Janus is Saturn’s moon that shares a horseshoe orbit with Epimetheus.
Janus and Epimetheus are unusual co-orbital moons. They travel around Saturn on nearby paths and exchange orbital positions approximately every four years. Their gravitational interaction prevents them from colliding: as one moon moves inward and speeds up, the other moves outward and slows down, after which their relative positions reverse.
Janus is named after the Roman god of doorways and transitions, while Epimetheus is named after the Greek mythological Titan associated with afterthought. Pandora is another mythological name used for a Saturn moon, but Pandora is connected with the F ring rather than the Janus–Epimetheus co-orbital pair.
The system is often described as a “horseshoe orbit” because the moons’ relative motion traces a horseshoe-shaped pattern in a rotating frame. The arrangement is one of the clearest examples of co-orbital motion observed in the Solar System.