Titan's shadow

By Phil Plait | December 18, 2009 8:53 am

Check this out! Cassini took a gorgeous shot of Titan casting its shadow on Saturn:

titan_shadow

[Click to embiggen]

Wow!

A couple of cool things to note in this image… one is that the shadow is fuzzy. That’s because Titan has a thick atmosphere, so there is no sharp edge to the moon to cast a sharp shadow.

[Update: A few folks have pointed out that I didn't consider the idea that the fuzziness may be due to the fact that the Sun is not a point source, and so it will cast a fuzzy shadow -- this same thing happens during solar and lunar eclipses on Earth. I did some quick trigonometry, and I get that the fuzzy outer part of the eclipse shadow (called the penumbra) should be about 1000 km across or so, while the deep shadow (the umbra) is a little bit bigger than the size of Titan itself, or about 5200 km (again, these are pretty rough numbers). That jibes well with what we see in this image (if you neglect the weird distortion of Titan's shadow being stretched since it's projected on Saturn's curved cloud tops), so I'm now thinking that the shadow is not fuzzy due to Titan's atmosphere at all -- it's just the penumbra of this Titanic solar eclipse! I stand corrected, and I thank my readers for pointing this out. Very cool.]

Another is that even though Titan’s orbit is almost exactly in the same plane as the rings, its shadow is really far south of the rings’ shadow. That’s because spring has started in Saturn’s northern hemisphere, so the Sun is shining down on the planet from just north of the equator. The rings are relatively close to the planet surface, so the shadow they cast is just south of the equator (and narrow since the Sun is still shining nearly straight down the ring plane).

Titan orbits Saturn much farther out (1.2 million km from Saturn, very roughly 10 times farther out than the main rings on the average). Over that distance, the angle of the moon’s shadow carries it farther south of the equator. It’s a like a tall tree’s shadow being longer than a short tree’s shadow.

I’ve said it before, but it bears repeating: we will get a vast amount of science out of Cassini data, and learn more about Saturn in a few years than we have in all the centuries of observations before we launched the probe. But sometimes it’s just the pure beauty of the images that gets to me.

Tip o’ the tweet to Carolyn Porco.

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, eclipse, Saturn, Titan

Comments (27)

Links to this Post

  1. La sombra de Titán | December 18, 2009
  2. Carnival of Space #134 « Cumbrian Sky | December 23, 2009
  1. I miss the old homestead. *sniffs*

  2. Has Cassini ever taken a non-gorgeous shot? I doubt it!

  3. Although science isn’t necessarily beautiful, and beauty definitely isn’t always science, I love when the two come together. I always hope that sort of concordance will inspire people to take more interest in science. Thanks for all the great pictures!

  4. Larian, I have to agree!! That shot of the Starship Enterprise rising out of Titans atmosphere was simply amazing! ;)

  5. Flying sardines

    BA? I think you really should have waited the extra week or two for the year to finish before posting your “Top 10 pictures for 2009″ so you could include this one! See what happens when you’re too impatient!? ;-)

    BTW. If I recall, you did say something about perhaps posting a *second* “best pictures of 2009″ yes? Please, do that BA, please! I’m sure I speak for many here saying we’d love that! :-)

    Also is it just my eyes or does Titan’s shadow appear elongated and a bit asymmetrical as well as just fuzzy there?

  6. Mike

    Awesome Pic, It brings up a lot of interesting questions. This is a full solar eclipse due to Titan. How often does that happen? The sun would be a lot smaller in the sky, and Titan would be smaller than our moon (I think, its 1.5 times as big, but 3.5 times as far away. I’m to tired to do the math) But the surface of Saturn is much larger. I would assume it would only be possible in early/late spring or fall, the shadow would fall above or below Saturn otherwise. The orbit of Saturn is 30 years, so maybe once every 15 years? Pretty amazing that we just happen to have a satellite there….

    Thats why I love Science, Every mystery solved yeilds another.

  7. Dj Storm

    @Flying sardines Says:
    “Also is it just my eyes or does Titan’s shadow appear elongated and a bit asymmetrical as well as just fuzzy there?”

