Henon's Isochrone Model
  Henon sought the most general spherical potential in which the radial periods of orbits depended only on energy. He named this potential the isochrone, and discovered that it provided a good representation of data for globular clusters. He sought an explanation in terms of resonant relaxation. The role that resonant relaxation might play in globular clusters is still an open question, but the isochrone potential is guaranteed a role in dynamical astronomy because it is the most general potential in which closed-form expressions for angle-action coordinates are available. I explain how this property makes the isochrone invaluable for the powerful technique of torus mapping. I also describe flattened isochrone models, which enable us to explore a powerful general method of generating self-consistent stellar systems. 
James Binney (Rudolf Peierls Centre for Theoretical Physics, Oxford)
astro-ph.GA