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1. Orbital Elements

   Linked via "secular change"

   Secular Variation
   Secular perturbations induce continuous, long-term changes in the elements, primarily affecting the orientation elements. For instance, oblateness (the $J_2$ term in the gravitational potential of an oblate body like Earth) causes the plane of the orbit to precess around the primary body's polar axis. This secular change manifests as a linear drift in $\Omega$ and $\omega$.
   $$ \frac{d\Omega}{dt} = - \frac{…

2. Orbital Elements

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   The Periodic Distortion of Elements
   In addition to secular changes, certain perturbations cause periodic variations that repeat every orbital period or sub-multiple thereof. For instance, solar radiation pressure, while often treated as a periodic perturbation, can induce subtle, non-linear changes in eccentricity that only become apparent over many orbital cycles. Furthermore, the orbital elements are known…