NASA / JPL-Caltech / SSI / animation by Emily Lakdawalla
Dancing rings, moons, spokes, clumps, and shadows
Everything in the Saturn system is in motion. Over a period of nine hours, Cassini gazed at one of Saturn's ring ansae while the rings and moons spun on their different-speed orbits around Saturn. The first moons visible are Mimas (bigger) and Pandora (small, near the thin rope of the F ring), then Enceladus (similar in size to Mimas) and Epimetheus make an appearance at about the same time, followed by Janus, and then toward the end, Prometheus right on top of the F ring. You can also see spokes rotating with the B ring (the lightest part of the ring system in the middle), and braids and clumps traveling with the F ring (the skinny line at the outer edge of the ring system). Watch closely for a moment about two thirds of the way through the animation, when Mimas reappears at the lower left; you'll see a single frame where its shadow passes across the rings as a long dark line.