Above: the Keeler Gap, with S/2005 S1. Note the waves on the ring boundaries, above left and below right of the moon. The inner particles (above the moon) are orbiting Saturn faster, so the disturbance moves ahead of the moon; while the outer particles (below the moon) are orbiting slower, so the moon moves ahead of the disturbance.
Below: a closer view of the moon, and the waves created by its gravitational effect on the ring particles. The moon is about 4 miles in diameter, and reflects about 50% of the light falling on it -- an amount comparable to the nearby ring particles.