Watch Saturn's shadow dancing

By Phil Plait | March 23, 2009 11:47 am

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.

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!

Comments (18)

  1. Stunning images and animation, thanks for sharing Phil.

  2. I’m being followed by a moon shadow
    moon shadow-moon shadow
    leaping and hopping on a moon shadow
    moon shadow-moon shadow
    and if I ever lose my hands
    lose my plough, lose my land
    oh, if I ever lose my hands
    oh, if…
    I won’t have to work no more
    and if I ever lose my eyes
    If my colours all run dry
    yes, if I ever lose my eyes
    oh if …
    I won’t have to cry no more.
    yes, I’m being followed by a moon shadow
    moon shadow – moon shadow
    leaping and hopping on a moon shadow
    moon shadow – moon shadow
    and if I ever lose my legs
    I won’t moan and I won’t beg
    of (oh)* if I ever lose my legs
    oh if…
    I won’t have to walk no more
    And if I ever lose my mouth
    all my teeth, north and south
    yes, if I ever lose my mouth
    oh if…
    I won’t have to talk…
    Did it take long to find me
    I ask the faithful light
    Ooh did it take long to find me
    And are you going to stay the night
    I’m being followed by a moon shadow
    moon shadow – moon shadow
    leaping and hopping on a moon shadow
    moon shadow – moon shadow
    moon shadow – moon shadow
    moon shadow – moon shadow

  3. As the ring plane crosses the Sun itself, wouldn’t the pieces of the rings themselves start casting shadows within the rings? Is there anything that could be learned from the shadows that the rings cast upon themselves?

    And how long of a shadow does Titan cast on the rings in this situation?

  4. The Mad LOLScientist, FCD

    Loaded with ringy, moony, shadowy goodness. AWSUMOSITY!

  5. IVAN3MAN

    @ Ken B,

    I don’t think CENTAF will let Larian LeQuella to watch that video.

  6. Damn you, LarianLeQuella!

  7. Yeah, no YouTube for me.

    Glad to oblige Karen! *Evil laughter* (Although truth be told, I have it in my head now too…)

  8. Brian

    Better Cat Stevens than the Bee Gees, which is what Phil’s title put in my head….

  9. IVAN3MAN

    Ken B:

    As the ring plane crosses the Sun itself, wouldn’t the pieces of the rings themselves start casting shadows within the rings? Is there anything that could be learned from the shadows that the rings cast upon themselves?

    Yes, the observations could reveal any deviations across the rings from a perfectly flat wafer-like disk and measure any vertical warping in the main rings.

  10. And how long of a shadow does Titan cast on the rings in this situation?

    It doesn’t. Not this equinox, anyway. Titan is on the wrong side of Saturn to cast a shadow on the rings close enough equinox to do so. Very sad.

    As the ring plane crosses the Sun itself, wouldn’t the pieces of the rings themselves start casting shadows within the rings? Is there anything that could be learned from the shadows that the rings cast upon themselves?

    Well, the particles shadow each other all the time, really. It gets more pronounced as the Sun gets lower on the rings. It also changes the photometry in some (hopefully) interesting ways. At equinox, the rings will be illuminated as much by Saturn’s reflected light as by the Sun (although there will be some Sun still).

    Phil: Thanks for sharing this! I had to bite my tongue last week when you posted the transit images because I know that the release was coming, but didn’t want to ruin the surprise. :-D

  11. Stark

    That is bloody gorgeous. Here I have to write up a presentation on Saturn, and Cassini/Huygens for class. Now I have to find a way to add this stuff! Thanks for spotting it!

  12. Hax Or

    I like how your blog says “DUH”.

    “DUH!!!!!”

    Nice choice of words. Are you 12?

  13. That is bloody gorgeous. Here I have to write up a presentation on Saturn, and Cassini/Huygens for class. Now I have to find a way to add this stuff! Thanks for spotting it!!

  14. JB of Brisbane

    @Brian – this may be a bit OT, but…
    “Shadow Dancin'” was not a Bee Gees song strictly speaking. Written by Barry Gibb, it was sung by younger brother Andy, who despite his Gibb brother status, never got to be a full-fledged Bee Gee before his untimely death.
    I was a bit worried that the Copyright Police (TM Regd.) would be on Larian LeQ’s case, but as Yusuf Islam has disowned his Cat Stevens songs quite publicly, I guess it’s all right.

  15. KBD*

    You know something, there is a such thing as photo-shop now. HaHa. What a crock, and seriously a waste of time.

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