Apsidal precession or a precessing elliptical orbit

    Caption: An animation of the apsidal precession of an elliptical orbit of a relatively low mass astro-body about a much more massive astro-body that is effectively at rest at the center of mass of the orbital system.

    Features:

    1. Apsides (singular apsis) are two points (apoapsis and periapsis) on an elliptical orbit intersected by the major axis of the orbit.

    2. The periapsis/apoapsis is the closest/farthest point on the orbit from the ellipse focus at the center of mass of the gavitational two-body system.

    3. Apsidal precession is a rotation of the apse line (AKA line of apsides) which connects the periapsis and apoapsis.

    4. There would be NO apsidal precession in an exact gravitationally-bound gavitational two-body system determined by Newtonian physics.

    5. In an orbital system that is NOT exactly a Newtonian physics gravitationally-bound gavitational two-body system, apsidal precession of the two-body system (which is part of the overall orbital system) can be caused by many kinds of gravitational perturbation.

      Usually, the largest is cause is gravitational perturbations from other astro-bodies in the orbital system that are NOT part of the two-body system.

      Another kind of gravitational perturbation is due to general relativity (GR). GR dictates a force in addition to Newtonian gravitation (which is a 1/r**2 force or inverse-square law force). The additional force varies as 1/r**4 and leads to an apsidal precession (see Wikipedia: Two-body problem in general relativity: Effective radial potential energy).

      In the Solar System, this additional GR force is overwhelmingly largest for orbit of Mercury (see Wikipedia: perihelion shift of Mercury).

    6. Note apsidal precession is NOT the same as axial precession. As shorthand, both processes are often just referred to as "precessions".

    Credit/Permission: © UserWillowW, 2008 / Creative Commons CC BY-SA 3.0.
    Image link: Wikipedia: File:Precessing Kepler orbit 280frames e0.6 smaller.gif.
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