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SYFY WIRE Bad Astronomy

A moon casts a looooong shadow

By Phil Plait

On January 21, 2012, the Cassini spacecraft was about 2.5 million km (1.6 million miles) from Saturn when it took this shot of the planet's clouds:

That long, distorted black smudge is actually the shadow of Mimas, one of Saturn's moons! Mimas was way off to the side when this picture was taken, so the shadow it cast was stretched out due to the curvature of the planet itself. Think of it this way: if Mimas had been directly between Saturn and the Sun, the shadow would be a nice circle right in the middle of the planet's face. But because it was off to the side, the shadow happened to fall where the planet was curving away, so it got elongated.

This has been seen many times before by Cassini as Saturn's fleet of moons dance around the giant planet. Here's one from Titan, and another from tiny Epimetheus casting a shadow on the rings!

In the image I noticed a faint, circular feature above and to the right of the shadow that looked like either a storm or perhaps a camera defect -- sometimes dust in the camera makes circular donut shapes in the pictures, which have to be corrected. So I went to the Cassini raw image database and found the shot -- it's real! It's a storm of some sort, and it just didn't show up well in the first picture (note I had to rotate the image to match the one that was released of Mimas's shadow). What's funny is, you can see one of those small donuts in the picture, off to the left!

Pretty cool. And I love that the raw images are so accessible, so if someone has a question like that, in many cases the answer is just a few clicks away.

Images credit: NASA/JPL-Caltech/Space Science Institute

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