1. The problem statement, all variables and given/known data For an ellipse, if we take the cross product of the velocity vector with the radial vector (distance from center of mass), it is equal to a constant h. Is this true? If so, what is the proof? 2. Relevant equations n/a 3. The attempt at a solution Conceptually it makes sense. When a comet is farther from the sun, the radial vector is longer, but the velocity vector is less. Likewise, at perihelion, the comet has more velocity but less radius. So it should be a constant? I can't justify it mathematically though.