  A Hamiltonian model is constructed for the spin axis of a planet perturbed by
a nearby planet with both planets in orbit about star. We expand the
planet-planet gravitational potential perturbation to first order in orbital
inclinations and eccentricities, finding terms describing spin resonances
involving the spin precession rate and the two planetary mean motions.
Convergent planetary migration allows the spinning planet to be captured into
spin resonance. With initial obliquity near zero, the spin resonance can lift
the planet's obliquity to near 90 or 180 degrees depending upon whether the
spin resonance is first or zero-th order in inclination. Past capture of Uranus
into such a spin resonance could give an alternative non-collisional scenario
accounting for Uranus's high obliquity. However we find that the time spent in
spin resonance must be so long that this scenario cannot be responsible for
Uranus's high obliquity. Our model can be used to study spin resonance in
satellite systems. Our Hamiltonian model explains how Styx and Nix can be
tilted to high obliquity via outward migration of Charon, a phenomenon
previously seen in numerical simulations.
