1. The problem statement, all variables and given/known data Given the semimajor axis of Jupiter's orbit: 5.2 AU, and the eccentricity: .048 and the period: 11.86 years, find the total angular momentum of the Jupiter-Sun system. Assume it is an isolated system - ignore interactions from other planets etc. 2. Relevant equations The first equation at the top of this page:http://en.wikipedia.org/wiki/Angular_momentum plus various geometric equations concerning ellipses. 3. The attempt at a solution I wish I had one. My thought process is that I should find the angular momentum of each mass about the location of the center of mass which could be calculated easy enough. Since angular momentum is conserved, I can pick any arbitrary location and then calculate it. The problem I'm running into is mainly - assuming the above approach is correct - how to find the velocity of either Jupiter or the Sun at a given point on its orbit. With enough time perhaps I could derive an equation using Newton's universal gravitation law and what not, but I've been staring at this problem for a while and nothing is coming to me. Maybe my approach is inherently flawed...any help is appreciated. Thanks. edit: this is problem 2.6 in An Introduction to Modern Astrophysics, 2nd Ed.