Cassini is now well into the “Cassini Equinox Mission”, a two-year mission following the completion of its successful four-year prime mission last summer. The scientific theme driving the equinox mission is to observe seasonal changes throughout the system as Saturn has its vernal equinox (the moment when the Sun crosses Saturn’s equator, moving from south to north). The equinox occurs shortly after midnight (Universal Time) on August 11, 2009 (the evening of August 10 in American time zones). The actual moment of equinox is not critically important because of the finite size of the disk of the Sun as seen from Saturn and the relatively slow pace of Saturn’s orbit around the Sun (29.66 years for a Saturnian year). This means there is an equinox season of weeks to months during which the rings are nearly edge-on to the Sun. One aspect of this of interest to ring scientists is that any deviations out of the ring plane will result in long shadows on the rings. By measuring the shadows we can get very accurate measurements of these small vertical perturbations which are otherwise hard to see. An early look at this sort of phenomenon is visible in the picture below in which the shadow of one of Saturn’s moons is clearly visible on the ring. The extent of the moon shadow, combined with our already precise knowledge of the relative positions of the Sun, Saturn, and the moon, will enable imaging team scientists to better constrain the inclination of the orbit of the moon. Soon, small ripples in the rings will cast shadows and, in a sense, become visible for the first time.
