 Most of the known perturbations to motion in stable, regular, and well-determined dynamical systems tend to be periodic at some level, but in many-body systems, chaotic dynamics result in some effects which are one-way (for example, planetary migration).Secular phenomena create variations in the orbits of the Moon and the planets.The solar emission spectrum and the solar wind are undergoing secular trends due to migration through the galactic plane, leading to effects that may impact on climate and cause extinction events.