crest and trough

As Kottke informs, during one of its final passes of the Saturn system, the Cassini probe (previously) delivers an image of a spiral density wave in one of the planets rings that illustrates the complexity and nuance of gravitational relationships. The ripples are caused by the waltz of two satellites, Janus and Epimetheus, which share the same orbital path and trade places as leader and follower with a period of four years. Like extracting historical data from tree growth rings, gauging the distance between the waves reveal details about past trajectories.