We report results of a systematic study of the energy-field dependence
of the four elementary periodic orbits of the electron motion of the
hydrogen atom in crossed magnetic-electric fields from weak to strong
perturbation, up into the continuum regime. We find the classical dyna
mics of the crossed-field atom to be intimately connected to that of t
he Sun-Earth-Moon three-body problem in Hill's approximation, exhibite
d by striking similarities of the four elementary orbits of the atomic
and celestial systems in their energy-field dependence.