Perspective on four moons

By Phil Plait | September 8, 2010 7:00 am

The Cassini probe orbiting Saturn returned an interesting picture yesterday. It shows four tiny moons and the rings seen nearly edge-on. Take a look:

cassini_4moons_rings

[Click to enjovianate.]

From left to right the moons are Epimetheus (113 km/70 miles across), Janus (179 km/111miles), Prometheus (86 km/53 miles) and Atlas (30 km/19 miles). Like I said, tiny.

When I see images like this I like to amuse myself by fiddling in my mind with their perspective. For example, is Epimetheus closer to us (well, to Cassini when the picture was taken) than Janus was? Even more interestingly, are we looking down on, or up at the rings?

Images like this don’t give us the clues we usually get here on Earth to figure out distance. Look at the picture: the rings make a tight curve across the field. We know we’re seeing a circular ring nearly edge-on… but are we looking down on it, so that the top of the curve is farther away, or are we looking up at it, so that the bottom of the curve is farther away?

For example, take a DVD and hold it so that you’re looking at it almost edge-on. Tilt the near edge down a bit so you’re looking down on the top side. Now tilt the near side up so you’re looking up on the bottom side. See the issue? Without lighting, focus, or other cues, it’s hard to tell which way you’re seeing an object.

So for the Saturn picture, which is it? I’ll tell you below, but see if you can figure it out.

Did you guess? I’ll tell you a secret: you can’t tell from just looking. It so happens that Cassini was above the ring plane looking down on them, so the top part of the curving rings is farther away. But in reality, the picture gives you no clue of this*.

As it happens, too, Epimetheus was slightly closer than the other three moons when this shot was snapped. It was 1.2 million kilometers from Cassini, while the other three were 1.3 million. Since the moons are irregular and lumpy, there are very few distance clues either. One thing that helps is that we’re seeing this vista from an angle, so if a moon were a lot farther away than the others, it would look like it’s way off the ring plane. So that’s a clue that they are all pretty much at the same distance, but other than that there’s not much you can determine.

That tells you how important spacecraft telemetry is. We get a constant stream of information from Cassini, telling us where it is, where it’s pointing, what filters and camera it’s using. Without these bits of data we’d have a hard time actually doing any science with these images.

And while it’s very pretty, there’s a whole lot more going on here than just an interplanetary tableau. The picture is only one piece of it! Scientists are learning boatloads about the outer solar system with Cassini and other probes, and it’s only by diving into the actual data that this can be done.

Image credit: NASA/JPL/Space Science Institute



* As it happens, there might be a way to tell. If the outer ring were not so thin, and actually had a visible width, you’d see the far side as narrow and the near side as wider due to perspective. However, in this small picture and with that narrow a ring, I don’t think in this case that’s measurable.

CATEGORIZED UNDER: Pretty pictures, Science

Comments (52)

  1. MattF

    Actually, in this instance, you can tell. Look at the (very small and faint) shadow that the rightmost moon is making on the rings. Look at the moons to find out where the Sun is. Based on the Sun’s position, the placement of the shadow only makes sense if the small moon is “in front of” the rings (and casting the shadow away from us) rather than “behind” them (and casting the shadow toward us). Therefore, we’re just above the ring plane, looking down on them.

    But your main point is still a valid one.

  2. Stuart

    Is it just me, or does it look like the Apollo CSM between the rings?

  3. Oli

    They should send a Cassini 2 to Uranus and Cassini 3 to Neptune. We could learn so much from doing that…

  4. Jenny

    Oops I said something wrong here. How do I delete a comment?!

  5. John Sandlin

    I certainly couldn’t guess. Not knowing specifically where the sun is, other than mostly behind Cassini, it’s hard to be certain which shadow is which. So I figuratively flipped a coin. It was wrong.

  6. Stacey

    With a 50/50 shot at being right, I guessed correctly based on the brightness and shadows on the moons. Which was probably an incorrect basis given my limited knowledge of space in general! Darn, wish this was a winning lottery ticket instead. :) Thanks for always sharing these cool images with us. Your brain amazes me.

  7. Andrew
  8. lagomorph

    enjovianate? Which planet is this anyway? :)

  9. Egaeus

    Considering that it was recently “spring on Saturn,” and that such as statement assumes the traditional northern hemispherical bias of most Earthlings, I figured that since the rings seemed to be illuminated from the sun slightly above them that we were looking down on the rings. Also considering that most published photos of Saturn itself are oriented according to how it is viewed from Earth, I correctly assumed that the top of the curve was farther away.

    However, that doesn’t strictly rely on the picture, but rather human biases in how it is presented. Was that wrong? Should I not have done that?

  10. BJN

    I agree with MattF, assuming that the image has celestial north at the top of the image, the illumination of the moons gives a clear indication that the ring is tilted down to be illuminated too.

