I’ve seen the tiny black shadows cast by Jupiter’s moons. Do Saturn’s moons cast observable shadows on that planet?

Cassini was 603,000 kilometers from Dione when it captured the moon transiting in front of Saturn on December 14, 2004.

Shadow transits of Titan, Saturn’s largest moon, have been seen with a telescope as small as 2 7/8-inch (73-millimeter) aperture. In 1891, two English observers saw the shadow of Rhea on Saturn with 6 1/2-inch instruments. They also claimed to have seen the shadow transits of several other moons, but experts are skeptical about those.

The catch is that Saturn’s bright inner moons follow orbits that are inclined like the rings, so their shadows usually miss Saturn’s disk. But transits can occur in a four-year interval centered on a date when the rings are edge-on to our view (about every 15 years). The rings will next be edgewise in September 2009, so the shadow transits of Saturn’s inner moons will start up in 2007.

— Roger W. Sinnott


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