Janus and Epimetheus travel in nearly the same track, about 94,100 miles (151,500 km) out from Saturn. They occasionally pass each other, their gravity causing them to switch speeds and positions as they do; Janus goes faster and higher one time, slower and lower the next – but the two never come within more than about 6,200 miles of each other. The two moons switch positions roughly every four years. This scenario is referred to in astrophysics as a 1:1 resonance. Astronomers were initially confused when the moons were discovered in 1966 as it wasn’t known at the time that...