In a sure sign that Saturn is approaching that special time - called equinox - during its travels around the sun, images taken by the cameras of NASA's Cassini spacecraft have captured, for the first time, the shadows of the planet's moons cast onto its broad expanse of rings.

Like Earth and most of the other planets, Saturn's spin axis is tilted relative to its motion around the sun. This condition results in the cyclical passage of the sun, seen from Saturn, from the southern hemisphere to the north and back again, and the full sweep of seasonal changes on Saturn and its rings and moons, over the course of Saturn's year, equal to 29.5 Earth years.

Thus, about every 15 Earth years, or half-Saturn-year, the sun passes through the plane containing the planet's rings.

During these times the shadows of the planet's rings fall in the equatorial region on the planet, and the shadows of Saturn's moons external to the rings, especially those whose orbits are inclined with respect to the equator, begin to intersect the planet's rings.

When this occurs, the equinox period has essentially begun, and any vertical protuberances within the rings, including small embedded moons and narrow vertical warps in the rings, will also cast shadows on the rings. At exactly the moment of equinox, the shadows of the rings on the planet will be confined to a thin line around Saturn's equator and the rings themselves will go dark, being illuminated only on their edge.

The next equinox on Saturn, when the sun will pass from south to north, is Aug. 11, 2009.

Because of these unique illumination circumstances, Cassini imaging scientists have been eager to observe the planet and its rings around the time of equinox, and Cassini's first extended mission, which began on July 1, 2008, and extends to Sept. 30, 2010, was intended to gather observations during this time. Hence, its name: Cassini Equinox Misson.

The onset of this novel geometry begins with the appearance of the shadows of Saturn's moons on the icy platform of its rings.

Cassini imaging scientists first predicted when and where the moons' shadows would fall on the rings and then planned special imaging sequences to target those locations.

And those predictions paid off. Images and a movie of moon shadows skating across the rings are being released to celebrate these sights.

More than just pretty pictures, these observations and others to come could provide valuable information regarding the presence of any deviations across the rings from a perfectly flat wafer-like disk. Working outward from the planet, the main rings are named C, B, and A. Saturn's ring system is wide, spanning hundreds of thousands of miles or kilometers.

But the main rings are perhaps only 10 meters (30 feet) thick, and they lie inside the F ring which is vertically thicker than the A, B and C rings, making the determination of interior vertical deviations difficult when imaging the rings edge-on.

"We hope that such images will help us measure any vertical warping in the A and B rings," said John Weiss, an imaging team associate who planned the observations at the Cassini Imaging Central Laboratory for Operations (CICLOPS) within the Space Science Institute in Boulder, Colo.

"Because we know how big the moons are, and where they are in their orbits around Saturn when they cast these shadows, we have all the information we need to infer any substantial vertical structure that might be present."

On Jan. 8, Epimetheus, a small moon 113 kilometers (70 miles) across, was the first moon observed casting a shadow onto the outer edge of the A ring. Next Pan, 30 kilometers (20 miles) across and orbiting within the rings, was caught casting a shadow on the A ring on Feb. 12.

Eventually, more moons will cast shadows on the rings and all shadows will grow longer as exact equinox approaches. The shadows will be their longest just before and just after equinox when the sun exactly crosses the ring plane on Aug. 11, 2009.

"One of the best things about being in orbit around Saturn are those mind-expanding opportunities that arise every now and again to see some celestial phenomenon you couldn't possibly see here on Earth," said Carolyn Porco, leader of the Cassini imaging team in Boulder, Colo.

"It's at those times you feel a real sense of privilege to be alive ... now... to witness such remarkable sights. And from the looks of it, the next year is going to be one remarkable sight after another."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA's Science Mission Directorate, Washington.

The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team consists of scientists from the U.S., England, France, and Germany. The imaging operations center and team leader (Dr. C. Porco) are based at the Space Science Institute in Boulder, Colo.