APOD: In the Shadow of Saturn (2013 Nov 13)

[neufer]

Absolutely amazing image, and in natural colors. Is there a way to get a print of it, preferably in its highest resolution?
I'd love to have it up on my office wall to stare and marvel at.

All NASA images are in the public domain. You could download a high resolution image and take it to your local print shop or use an online printing service to make yourself a poster-sized print. You could even add an inspiring quote, and sell prints online. I might buy one.

http://www.zazzle.com/nasa_cassini_satu ... 1572874632

[Anthony Barreiro]

Absolutely amazing image, and in natural colors. Is there a way to get a print of it, preferably in its highest resolution?
I'd love to have it up on my office wall to stare and marvel at.

All NASA images are in the public domain. You could download a high resolution image and take it to your local print shop or use an online printing service to make yourself a poster-sized print. You could even add an inspiring quote, and sell prints online. I might buy one.

http://www.zazzle.com/nasa_cassini_satu ... 1572874632

Darn, somebody got there first!

[neufer]

---

Every time I look at this image, I'm intrigued by the fact that the rings seen against the dark face of Saturn do not line up with the rings as seen against the background of space. I understand that the rings between Cassini and Saturn are being illuminated by light reflected and perhaps refracted from the rings that are in direct sunlight. But it tickles my mind to notice that I automatically try to decide where the rings "really" are.

The dim northern face of Saturn (illuminated by all the rings) basically just show the shadows of the thick A & B rings

The back-lit rings on either side are much more complicated.

[Nitpicker]

... I've noticed that every time we've attempted to correct each other previously, I was wrong and you were right.

Well, in this example I thought I was just agreeing with you

So, finally, we'll either be both wrong or both right.

The black crescent is Saturn's shadow on the half of the rings closest to Cassini. The shadow shows up only above Saturn because the rings below Saturn are on the other side of Saturn from Cassini's point of view, closer to the Sun, where Saturn can't cast a shadow.

The thin white ring that encircles Saturn is sunlight shining through Saturn's upper atmosphere. The black crescent shadow appears on the outside of the thin ring of atmosphere because the crescent shadow is the shadow of Saturn on the rings closest to Cassini. Parts of the thin ring are less bright because it's blocked by shadows cast by the rings, on the lower half of Saturn, or by the rings themselves, on the upper half.

I must confess to not understanding how that is possible. Does this mean that wherever the rings are unilluminated by direct sunlight, we cannot see them, but can effectively see through them onto the planet, where we can see ringshine from the illuminated parts of the rings?

[Anthony Barreiro]

---

Every time I look at this image, I'm intrigued by the fact that the rings seen against the dark face of Saturn do not line up with the rings as seen against the background of space. I understand that the rings between Cassini and Saturn are being illuminated by light reflected and perhaps refracted from the rings that are in direct sunlight. But it tickles my mind to notice that I automatically try to decide where the rings "really" are.

The dim northern face of Saturn (illuminated by all the rings) basically just show the shadows of the thick A & B rings

The back-lit rings on either side are much more complicated.

Huh. So the rings we see against Saturn are illuminated by light coming from the Sun, reflecting off the rings, reflecting off the nighttime surface of Saturn, and shining through the otherwise invisible rings. That makes sense, and it's complicated enough for me. One thing threw me off: the A ring, which I'm used to seeing as the brightest through a telescope, is darker than the other rings in today's apod -- because it's the densest ring it reflects the most light back toward Earth, and because it's the densest ring it blocks the most light when backlit! Pretty cool.

[MarkBour]

I think this is a really marvelous picture. But after just enjoying it a bit, and then reading this discussion, I have some questions.

