Technical
J2 Perturbation
The dominant gravitational perturbation caused by the Earth's equatorial bulge, which steadily rotates an orbit's plane and apsides.
The Earth is not a perfect sphere; its equatorial bulge is captured by the J2 term in the gravity field. This oblateness produces the largest non-spherical perturbation on most satellite orbits.
Its main effects are nodal precession (rotation of the RAAN) and rotation of the argument of perigee. Mission designers exploit J2 to achieve sun-synchronous orbits and must account for it in long-term orbit prediction and constellation maintenance.