    That’s (probably) because Titan’s shadow doesn’t “hit” Saturn head-on, but somewhat off-center.

    When I saw the image I thought that the fuzziness is Titan’s penumbra; if my calculations are correct, Titan’s shadow on Saturn should have a diameter of ~3800 km, and its penumbra a diameter of ~6200 km. I don’t know why the penumbra isn’t more visible, maybe the image was enhanced?

  8. George

    Is the reason for the fuzzy outline of the shadow necessarily due to the atmosphere and not also because the sun is a distributed source (not a point source)? I don’t know what its apparent size is out by Saturn, so maybe it’s far enough away to act like a point source, but you certainly won’t get sharp shadows from objects blocking the sun on earth…

  9. Yes George, I think you are right.
    The diameter of the Sun is about one fifth of that of Titan seen from Saturn’s cloud tops during a typical occultation. So the Sun is far from a point source and the shadow will have a sizable penumbra no matter the atmosphere.

  10. Cassini’s RTGs contain aggregate 72.3 pounds of plutonium-238, 0. 61 megacuries. It represents an unconscionable exterminative hazard to undiscovered fragile and endangered Saturnian cryogenic lifeforms both for the radiation and for its 16 kilowatts of heat emission. Cassini must be immediately returned to Earth and an environmental impact statement filed before it is allowed to return to Saturn. Saturnian system global warming cannot be tolerated!

  11. More on the penumbra:

    http://gallery.me.com/julesstoop/100046/saturnoccult/web.jpg

    This simulation was made with Celestia (which doesn’t take absorption and scattering by atmospheres into account when rendering shadows).
    http://shatters.net/celestia

  12. Nephew Jeff

    Branching out from trolling sci.chem and sci.physics, are we, Uncle Al?

  13. George

    Wow, Jules. That’s quite the simulation. …pretty much reproduces the photo (at least, to the extent that I can analyze the image by eye)

  14. Seabear70

    That’s no moon….

    It’s a space station…

  15. MAC

    I’m no astronomer, but it seems to me that if Titan’s shadow is fuzzy because of its penumbra, the rings’ shadows should be similarly out of focus. Or am I missing something obvious?

  16. Thanks George,

    But it’s actually pretty bad (shame on me). As you can see from the cloud bands in Saturn’s atmosphere not being (almost) parallel in my picture (as they are in the original), my virtual viewpoint was much closer to the planet compared to cassini’s original shot (I was actually mistaken by a factor of 10!). Here is a better approximation:

    http://gallery.me.com/julesstoop/100046/saturnoccult2/web.jpg

    Upon close examination of this one you’ll notice that the orbital data of the saturn-titan system are a bit off in Celestia.

  17. @MAC:

    The rings are much closer to the planet than Titan, so their angular size – as seen from the top of saturn’s atmosphere – relative to the angular size of the Sun, is a lot larger. So the penumbra is smaller by the same factor.

    If you’re not convinced, try it on earth:
    Wait for a sunny day and take a golf ball to make some shadows from the sunlight on a piece of paper. The bigger the distance between the ball and the paper, the larger the relative and absolute size of the penumbra.

    Once you are a few meters away, the shadow will all but disappear. Why? The angular size of the sun, as you would see it from the paper’s point of view, is now larger than the angular size of the golf ball. This sometimes happens with solar eclipses on earth as well (when the moon is relatively far from earth and the sun is relatively close). It’s called annular eclipse.

  18. MAC: Titan is, as Phil pointed out, 10 times as far away from Saturn as the rings, so the effect of the penumbra is much greater.

  19. Torbjörn Larsson, OM

    Branching out from trolling sci.chem and sci.physics, are we, Uncle Al?

    That is how life propagates among them tin foil hats. It gets moldy – and hyphaenated – under there…

  20. Flying sardines

    @9. Dj Storm Says:

    @Flying sardines Says: “Also is it just my eyes or does Titan’s shadow appear elongated and a bit asymmetrical as well as just fuzzy there?”

    That’s (probably) because Titan’s shadow doesn’t “hit” Saturn head-on, but somewhat off-center.

