A shepherd moon (also herder moon or watcher moon) is a small natural satellite that clears a gap in planetary-ring material or keeps particles within a ring contained. The name is a result of the fact they limit the “herd” of the ring particles as a shepherd.

How is a shepherd moon different than a regular moon?

Shepherd satellites are small moons that orbit within, or just beyond, a planet’s ring system. They have the effect of sculpting the rings: giving them sharp edges, and creating gaps between them.

How many shepherd moons are there?

Portion of Uranus’s ring system with the bright Epsilon ring flanked by its two shepherd moons, Cordelia and Ophelia, in an image obtained by Voyager 2 on Jan.