The more distant moon

By Phil Plait | January 18, 2011 10:36 am

Quick! Which of these two moons was closer to the Cassini spacecraft when it took this image?

Hard to tell, isn’t it? Actually, it’s impossible to tell without knowing what’s what.

The gray moon to the upper right is Dione, and the blindingly white moon to the lower left is Enceladus (famed for its geysers of water erupting from its south pole). When I look at this picture, I can mentally swap the two moons in distance, making one seem farther away, then closer. It’s a bit like the Necker cube illusion.

But here’s a big hint: Dione is Saturn’s third largest moon at 1123 km (698 miles) across. Enceladus is comparatively small at 504 km (313 miles) in diameter. Since Dione is not twice the size of Enceladus in the picture, it must be farther away. In fact, Cassini was 510,000 km (317,000 miles) from Enceladus when it took this picture, but Dione was a more distant 830,000 km (516,000) — more than twice as far as our own Moon is from Earth.

Note that Dione is much duller and dimmer than Enceladus; its surface is rocky while that of Enceladus is highly reflective water ice. That’s why the smaller moon’s image is washed out; the exposure was set for darker objects. Both moons look slightly non-circular, but that’s because the Sun was very slightly off to the side when this picture was taken, so neither moon is quite "full".

Engineers and scientists know precisely where Cassini, Saturn, and its moons are at any given time by first knowing celestial mechanics: the math and physics of orbital motion. But they also get telemetry from Cassini that gives them its position. Physics is all well and good, but nothing beats a measurement in the field. And that’s how they know how distant those moons were when the shot was taken. Without that knowledge, these pictures would still be amazing and important to planetary astronomy, but would hold a lot less scientific value.

As usual, you have to be careful when looking at pictures of objects in space. Your brain is accustomed to all sorts of cues for judging size and distance like sunlight angle, amount of detail seen, the amount of haze in the air… all of which are totally absent in space. You might think you’re not easily fooled, but your brain has other ideas.

Image credit: NASA/JPL/Space Science Institute


Related posts:

Dione and Rhea sitting in a tree
An otherwordly eclipse
Enceladus sprays anew
Enceladus on full afterburner


CATEGORIZED UNDER: Cool stuff, illusion, Pretty pictures
MORE ABOUT: Cassini, Dione, Enceladus, Saturn

Comments (21)

  1. Without knowing a thing about the moons, it would be very easy to tell which is closer…if the photo was in 3D!
    Man, I wish this 2D fad would pass. It’s just not natural.

  2. Chris

    I knew right away. Always pick the not obvious answer to questions like that.

  3. Zucchi

    You can’t see any stars in that picture — is Bart Sibrel going to claim it’s fake?

  4. Totally faked. No stars. ;)

  5. That’s funny, I got it right on first glance.

  6. I was about to argue the point about there not being stars when I realized it was just my dusty monitor. Ugh. Faked picture AND aliens putting dust on my monitor! Conspiracy!!

  7. Unaspammer

    @ Chris,

    Which answer is the obvious one? My brain naturally wants to see Enceladus as closer, due to the visual cue that it is lower in the image. So if I had gone “non-obvious” I would have picked Dione and been wrong!

  8. Mark

    Does anybody have a picture that shows moons at the same scale as Earth’s Moon? This post made me wonder what Dione would look like if it was orbiting Earth.

  9. Sam H

    For me Dione was closer. Speaking of which – due to the shape of the canyon system and colour contrast between it and the rest of the moon, did you ever think it looked a little like a marble from certain angles, such as this one?

    Mark: This should help. Some images are dated, Dactyl would be invisible and Deimos is smaller than Phobos. But aside from that it should be good.

    http://upload.wikimedia.org/wikipedia/commons/1/18/Moons_of_solar_system.jpg

  10. dcsohl

    Dione is 1/3 the diameter of our Moon, so it would be about 10 arc-minutes or 1/6 degree across if it were orbiting at the same distance as the moon.

  11. andy

    Dione is an icy moon, so if it were orbiting Earth it would begin to sublimate. With less than 1.5% of the Moon’s mass it would probably do a pretty poor job of holding onto its atmosphere and become a “super-comet”. I reckon this would be quite impressive to see. Anyone prepared to wait a few billion years for the Sun to become a red giant might be in for quite a show.

  12. Matija

    Just a nitpick: Dione is not the third-largest moon of Saturn, as Titan, Rhea and Iapetus are all larger. Unfortunately, being too familiar these moons ruined the illusion for me, I knew right away that Enceladus must be closer.