    When I saw the image I thought that the fuzziness is Titan’s penumbra; if my calculations are correct, Titan’s shadow on Saturn should have a diameter of ~3800 km, and its penumbra a diameter of ~6200 km. I don’t know why the penumbra isn’t more visible, maybe the image was enhanced?

    Thanks – that makes sense. :-)

    @ 16. Seabear70 Says:

    That’s no moon…. It’s a space station…

    No, wait it is a moon after all. ;-)

  21. Messier TidyUpper

    One idea for an awesome image combination would be combining this with a few lines out to an inset close up of Titan in colour showing the glint of Kraken Sea (mare.) You know the one the BA posted before here :

    http://blogs.discovermagazine.com/badastronomy/2009/12/17/a-titanic-wink-confirms-otherwordly-lakes/

    (Actually either the Titan photographic map or the crescent glint could work there.)

    A colour version of this image would be neat too. :-)

    Thanks Cassini & BA for getting & sharing this.

  22. Flying sardines

    @ 12. Uncle Al Says:

    Cassini’s RTGs contain aggregate 72.3 pounds of plutonium-238, 0. 61 megacuries. It represents an unconscionable exterminative hazard to undiscovered fragile and endangered Saturnian cryogenic lifeforms both for the radiation and for its 16 kilowatts of heat emission. Cassini must be immediately returned to Earth and an environmental impact statement filed before it is allowed to return to Saturn. Saturnian system global warming cannot be tolerated!

    Ok Uncle Al, you’re now appointed to go bring Cassini back – find yourself a rocket & go fetch! ;-) :-P

    I think fictional Saturnian lifeforms have nothing to worry about as Cassini won’t be ending up on Titan when its mission ends – hopefully in many years time.

    What would actually be seriously cool in my view would be if instead of sending Cassini into Saturn when this day comes, if they landed it on one of Saturn’s smaller moons instead – one of the asteroidal-size small icy rocks like Hyperion, Phoebe or Epimethus a la NEAR-Shoemaker landing on asteroid Eros at the end of its mission. Just a thought. :-)

  23. Plutonium being from Pluto

    @ 16. Seabear70 Says:

    That’s no moon….
    It’s a space station…

    Nice one – but it’d work a lot better if the shadow casting moon was Mimas instead!

    Check out : http://en.wikipedia.org/wiki/Mimas_(moon) to see what I mean.

    A Xmas carol related take on this would be the following “sabotage” of that traditional “Star” – don’t know if that’s its official title – Xmas carol :

    (Nb. If you happen to love this carol & don’t want it spoiled then you may want to skip this next bit & go onto the next post. OTOH, if you’ve had to listen to it on constant loop & found it drives you crazy then you may enjoy this sabotage of its lyrics! ;-) )

    *
    *
    *
    *
    *
    *
    *
    *
    *
    *
    *
    O star of wonder! Star of might!
    Star that shoots a laser bright!
    Its the Death Star come to get ya!
    Vader rules this Xmas night! ;-)

    (or alternate last line : Your planet will be killed tonight!)

    ****
    *
    *
    *
    *
    *
    *
    *
    *
    *
    *
    *

    Safe to look again now … ;-)

  24. JohnDoe

    [i]the deep shadow (the umbra) is a little bit bigger than the size of Titan itself, or about 5200 km[/i]

    Since the sun is larger than Titan, should the shodow not be smaller? My calculation would have the radius of the umbra at Saturn’s aphelion at about 560km (and ~625km at perihelion) less than Titan’s radius. The difference in radius between the core shadow and Titan should be:

    (Sun_radius-Titan_radius)/(Distance_Sun_Saturn)*Distance_Titan_Saturn.

    That would be between 3900 and 4030km The penumbra should be slightly more than 2x this, since the first part of the Equation should be (Sun_radius+Titan_radius). If you assume that the clouds are completely opaque, Titan_radius would have to be increased accordingly (less than 2%).

    Another possible reason for the fuzziness would be the that Saturn’s cloud layers. I have no idea how translucent the top layers are.

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