    I wouldn’t presume to guess the relative positions of the moons, however.

  11. Messier Tidy Upper

    Well this goes to show the old saying about there being no up or down in space rings true. Gotta admit I couldn’t tell at all.

    Great picture and good to see some of the smaller moons that usually don’t get the limelight. :-)

    Atlas being the smallest of those moons there would be good news for the mythical giant named Atlas who supposedly holds the world on his shoulders and also make life easy for the cartographic atlas makers – and from there I wonder if you could spot the member of the Plieades star cluster called Atlas too! ;-)

    @3. Oli Says:

    They should send a Cassini 2 to Uranus and Cassini 3 to Neptune. We could learn so much from doing that…

    Seconded by me but they should call them different names to avoid confusion over which “Cassini” is the “Cassini” that you’re talking about. I’d call the Ouranos “Cassini 2″ one Herschel and the Neptune bound “Cassini 3″ spaceprobe Lassell myself.

    I’d really love to see dedicated long duration orbiter missions to Ouranos and Neptune. There’s so much we’ve yet to see and discover about these outer ice giant planets; for instance, Voyager II only imaged 40% of the surface of Triton because the rest of that moon was in shadow and seeing what the rest of Triton – & many others in a similar situation – harbours would be fascinating and, I think, surprising.

    @2. Stuart Says:

    Is it just me, or does it look like the Apollo CSM between the rings?

    Not just you, I see it too. The moon (or moonlet) Promethus very much looks Apollo shaped there.

    Reminds me that Stephen Baxter used Apollo (type?) crew capsules combined with shuttle tech to travel to the Saturnian system in his well-written, evocative but very gloomy ‘Titan’ novel.

  12. Ooh. I wish they’d taken two images with some separation, that would make a wonderful (exaggerated) stereo picture.

    Possibly related: http://www.pbfcomics.com/archive_b/PBF248-Transmission.jpg

  13. For example, is Epimetheus closer to us than Janus was?

    I don’t know about you, but they all look like they’re about 24 inches away from me.

    Stuart:

    Is it just me, or does it look like the Apollo CSM between the rings?

    Sorry, but it’s clearly a goldfish.

  14. kevbo
  15. Kevin Devine

    In space, I learned this from a book: The enemy’s gate is down!

  16. Levi in NY

    Well, technically Saturn is a Jovian planet (also called a gas giant).

  17. Brian Schlosser

    @kevbo #10: Encroniate!

  18. Oli

    @8. MTU

    what about sending a Triton-rover with them? Huygens 2 (yay for original names ^_^)

  19. Messier Tidy Upper

    @ ^ Oli : I’d love that! :-D

    Not so sure about the aptness of that name though – there’s a Huygens lander already & so let’s call it something else to avoid confusion and spread the naming honours. Come to think of it the early scientist discoverers names have been commemorated already in the Neptunean ring names.

    Lets see, given its Neptune and a moon in ice cold storage how about Davy Jones for the lander after the sailors “Davy Jones locker” and Jack Sparrow Poseidon or Argo for the orbiter? Feel free to come up with better ones! ;-)

  20. Messier Tidy Upper

    For the naming of the Neptunean rings – see :

    http://en.wikipedia.org/wiki/Rings_of_Neptune

    Neptune’s rings are named after astronomers who contributed important work on the planet: Galle, Le Verrier, Lassell, Arago, and Adams.

    I forgot about that when I thought of using the name Lassell (Triton’s discoverer – there was considerable controversy over the planet’s discovery based on competing English and French calculators of its position so rather than singling one of them out I thought I’d use the less controversial more clear-cut choice.) for the orbiter and that could result in confusion and repetition.* A bit like my posts sometimes unfortunately! ;-)

    * Eg. “Lassell observes Lassell” when the orbiter is imaging the ring, etc …

    ————-

    PS. As for Davy Jones see : http://en.wikipedia.org/wiki/Davy_Jones%27_Locker

    Plus the Pirates of the Caribbean movies natch. ;-)

  21. Gary

    The exact same observation was made in the Gerry Anderson TV series UFO. In an episode called ‘Close Up’ (series 1, episode 13) they got a space probe to follow a UFO back to it’s home planet and it took loads of photographs, but these all turned out to be completely useless as the data showing the distance of the planet was lost, so they had no way of telling what the surface features really were. The point was illustrated in a saucy fashion by showing Commander Straker close ups of Lt Gay Ellis’s (Gabrielle Drake) leg! :)

  22. Oli

    @ MTU: Thetis? Oceanus/Okeanos?

    I know about Cassini-Huygens, I just referred to it as Huygens 2 as a temporary name, to make it clear what I meant ;-)

  23. MattF

    John Sandlin: Not knowing specifically where the sun is, other than mostly behind Cassini

    Turns out that’s all you need.

