  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.
