Above: the Keeler Gap, with S/2005 S1 (named Daphnis in 2006). 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. (In these images east is to the right, so the motion of the moon and the ring particles is to the right, and "waves" to the right of the moon are ahead of it because of their faster orbital velocity, while "waves" to the left of the moon are behind it because of their slower orbital velocity.)(Both images by Cassini Imaging Team, SSI, JPL, ESA, NASA, apod050523)
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.