    BJN: assuming that the image has celestial north at the top of the image

    Well, yes. Elements like “above the plane of the rings” in my description makes little sense unless celestial north is “up”. Thanks for clarifying.

  24. Hopkirk (Deceased)

    Just to the right of the moon furthest on the right is a small shadow in the ring. Is that an actual shadow or a small lump of something?

  25. usagi

    @15 Darn, you beat me to it…

  26. QuietDesperation

    Considering that it was recently “spring on Saturn,”

    Fall is really the better time to visit because that’s when the colors of the atmospheric belts change.

  27. Captn Tommy

    By the way in the cassini photo with the distances Epimetheus is in front of the rings (the apparent corner of the elipse) leading the train, which adds to the optical confusion.

    I am not sure whether Epimetheus or Janus is in the outside track of their co-orbit. so Epimetheus may be pulling away from Janus or Janus catching up.

    I have the picture up as my background this week.

    Captn Tommy

  28. They should send a Cassini 2 to Uranus

    Mustn’t make oldest joke in world… mustn’t make oldest joke in world…

  29. Sean H.

    The white inner ring kind of made it easy for even me to tell that it was looking down at the angle.

  30. Ross

    I thought for sure there would be some chatter here about this:
    http://neo.jpl.nasa.gov/news/news169.html
    Two Small Asteroids to Pass Close by Earth on September 8, 2010

  31. Crudely Wrott

    I used the apparent angle of illumination of the moons to get a rough bearing on the sun.
    The sun seems to be off to the left and the angle of illumination appears to be nearly parallel to the top and bottom edges of the picture. Knowing that recently the rings were edge on to the sun was the big clue.

    Since the ring plane is clearly tilted “down” (to the left) relative to the angle or illumination our POV is clearly defined. We are just above the ring plane and up is toward the top of the image as is Saturn’s north pole and the upper arc is the farthest away.

  32. I used the seperation line of the outer ring when looking at the left corner. The line looekd to me like it was angling up, indicating that the top half was farther away.

  33. The top line of the ring is slightly dimmer than the bottom line indicating it is further away from the perspective of the camera. It must therefore be from the top looking down.
    From a bottom up perspective the top line of the ring would be slightly brighter indicating it was closer.
    But then again the camera could aways be upside down.

  34. lordbubonicus

    @Messier Tidy Upper:

    You couldn’t use Herschel either. That’s already taken by ESA’s infrared satellite that launched with Planck. Unfortunately.

  35. Nigel Depledge

    Apologies if someone beat me to this (I’ve not yet read all the comments)…

    Messier Tidy Upper (11) said:

    Atlas being the smallest of those moons there would be good news for the mythical giant named Atlas who supposedly holds the world on his shoulders . . .

    Atlas was a Titan (yes, I know that’s confusing for the names of Saturn’s moons), and he held the heavens upon his shoulders, not the world. The first time he was depicted holding the world on his shoulders was on the cover of a map book, but I don’t know when that was.

  36. Nigel Depledge

    Messier Tidy Upper (11) said:

    @3. Oli Says:

    They should send a Cassini 2 to Uranus and Cassini 3 to Neptune. We could learn so much from doing that…

    Seconded by me but they should call them different names to avoid confusion over which “Cassini” is the “Cassini” that you’re talking about. I’d call the Ouranos “Cassini 2″ one Herschel and the Neptune bound “Cassini 3″ spaceprobe Lassell myself.

    Agreed!

    EDITED TO ADD:
    D’oh! I just read comment #33, so Herschel’s out.

  37. Nigel Depledge

    Messier Tidy Upper (19) said:

    Lets see, given its Neptune and a moon in ice cold storage how about Davy Jones for the lander after the sailors “Davy Jones locker” and Jack Sparrow Poseidon or Argo for the orbiter? Feel free to come up with better ones!

    Maybe Argo would be a more appropriate name for a probe to explore the Kuiper belt. That would be a seriously long mission!

  38. Nigel Depledge

    Ross (30) said:

    I thought for sure there would be some chatter here about this:
    [url omitted]
    Two Small Asteroids to Pass Close by Earth on September 8, 2010

    If only there were some astronomer dude who could write a book about this kind of stuff, and whether it poses a risk to us on Earth . . .

    ;-)

  39. MTU, Nigel et al:
    There’s no law against using the same name twice for spacecraft! After all, three of the shuttles ( Columbia, Challenger and Endeavour ) have/had the same names as Apollo spacecraft ( Apollo 11 CSM, 17 LM and 15 LM, respectively ). So I would still go with Herschel for a Uranus probe – especially as the other Herschel is likely to be defunct long before it happens. Or call it “William Herschel” to distinguish it. I can’t think of any better name.
    For a Neptune probe, I’d go with Le Verrier. Irrespective of the controversy over whether Adams or le Verrier predicted Neptune’s existence first, the fact is that it was le Verrier’s calculations which Galle and d’Arrest used to find it. And Lassell for the Triton lander, as he discovered Triton.

