Janus and Epimetheus orbit Saturn once every 16 hours. They share the same orbit.

One is slightly closer to Saturn than the other and moves slightly faster. Every four years, the inner moon catches up to the outer slower moon. As they pass in gravitational embrace, the outer moon moves into the inner orbit and vice versa.

On January 21, 2006, Epimetheus caught up and swapped position with Janus which will remain the innermost of the pair until 2010, when the next switch will occur.

Right: Artist's impression of orbits. Satellite sizes and orbit separtion are exagerated in scale.