The Cassini spacecraft has captured its first snapshots of moon shadows on Saturn's rings. The shadows are a sign that the Sun will soon cross Saturn's equator, bringing spring to the planet's northern hemisphere for the first time in almost 30 years.
Like Earth, Saturn experiences seasons because its equator and rings do not lie in the same plane as its orbit – they are tilted by some 27°. Twice during the planet's 29.5-year orbit, the Sun crosses its equator, illuminating the planet's rings edge-on.
The next such equinox will be on 11 August 2009. But NASA's Cassini spacecraft caught one of the first signs of the coming alignment in January, when it snapped images of moon shadows on the planet's rings. Shadows of Saturn's moons are typically seen on the planet itself.
Cassini's mission was extended to 30 September 2010 in order to watch the equinox, which occurs once every 15 years. The team hopes the change in light will offer new insight into the planet's weather as well as reveal more about Saturn's rings.
"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, Colorado.
