'Tis the Season for Spokes
Source- http://saturn.jpl.nasa.gov/video/ 'Dark spokes dance around Saturn's B ring in this series of movies comprised of images taken with Cassini's wide-angle camera. This animation is a concatenation of spoke movies acquired by NASA?s Cassini spacecraft in the second half of 2008 on Aug. 21, Sept. 19, Sept. 26, Oct. 11, and Nov. 25. As Saturn nears equinox in August 2009 and the sun angle on the ring plane decreases, spokes become common sights in Cassini images, just as they were in Voyager images (See Saturn's rings - high resolution). The planet's orbital period is 29.5 years, so Saturn has nearly made one complete trip around the sun since the flybys of the two Voyager spacecraft in 1980 and 1981, allowing Cassini to closely match Voyager's viewing geometry. Each of these five movies shows the sunlit side of the rings at low solar phase, or spacecraft-rings-sun, angles. The spokes appear dark against Saturn's B ring at low phase angles because the particles within them scatter light more efficiently in the forward direction (away from Cassini) than the surrounding larger ring particles. In the opposite viewing geometry, at high phase angles, spokes appear bright relative to surrounding ring particles (See Spokes Sighted Again!). The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo. For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov. The Cassini imaging team homepage is at http://ciclops.org. '