Nodal precession

Nodal precession is the precession of the orbital plane of a satellite around the rotation axis of an astronomical body such as Earth. This precession is due to the non-spherical nature of a spinning body, which creates a non-uniform gravitational field. The following discussion relates to low earth orbit of artificial satellites which have no describable effect on the motion of the Earth. The nodal precession of more massive, natural satellites such as the Moon is more complex.

Around a spherical body, an orbital plane would remain fixed in space around the central body. However, most bodies rotate, which causes an equatorial bulge. This bulge creates a gravitational effect that causes orbits to precess around the rotational axis of the central body.

The direction of precession is opposite the direction of revolution. For a typical prograde orbit around Earth ( ie. in the direction of central body rotation ) , the longitude of the ascending node decreases, i.e., node precesses westward. If the orbit is retrograde, this increases the longitude of the ascending node, i.e., node precesses eastward. This nodal progression enables Sun-synchronous orbits to maintain approximately constant angle relative to the Sun.