  13. andy

    Dione is not the third-largest moon of Saturn, as Titan, Rhea and Iapetus are all larger

    Yes but we now know thanks to the tireless efforts of Richard Hoagland that Iapetus is not a moon but an artificial construct. Therefore BA correctly asserts that Dione is the third largest moon of Saturn.

    :-)

  14. Also a nitpick – I don’t believe Dione’s surface is rocky. It’s not as pure ice as Enceladus’ surface (Enceladus gets the benefit of its own geysers), so it’s darker, but it’s still mostly ice.

    This comment brought to you by the xkcd 386 service*

    *Making your internet less wrong every day.

  15. CB

    When I looked at the picture, my brain told me the brighter object was closer, and thus much smaller than the bigger one.

    And yeah, my brain’s reasoning is false, and the only real way to know is to know what objects those are. I knew which was closer when you said which moons those were, but even then I only know that therefore Enceladus must be closer because it’s so much smaller because of the work of many astronomers et. al.!

  16. Messier Tidy Upper

    You might think you’re not easily fooled, but your brain has other ideas.

    Oh, I *know* my brain (& I) can be easily fooled. ;-)

    I’m all too sadly familiar with being fooled and wrong from past experiences.

    Now I’m at the stage when I always try to keep in the back of mind (even the side or front sometimes! ;-) ) the notion that : “Hey, maybe I’m wrong about this. Or maybe there’s a lot more to it and its not as simple as it appears?”

    Perhaps a big part of growing up is realising that things are usually a lot less certain and a lot more complex than they first appear? Or perhaps I’m wrong about that too! ;-)

    Great image there – guessing or knowing Encleadus is in the picture there is the key.

    Thankyou Cassini team wonderful work as always. :-D

  17. Messier Tidy Upper

    @14. Vagueofgodalming :

    Also a nitpick – I don’t believe Dione’s surface is rocky. It’s not as pure ice as Enceladus’ surface (Enceladus gets the benefit of its own geysers), so it’s darker, but it’s still mostly ice.

    I vote we send a spacecraft – preferably with real live planetary geologists there – to find out directly! ;-)

    @13. andy :

    Yes but we now know thanks to the tireless efforts of Richard Hoagland that Iapetus is not a moon but an artificial construct. Therefore BA correctly asserts that Dione is the third largest moon of Saturn.

    Also Iapetus isn’t a moon – but Japetus is – just ask Arthur C. Clarke in the original Space Odyssey</inovel! ;-)

  18. Messier Tidy Upper

    @8. Mark :

    Does anybody have a picture that shows moons at the same scale as Earth’s Moon? This post made me wonder what Dione would look like if it was orbiting Earth.

    Well, that’s a tough one – maybe someone with better computer graphics skills than me can create one?

    Via a ‘google images’ search I’ve found this one :

    http://www.flickr.com/photos/bluedharma/89186149/

    which has the Saturnian satellites next to Earth (but NOT Moon) to scale.

    Wikipedia has this chart of the moons of the ringed butterscotch planet to scale :

    http://en.wikipedia.org/wiki/File:Moons_of_Saturn_2007.jpg

    But the writing is almost all illegaible (ironically enough – wrong spelling?) even at the 400% zoom on my computer & its very cluttered and, sad to say, not that great.

    But – aha! – try this one :

    http://www.astro-observer.com/solarsystem/compare/comparemoons.html

    which compares many more moons than just those main ones of Saturn & is probably about the closest thing to what you’re after I’ve found yet. :-)

    From there you can see Dione is far smaller than our Moon & as (#11.) andy
    has noted already moving Dione from it is now to where we are would have some pretty dramatic consequences for it! Our solar system will, indeed, look totally different in the future with a far huger and thus far brighter red giant Sun. But I wouldn’t want to see it from the Earth’s to-be-then molten surface that’s for sure! :-)

    PS. D’oh! Just noticed that (#9.) Sam H has answered this as well. Oh well, different links from each of us so I’ll leave it.

  19. JCF

    OMG! I can totally see the image of Jesus Christ (or some guy with Mr. Magoo style glasses) on the surface of Dione! And the image is looking down at Enceladus… This, undoubtedly, is solid evidence that the world will end in 2012!!!

  20. Crux Australis

    Having never heard it spoken before, how does one pronounce “Dione”?

  21. I made this comparison between the Earth, our moon and several moons in the solar system.
    Perhaps this will help.??

    http://www.flickr.com/photos/bluedharma/4236625022/

    A larger size is here…
    http://farm5.static.flickr.com/4050/4236625022_8e44423301_b.jpg

    Bluedharma

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