Moon shadow on Saturn's rings
Epimetheus (not visible), a tiny moon of Saturn, casts its shadow [streak above center] across the giant planet's rings. The Cassini spacecraft captured this image in January. Twice during every 29.5-year orbit of Saturn around the sun, the plane of the gas giant's rings aligns with the sun in a Saturnian equinox—the next such event will take place in August. At equinox the rings appear dark, as they are only illuminated on the outer edge. But as the rings ease into alignment with our star, Saturn's moons cast their shadows across the rings, growing longer as equinox approaches. Epimetheus is an oddball moon, about 90 miles (145 kilometers) across in its longest dimension, in that it exchanges orbits with a larger moon, Janus, every four Earth years or so. Currently Janus orbits 30 miles (50 kilometers) closer to Saturn than Epimetheus does, but in 2010 gravitational effects between the moons will draw Epimetheus inward and push Janus outward, effectively swapping their places relative to their planet.