A moon casts a looooong shadow

By Phil Plait | April 10, 2012 12:00 pm

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

Related Posts:

Saturn’s rings cast long shadows
Side view of a Death Star moon
Titan’s shadow
Watch Saturn’s shadow dancing

MORE ABOUT: Cassini, Mimas, shadow

Comments (6)

  1. Paulino

    Doesn’t it (the storm) look like the marks left by Shoemaker-Levy 9 on Jupiter?

  2. Joseph G.

    Longmoon is loooooooong.

  3. Muz

    That donut is bizarre! what a strange and wonderful universe.

  4. D

    Sorry but thats a poor description of why that shadow appears long – it has to do with the angle of the surface of Saturn relative to the direction of the sun, not the curvature of saturn. It’s no different than a shadow of a tree on earth being long in the evening as the sun sets.

  5. Shadow of the “Death Star moon”* – awesome! 8)

    Reminds me of an online cartoon I’ve just seen via a friend on facebook (can’t recall how or know if we still can embed images here alas) showing Cat Stevens in therapy. He’s telling his pyschiatrist that he’s being followed by a moon shadow, moon shadow! ūüėČ

    * For those who don’t know (very few? Anybody?), click on my name for an ‘Astronomy Picture of the Day’ image illustrating the resemblence. :-)

  6. The best thing to do with raw images is to zoom in on them way, way beyond the limits of their actual resolution, and convince yourself that you see space shuttles and hotel buildings.

    It worked for Piper, and it can work for you!


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