Angular momentum is defined asorwhereis the angle as shown below.

All bodies in motion have angular momentum about some point, even if the the particle is travelling in a straight line. As the particle moves from A to B above the quantitystays constant. Angular momentum may be more usefully visualised the particles moving in a circle.

For a particle moving in a circleandare at right angles, andare at right angles to bothandThis means that

Kepler's Laws of Motion are a direct consequence of the Law of Conservation of Angular Momentum. The Law applies on all scales, including the quantum mechanical scale, and is responsible for the stability of atoms.