|
In astronomy, secular phenomena are contrasted with phenomena observed to repeat periodically. In particular, astronomical ephemerides use secular to label the longest-lasting or non-oscillatory perturbations in the motion of planets, as opposed to periodic perturbations which exhibit repetition over the course of a time frame of interest. 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).
Depending on what time frames are considered, perturbations can appear secular even if they are actually periodic. An example of this is the precession of the Earth's axis considered over the time frame of a few hundred or thousand years. When viewed in this time frame the so-called "precession of the equinoxes" can appear to mimic a secular phenomenon since the axial precession takes 25,771.5 years and monitoring it over a much smaller timeframe appears to simply result in a "drift" of the position of the equinox in the plane of the ecliptic of approximately one degree every 71.6 years.
|
