Check out that flat shadow crossing Saturn's rings. It's the shadow of Mimas, one of Saturn's moons, and its appearance means we're getting closer to Saturn's equinox, when it'll be spring in the North and the rings will go dark.
Saturn's equinox only happens about once every fifteen years, and it's when the sun crosses the plane of Saturn's rings. Before that happens, you start to see the shadows of Saturn's moons across the rings. According to Astronomy Now:
During this celestial alignment, the shadows of the planet's rings fall in the equatorial region on the planet, and the shadows of Saturn's moons external to the rings, especially those whose orbits are inclined with respect to the equator, begin to intersect the planet's rings. Any vertical protuberances within the rings, including small embedded moons and narrow vertical warps in the rings will also cast shadows on the rings. At exactly the moment of equinox, the shadows of the rings on the planet will be confined to a thin line around Saturn's equator and the rings themselves will go dark, being illuminated only on their edge.
So you can expect to see some more spectacular vistas from Saturn, thanks to the Cassini Space Probe's Imaging Science Subsystem, headed up by Star Trek science advisor Carolyn Porco. Right now, Cassini's site includes some more images, and some movies of the Mimas' shadow crossing the rings, and fellow moon Enceladus eclipsing Mimas. Check it out. [CICLOPS]