Captain's Log March 23, 2009 The Dance of the Moon Shadows We are almost there. Saturn and we, its companions, have journeyed together now for nearly five years, in a circumnavigation of the outer solar system. Like Earth, Saturn is tilted with respect to its orbital motion and hence blessed with changing seasons, as the Sun's position, seen from Saturn, varies from north to south and back again over the course of a Saturnian year. In four and a half months' time, we will arrive at equinox, one of two special places in Saturn's orbit when the Sun's position in the sky at noon is exactly aligned with the plane that coincides with the equator and its rings. At that time, in mid-August, the Sun will transit from the southern to the northern hemisphere. It will be northern vernal equinox on Saturn ... the start of northern spring. Since our arrival in mid-2004, we have watched the shadows of the planet's rings and moons slowly slip southward across the face of the planet as the Sun has steadily marched northward. In January, in a sure sign that the equinox season had begun, the shadows of Saturn's moons slipped off the planet and onto its rings. In anticipation of this event, sequences of images were designed and executed to capture the motions of these dancing shadows ... a remarkable sight that is not even possible here in the inner solar system where the terrestrial planets are unadorned with rings. From now until August, big visual changes will come to this place. The shadows of the rings on the planet will slip even farther south until they collapse into a line encircling the equator, and the rings themselves will continue to darken as the rays of the sun progressively lengthen. Finally, on August 11, the sun will be illuminating the rings edge-on. Large imaging mosaics of the entire ring system are planned for this very special occasion. Here on Earth, we welcomed the arrival of yet another northern vernal equinox only three days ago. Now, like the Druids of old, in reverence and celebration for the motions of the Sun, we prepare to greet the arrival of spring to the northern reaches of Saturn. It will be a time for revelry and wonder ... and the only Saturnian spring we Cassinians will ever know. Carolyn Porco Cassini Imaging Team Leader CICLOPS Space Science Institute Boulder, CO

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