Watch Saturn's shadow dancing

Bad Astronomy
The entire universe in blog form
March 23 2009 11:47 AM

Watch Saturn's shadow dancing

As I mentioned in a recent blog post, Saturn is currently presenting itself to us with its rings and moon orbits nearly edge-on. I knew this would mean we'd see transits of the moons: from our view, the moons seem to pass directly over the face of Saturn.

Cassini animation of the moon Epimetheus’s shadow on the rings
What I didn't think of is this also means the moons will cast shadows on the rings themselves! This is starting to happen now, and Cassini, our robot-on-the-spot, is now sending back spectacular pictures (like it ever sends back any other kind, duh) of these events! The animation you see here (click to embiggen) shows the tiny moon Epimetheus -- only 113 km (70 miles) across -- casting its own shadow on the rings. While it was still a million kilometers from the tiny world, Cassini took a series of images that the ground team strung together into this beautiful and somewhat eerie animation. The shadow moves across the rings because Epimetheus's orbit isn't precisely aligned with the rings, it's tilted by less than a degree, but that's enough to send its stretched-out shadow drifting across the rings like a ghost as the moon bobs above the ring plane.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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If you look at this still frame from the animation, you can see incredible detail in the rings, and even that the shadow is not quite symmetric; probably a reflection (so to speak) of Epimetheus' irregular shape.Cassini picture of the moon Epimetheus’s shadow on the rings

Some of the other moons are creating these dances as well; here is the even smaller flying-saucer-shaped Pan (just 20 km (12 miles) across) as it orbits in a gap in the rings, casting its own shadow across the rings:

Cassini picture of the moon Pan’s shadow on the rings

Can you see the moon Pan in the gap in the rings and its shadow? It's tiny in that version, so take a look at this zoom:

Cassini zoomed picture of the moon Pan’s shadow on the rings

Whoa, cool. Pan is actually orbiting in that ring gap, so the rings have to be almost perfectly edge-on to the Sun to get that shadow. Right now that's not quite the case; there's still a bit of a tilt. But as Saturn orbits the Sun that angle will diminish, and in a few months (in August) it'll be precisely 0. Then we'll see the shadows stretching out along the rings, lengthened the same way your own shadow is elongated at sunset. As we approach this point in time -- what's really the Equinox on Saturn, the same as the Equinox we just had on Earth -- well see this more and more, so expect a ton more devastating animations and images from Cassini in the months to come!

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