Editing Orbital elements (section)

== Perturbations and elemental variance ==
{{Main|Perturbation (astronomy)}}
Unperturbed, [[Two-body problem|two-body]], [[Newtonian gravitation|Newtonian]] orbits are always [[conic section]]s, so the Keplerian elements define an unchanging [[ellipse]], [[parabola]], or [[hyperbola]]. Real orbits have perturbations, so a given set of Keplerian elements accurately describes an orbit only at the epoch. Evolution of the orbital elements takes place due to the [[gravitational]] pull of bodies other than the primary, the [[Sphericity|non-sphericity]] of the primary, [[atmospheric drag]], [[Theory of relativity|relativistic effects]], [[radiation pressure]], [[electromagnetic force]]s, and so on.

Keplerian elements can often be used to produce useful predictions at times near the epoch. Alternatively, real trajectories can be modeled as a sequence of Keplerian orbits that [[Osculating orbit|osculate]] ("kiss" or touch) the real trajectory. They can also be described by the so-called [[planetary equations]], differential equations which come in different forms developed by [[Joseph Louis Lagrange|Lagrange]], [[Carl Friedrich Gauss|Gauss]], [[Charles-Eugène Delaunay|Delaunay]], [[Henri Poincaré|Poincaré]], or [[George William Hill|Hill]].