Which of Kepler's Laws of Planetary Motion Is a Consequence of the Conservation of Angular Momentum?

Angular Momentum with Reference to Orbits

• Angular momentum is a product of the mass of an object (which in the case of planets will not change), its velocity perpendicular to the line joining it to the axis of rotation (the line between it and the sun) and its distance from the axis (the sun).

Conservation of a Planet's Angular Momentum

• Because angular momentum is conserved and the mass of a planet does not change, the varying quantities are the distance to the sun and the orbital velocity. Because the equation is a product, as the distance increases the speed must decrease, and vice versa.

Kepler's Knowledge of Angular Momentum

• The idea of angular momentum had not been derived at the time Kepler was working, so his law was not intended as a specific expression of angular momentum, but rather as a tidy way of describing the observed motions.