JanusFrench astronomer Audouin Dollfus discovers Janus, the tenth discovered natural satellite of the planet Saturn, orbiting just beyond the outer edge of Saturn’s ring. Confusion is caused when, mere days later, an American astronomer discovers another body in a near-identical orbit that cannot be reconciled with the orbit calculated for Janus. Only in the late 1970s will a theory emerge that explains the inconsistencies as the presence of two moons moving around Saturn as a co-orbital pair, occasionally swapping positions. Janus’ existence will be verified in 1980 by Voyager 1. (Janus is shown here in a 21st century Cassini photo.)