Based on the principle that causes needed to be sought for observed planetary motions, both regular and exceptional, Johannes Kepler (1571–1630) posited both a force between planets and the Sun and also a force to propel the planets. Isaac Newton (1643–1727) was to show that a principle of inertia could be used instead of the force to propel the planets. Kepler’s most famous contribution was the discovery that the planets moved in elliptical, rather than circular orbits.