The Sciences

Titan's shadow

Bad AstronomyBy Phil PlaitDec 18, 2009 1:53 PM


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

[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.

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