  40. Mattf:

    I had kind of assumed that that dark spot was Encke gap at the ansa, not a shadow. I’m reasonably sure that we’re past the point where Cassini is returning moon-shadow images, although I haven’t looked at the planning stuff in over a year now I guess.

  41. Colin

    It’s the Keeler Gap. We’re only seeing a tiny portion of the outer A ring there.

  42. Pete Jackson

    OK, I got fooled. If you enjovianate the picture, and look at the extreme right, you see more black space between the thin (F?) ring and the bright ring at the upper gap on the page than at the lower gap. So, assuming total circular symmetry of the rings, and that they are coplanar, that should mean that the upper gap is closer than the lower gap and, hence, that we are below the ring plane looking up at it.

    So, I guess my assumptions of circularity and coplanarity were wrong.

  43. MattF

    John Weiss: I had kind of assumed that that dark spot was Encke gap at the ansa, not a shadow.

    Interesting. I hadn’t considered that. I can’t seem to resolve with my eyeballs alone whether it’s definitely one or the other.

    John Weiss: I’m reasonably sure that we’re past the point where Cassini is returning moon-shadow images, although I haven’t looked at the planning stuff in over a year now I guess.

    I’d need to do some trig for that one, and I don’t have tables handy to find sun angles and stuff. We’re only past the Saturnian equinox by a year or so (2009 Aug. 11 or so, IIRC); roughly speaking, that puts us about a seventh of the way to maximum axial tilt (which is about 27 degrees). I don’t know if a few degrees is enough to remove the ability of moons to cast shadows on the rings.

    Thanks for the double-check, though.

  44. Tribeca Mike

    Nigel Depledge — perhaps you’re thinking of Mercator’s famous atlas of 1570. He actually didn’t call it an atlas, but a “Theatre of the Round World.” A most delightful title, no? Here’s the frontispiece…

    http://www.uh.edu/engines/atlas3.jpg

  45. becky's thoughts

    @ #11 “rings true” hahaha

  46. “roughly speaking, that puts us about a seventh of the way to maximum axial tilt (which is about 27 degrees). I don’t know if a few degrees is enough to remove the ability of moons to cast shadows on the rings.”

    Not quite. The Sun moves faster near the equinox than near the solstices, where it turns around. The Sun should be around 5.6 degrees out of the ring-plane now. I think that that exceeds all of the ringmoons’ inclinations. (Even if you assume it’s linear between equinox and solstice, were’ around 1/7.5 of the way to solstice now, so at least 3 degrees up. Of course, seen from the rings, the moons aren’t necessarily just ~1 degree up at their maximum vertical excursion.

    In fairness, I should note that I seem to recall large part of why the shadows stopped at some point was that Cassini was diving into the ringplane, making it hard to capture them. So there may be some incidental, low-quality (for shadow pictures, that is) lucky catches yet.

    If Colin says it’s Keeler (I’ll assume that that’s my colleague, Dr. Mitchell), it’s almost certainly correct. Scale fools even us ring scientists. :)

  47. Enjovianate? I thought Jove was Jupiter.

  48. Nigel Depledge

    Neil Haggath (39) said:

    There’s no law against using the same name twice for spacecraft! After all, three of the shuttles ( Columbia, Challenger and Endeavour ) have/had the same names as Apollo spacecraft ( Apollo 11 CSM, 17 LM and 15 LM, respectively ).

    And look what heppened to 2 out of those 3 vehicles.

    Even before the shuttle programme was closed down, Endeavour’s days were numbered, I tell ye!

    ;-)

  49. Nigel Depledge

    @ Tribeca Mike (44) –
    You could well be right.

    If I recall correctly, the figure in Mercator’s Theatre of the Round World was not intended to be Atlas, but was mistaken for Atlas – and hence when the next publisher produced a book of maps, it was called an atlas.

  50. Nigel Depledge

    Mike (47) said:

    Enjovianate? I thought Jove was Jupiter.

    Yup. We had this on a previous thread.

    Jove is Jupiter. Therefore, Jovian is big. Therefore, to enjovianate is to embiggen.

    It’s all perfectly cromulent.

  51. Tyler

    If we were looking up wouldn’t atlas appear above the, apparent, disc? Or did I just get lucky by thinking incorrectly?

  52. @Nigel 50
    Ah now it comes clear. I knew Phil couldn’t be confused, I just couldn’t follow the logic train. I guess Enjovianate could also mean making it happy and mirthful too.

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