Prometheus and Pandora, on the right, are shepherd moons whose gravity keeps the edge of Saturn's F-ring sharp. Janus is the third moon pictured in this Cassini space probe image. Credit: NASA/JPL/Space Science Institute).
A shepherd moon is a moon that constrains the extent of a planetary ring through its gravitational influence. Examples include Saturns Prometheus and Pandora, which shepherd the narrow outer F ring. Peter Goldreich and Scott Tremaine first proposed the idea of shepherd moons in 1979 to explain why the rings of Uranus were so narrow.