and optimistic outlook on the prospects for human progress. Human reason, in the guise of scientific advance, could not only understand the world but also change it. Kepler and Newton in the seventeenth century had uncovered the elegant and simple principles that lay behind the cosmos. The puzzling trajectories of the planets against the background of the fixed stars proved impossible to account for on the old Ptolemaic astronomy without the introduction of ad hoc complexity. Planetary motion, however, became completely comprehensible once it was understood that they moved in elliptical orbits round our sun.