Cassini Division

Cassini Division is the area between Saturn's A and B ring, which contains very little matter and looks like a dark gap in the ring. Width of Cassini Division is 4800 km. Cassini gap is most easily seen in parts of the ring that are farthest from Saturn's disk. Cassini Division looks like a thin black line that divides the brightest part of a ring on the inner B-ring and the outer A-ring. The outer A-ring is noticeably darker than the B-ring and the inner part of the B-ring is darker than the outer part of the same.
Jean-Dominique Cassini discovered four of Saturn moons and a dark line along the rings of Saturn, called Cassini Division(1675).
It is now known that the gravitational influence of Saturn’s moon Mimas is responsible for the Cassini Division, which is 4,800 kilometers (3,000 miles) wide. The Cassini division in between Saturn’s A and B rings is a gap caused by a 2:1 resonance with the moon Mimas.

Inner edge of Cassini Division is the place where the ring particles have orbital time of less than half of Mimas orbital time. For each orbit of Mimas those particles made two tour around Saturn, and we say, that they are in resonance with that moon. Whenever a body in the ring goes under the Mimas, the moon slightly flushes by gravity   upward. In the case of debris in Cassini Division, it comes to pulling in every other tour around Saturn, always in the same place on the course and always in the same direction. These small shifts are moved after a long time most of the rocks of crack on its outer edge and there are no longer mentioned resonances. In the area where debris make three touring around the planet for one of Mimas, a similar mechanism is formed and less conspicuous border between ring B and ring C.
Nearly two hundred years after Cassini's discovery, the German astronomer Johann Encke was in 1837th on the outer edge of the ring A noticed much less pronounced gap that has received its name(Encke Gap).