It’s Saturnian Groundhog Day!

Springtime is eminent in Saturn. Saturn will reach its vernal equinox in August 2009 and the signal for this is the casting of the moon’s shadow in the brilliant platform of Saturn’s ring. This is the only time when the moons cast their shadows on the ring rather than on the planet.

The first shadow-casting caught by Cassini is the 130 km Epimetheus. In the coming months, other moons will join Epimetheus and dotting the rings with their own shadows.

The next vernal equinox will be the next 29 years.

   (left) the shadow of Epimetheus on A ring, separated at about less than 20,000 km; (right) the second image of the Saturn’s-equinox-shadow-casting is the shadow of Pan, a small moon that orbits the Encke Gap of Saturn’s A ring. NASA/JPL/Space Science Institute