An otherworldly eclipse

By Phil Plait | January 25, 2010 12:00 pm

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

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Dione, Tethys

Comments (18)

  1. Astronomynut

    Phil, I think you may have mistyped. Odysseus crater is on Tethys, not Dione.

  2. hjb

    “It’s hard to miss the vast Odysseus crater on Dione.” Since this crater is actually on Tethys, I cannot concur with your assertion. :-)

  3. Sorry, that looks just too spectacular. I’m sure it was “manufactured” by the same special effects shop that did the Moon Landings. 😉

    Yeah, that is absofrikkinloutly beautiful! Love it!

  4. Dagnappit, and after I went through the post twice to make sure I had the two moons straight. :) OK, thanks, I fixed it.

  5. Flavio

    “I want to go to there”! :)

  6. As a native Ohioan (now a New York City transplant lost in a jungle of concrete, glass, booze, dejected brokers, etc.), I heart your Tethys comparison.

    Yet, I must play the devil’s advocate *cue creepy devil’s advocate music*:
    Does an impact crater of that size on a moon with essentially no atmosphere and presumably less dense surface material necessarily translate to an impact able to wipe out most life on this here awesome planet?

    Yes, I dare ask ze Skeptic of all Skeptics :)

  7. carlos

    Dione in this pcture is the anti-Tethys. :-)

  8. Stan9FOS

    Said it before, no doubt will say it again: Saturn – it’s the ultimate vacation destination. Phil has the right idea – park Kubrick’s orbital Hilton somewhere in the neighborhood, with nice day trips to assorted moons & the ring plane, and I’m there!

  9. TrippinBalls

    ‘Dagnappit’? Really, who says that nowadays?

    Cool pics.

  10. DeepField

    “…But we don’t see any reflected light on Dione. Why not?”

    There may be another factor: From this pictures’ standpoint, we can see light on Tethys that is reflected back from *Dione*, but the corresponding light that goes from Tethys to Dione is on its other side…

  11. NelC

    Didn’t I see this scene in 2001: A Space Odyssey? Can anyone see a black monolith and an abandoned long-range, manned space probe?

  12. Asimov Fan

    @ 13 NelC : Sorry, wrong moon. (hmm .. a lot of that on this thread.)

    The Monolith in 2001 : A Space Odyssey was located on “Japetus” or Iapetus as it is now spelt. At least it was in the Arthur C. Clarke novel – in the Kubrick movie, the Monolith was just floating around near Jupiter instead. 😉

    Awesome photo, thanks BA. :-)

  13. Petrolonfire

    11. TrippinBalls Says:

    ‘Dagnappit’? Really, who says that nowadays?

    Everyone here because we’re restricted by the BA’s prudish Ned Flanders-esque no swearing policy that’s who. I suppose it has to be to keep it safe for work & schools but still, it is pretty dang-diddly-danged silly. :-(

  14. DataJack

    Cassini is, apart from the Mars Rovers, the coolest thing in the solar system.


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