Discovery of Janus

Janus is a moon of Saturn, about 180 kilometres across, which shares almost the same orbit as the smaller moon Epimetheus. The French astronomer Audouin Dollfus observed it in December 1966, and the confusion over whether there was one body or two was settled in 1978, when Stephen Larson and John Fountain showed that the observations were explained by a pair of moons sharing an orbit. Every four years the two come close, and swap places, so that each travels on a horseshoe-shaped path relative to the other. The moon is named after the Roman god Janus, who looked in both directions and presided over doorways and beginnings.

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