co-orbital moon · moon of Saturn
Janus
A small inner moon of Saturn that shares almost the same orbit with Epimetheus, the two regularly trading places.
- Distance from Earth today
- 1.28 billion km8.564 AU · light: 1 h 11 min
- Distance from the Sun today
- 1.41 billion km9.425 AU
- As of
- 3 November 2026position from NASA/JPL orbital elements
Where it is today
Top view of the plane of Earth’s orbit, compressed distance scale. Open on the big map with the time slider →
Facts
- Diameter
- 178 km
- Mass
- 1.90 × 10¹⁸ kg
- Density
- 638 kg/m³
- Mean distance from the planet
- 151,500 km
- Orbital period
- 16.7 h
- Orbital eccentricity
- 0.0070
- Orbital inclination
- 0.2°
Source of the figures: JPL SSD – Planetary Satellites (public domain).
Discovery: 1966 – Audouin Dollfus
Description
Janus is a small, irregular moon orbiting just outside Saturn's main rings. Audouin Dollfus reported it in 1966, when the rings were seen edge-on, but its orbit was confused for years because a second moon, Epimetheus, travels on almost exactly the same path. The two are co-orbital: their orbits differ so little that when the inner one catches up with the outer one, they exchange momentum and swap places, a dance that repeats roughly every four years. Voyager 1 helped untangle the pair, and Cassini later imaged Janus in detail.