Editing Janus (moon) (section)

== Orbit ==
[[File:PIA08170 Epimetheus and Janus.jpg|thumb|left|[[Epimetheus (moon)|Epimetheus]] (lower left) and Janus (right) seen on 20 March 2006, two months after swapping orbits. The two moons appear close only because of [[foreshortening]]; in reality, Janus is about 40,000 km farther from ''Cassini'' than Epimetheus.]]
Janus's orbit is co-orbital with that of [[Epimetheus (moon)|Epimetheus]]. Janus's [[semimajor axis|mean orbital radius]] from Saturn was, as of 2006, only 50&nbsp;km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with [[Kepler's laws of planetary motion]], the closer [[orbit]] is completed more quickly. Because of the small difference, it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and [[orbital period]] are increased, and in exchange the outer moon's are decreased. The timing and magnitude of the [[Gravity assist|momentum exchange]] is such that the moons effectively swap orbits, never approaching closer than about 10,000&nbsp;km. At each encounter Janus's orbital radius changes by ~20&nbsp;km and Epimetheus's by ~80&nbsp;km: Janus's orbit is less affected because it is four times as massive as Epimetheus. 
[[File:Epimetheus-Janus_Orbit.png|thumb|250x250px|[[Rotating reference frame|Rotating-frame]] depiction of the [[Horseshoe orbit|horseshoe orbits]] of Janus and Epimetheus]]
The exchange takes place close to every four years; the last close approaches occurred in January 2006,{{sfn|JPL/NASA: The Dancing Moons}} 2010, 2014, 2018, and 2022 and the next in 2026. This is the only such orbital configuration known in the [[Solar System]].{{sfn|El Moutamid et al|2015}}

The orbital relationship between Janus and Epimetheus can be understood in terms of the [[Three-body problem#Circular restricted three-body problem|circular restricted three-body problem]], as a case in which the two moons (the third body being Saturn) are similar in size to each other.{{Sfn|Llibre and Ollé 2011|}}