Relative angular momentum

In celestial mechanics, the relative angular momentum () of an orbiting body () relative to a central body () is the moment of ()'s relative linear momentum:

where:

For a body in an unperturbed orbit about a central body, the orbital plane is stationary, and the relative angular momentum () is perpendicular to the orbital plane.
For perturbed orbits where the orbital plane is in motion, the relative angular momentum vector is perpendicular to the (osculating) orbital plane at only two points in the orbit.