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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. Solar System ephemerides are essential for the navigation of spacecraft and for all kinds of space observations of the planets, their natural satellites, stars and galaxies.
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).
In the Solar System
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.
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, influencing the Milankovitch cycles.
Secular variation is a concept describing long-term trends in the orbits of the planets Mercury to Neptune. Several attempts have from time to time been undertaken to analyze and predict such gravitational deviations from ordinary satellite orbits. Others are often referred to as post keplerian effects.
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