Sunlight passing through the Cassini Division between Saturn's A and B rings sweeps across and illuminates the surface of the moon Janus in this movie captured shortly after Saturn's August 2009 equinox.
The novel illumination geometry that accompanied equinox lowered the sun's angle to the ring plane, significantly darkened the rings, caused out-of-plane structures to look anomalously bright and threw shadows across the rings. As this movie shows, the equinox period also allowed the rings to cast shadows on the moons. These scenes were possible only during the few months before and after Saturn's equinox, which occurs only once in about 15 Earth years. Before and after equinox, cameras on NASA's Cassini spacecraft spotted not only the predictable shadows of some of Saturn's moons (see Across Resplendent Rings), but also the shadows of newly revealed vertical structures in the rings themselves (see A Small Find Near Equinox).
This view looks toward the trailing hemisphere of Janus (179 kilometers, 111 miles across). North on Janus is up and rotated 8 degrees to the right.
The movie is a concatenation of 12 still images. The images were obtained in visible light with Cassini's narrow-angle camera on Aug. 27, 2009. The view was acquired at a distance of approximately 268,000 kilometers (167,000 miles) from Janus and at a Sun-Janus-spacecraft, or phase, angle of 25 degrees. Image scale is 2 kilometers (5,271 feet) per pixel.
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.
Credit: NASA/JPL/Space Science Institute