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Thursday, January 6, 2011

The magic of illumination geometry: Saturn's rings and shadows

The magic of illumination geometry: Saturn's rings and shadows
Click on the image to enlarge

The shadows of Saturn's rings cast onto the planet appear as a thin band at the equator in this image taken as the planet approached its August 2009 equinox. The novel illumination geometry that accompanies equinox lowers the sun's angle to the ringplane, significantly darkens the rings, and causes out-of-plane structures to look anomalously bright and to cast shadows across the rings. These scenes are 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, Cassini's cameras have spotted not only the predictable shadows of some of Saturn's moons, but also the shadows of newly revealed vertical structures in the rings themselves. For an earlier view of the rings' wide shadows draped high on the northern hemisphere. The planet's southern hemisphere can be seen through the transparent D ring in the lower right of the image. The rings have been brightened by a factor of 9.5 relative to the planet to enhance visibility. This view looks toward the northern, unilluminated side of the rings from about 30 degrees above the ringplane. Images taken using red, green and blue spectral filters were combined to create this natural color view. The images were obtained with the Cassini spacecraft wide-angle camera on July 18, 2009 at a distance of approximately 2.1 million kilometers (1.3 million miles) from Saturn. Image scale is 122 kilometers (76 miles) per pixel.
Credit: NASA/JPL/Space Science Institute

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