  The frozen-perigee behavior of elliptic orbits at the critical inclination is
usually displayed after an averaging procedure. However, this singularity in
Artificial Satellite Theory manifests also in the presence of short-period
effects. Indeed, a closed form expression relating orbital inclination and the
ratio anomalistic draconitic frequencies is derived for the main problem, which
demonstrates that the critical inclination results from commensurability
between the periods with which the radial and polar variables evolve in the
instantaneous plane of motion. This relation also shows that the critical
inclination value is slightly modified by the degree of oblateness of the
attracting body, as well as by the orbit's size and shape.
