The Sciences

An otherworldly eclipse

Bad AstronomyBy Phil PlaitJan 25, 2010 11:00 PM


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What's this? Two outstanding and surpassingly beautiful Cassini images in one day? Yeah, because this is how much I love you guys. Check. This. Out:

[Click to embiggen.] That is so cool! It's Saturn's moon Dione passing in front of Tethys as their mutual motion, combined with Cassini's, give us this incredible view of what astronomers call an occultation (also called a "mutual event"... but you can just think of it as an eclipse). There's so much awesomeness in this picture. For one, see the faint glow on the "dark" side of Tethys? That's reflected light from Saturn, which is well off to the right in this picture. But we don't see any reflected light on Dione. Why not? Even though the two moons look about the same distance away, that's very misleading: in this image, Tethys is 2.6 million kilometers (1.6 million miles) from Cassini, while Dione is 400,000 km (240,000 miles) closer. That's enough to significantly change the geometry of the situation, so that we don't see Saturnlight on Dione. Note that while both moons are roughly the same size (Tethys is 1062 km (660 miles) across, and Dione is 1123 km (700 miles) in diameter), Dione looks bigger because it was closer to Cassini when these shots were snapped. If they had been at the same distance from Cassini, you'd just barely be able to tell Dione is bigger. Sometimes distance matters, not size. And sometimes size matters too. It's hard to miss the vast Odysseus crater on Tethys, a terrifying 400 km (240 miles) across. That's the size of Ohio. An impact that large on Earth would pretty much wipe out every living thing on the planet's surface. For another little dose of coolness, these images were taken about one minute apart, covering just two minutes of Cassini's tour of Saturn and its armada of moons. Imagine what you'd see if you could be there, staying at a hotel orbiting Titan? What wonders would befall your eyes if you had years to explore? Hmmm. Come to think of it, we don't have to wonder about it. Cassini is showing us.Credit: NASA/JPL/Space Science Institute

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