Radial epicyclic frequency (${\kappa }_r$)

The radial epicyclic frequency describes small radial oscillations about a circular orbit. For specific angular momentum $h(r)$, ${\kappa }_r^2=r^{-3}d(h^2)/dr$.

Epicyclic motion is the small oscillation of a displaced fluid element or particle about a circular orbit. Its natural radial frequency is the radial epicyclic frequency.