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".