1. How large is the disk of the Sun and where would it fall in this picture, were it visible? (One poster already said a little below the center of the dark disk of Saturn).
2. I heard that Saturn was a gas giant. But does it have a rocky core? In the discussion, it appears that lots of sunlight is scattered through a thin circular band around the dark disk, and the contrast is pretty sharp, not fuzzy. So, does this image show us the "ground" on Saturn?
3. Why are the rings not symmetrically located around the center line of the planet ? (It was explained that the part of the rings that appear in the upper part of the image are the closer parts, so the upper half of the rings are on the away-from-the-Sun side of Saturn. Then perhaps this is simple perspective, that the nearer rings are farther from the center line, because they are much closer to the camera.) It's odd, because it gives the impression that the rings are actually above the center of Saturn.
4. Following along with the previous question, how coordinated is all of this stuff in the Saturn system? It would be too simple if the equator of Saturn were aligned to the Sun, the rings were right over the equator, and everything rotated together. But perhaps some things are aligned.
5. What causes the shifts from the location of the rings to the location of the dark bands in the atmosphere?
6. How is the image of the near-side rings transformed as we see them on the dark side of Saturn?
   (My understanding from previous posts is that light from the Sun is hitting rings on the "anti-solar" side of Saturn, parts of the rings that are nearer to the camera than the planet, but that are not in shadow, then that light is scattered back onto the dark side of Saturn, then the light reflects off of the spherical shaped cloud-tops on Saturn, and back to Cassini. Quite a transit.)
7. If Earth, Venus, and Mars were visible as small light disks from this location, does that imply that they were all farther away from Saturn at the time than the Sun? So that light could travel to them and then reflect back to Saturn? Or is it possible to see a planet that is between the Sun and Saturn? Perhaps at a great enough angle, even an object that is closer to Saturn than the Sun would be able to reflect and scatter light, and so be visible against the darkness of space.
8. And the hazy ring where Enceladus moves is cool to think about. I don't know how much is known about the history of Enceladus. I am guessing that material coalesced into the moon Enceladus, and there was no ring there, just a moon. But now, it seems to be spurting material back out (water? something else?) and making a cloud-like ring of material that you would say "came from Enceladus", at least that's its recent history.

So, quite a lot of questions, probably just getting started, too. I don't expect someone to run out and answer all of them. There is a sense in which I'm being lazy. I suppose I could research and figure a lot of this out for myself. I think a nice ray-tracing art program would be very helpful, too. But this is a fun place to ask these kinds of questions.

[geckzilla]

I heard that Saturn was a gas giant. But does it have a rocky core? In the discussion, it appears that lots of sunlight is scattered through a thin circular band around the dark disk, and the contrast is pretty sharp, not fuzzy. So, does this image show us the "ground" on Saturn?

Nope. Just like the edges of clouds look sharp and solid from afar, so does the edge of Saturn. It's also very slightly and very forgivably overexposed.

[Chris Peterson]

I think this is a really marvelous picture. But after just enjoying it a bit, and then reading this discussion, I have some questions.
How large is the disk of the Sun and where would it fall in this picture, were it visible? (One poster already said a little below the center of the dark disk of Saturn).

It's hard to answer that without a good scale reference. If this image has been scaled to the resolution of the narrow angle imager, the Sun would be 166 pixels across. That's 3.4 arcmin, or about 9 times smaller than it appears from Earth.

I heard that Saturn was a gas giant. But does it have a rocky core? In the discussion, it appears that lots of sunlight is scattered through a thin circular band around the dark disk, and the contrast is pretty sharp, not fuzzy. So, does this image show us the "ground" on Saturn?

Saturn probably has a rocky core, but it is small and very deep in the gas. It could never be observed optically.

Following along with the previous question, how coordinated is all of this stuff in the Saturn system? It would be too simple if the equator of Saturn were aligned to the Sun, the rings were right over the equator, and everything rotated together. But perhaps some things are aligned.

Both the orbital axis and rotational axis of Saturn are tipped with respect to the ecliptic, and with respect to the invariable plane of the Solar System. Within the Saturnian system, most of the moons orbit at low inclinations. The ring system is precisely located over the equator, as that is the only stable position that exists.

If Earth, Venus, and Mars were visible as small light disks from this location, does that imply that they were all farther away from Saturn at the time than the Sun? So that light could travel to them and then reflect back to Saturn? Or is it possible to see a planet that is between the Sun and Saturn? Perhaps at a great enough angle, even an object that is closer to Saturn than the Sun would be able to reflect and scatter light, and so be visible against the darkness of space.

When this image was made, both Earth and Venus were closer to Saturn than the Sun, although they were each not far from their maximum elongation, meaning that they would appear close to 50% phase if they could be resolved. Mars was farther away from the Sun- nearly in line with it, in fact, meaning that had the image been taken a few days earlier, Mars would have been behind the Sun (probably not in perfect eclipse, however), and not visible.

[Anthony Barreiro]

...
[*] Following along with the previous question, how coordinated is all of this stuff in the Saturn system? It would be too simple if the equator of Saturn were aligned to the Sun, the rings were right over the equator, and everything rotated together. But perhaps some things are aligned.

The main rings and moons orbit Saturn's equator. Saturn's axis of rotation is tilted about 27 degrees relative to the plane of Saturn's orbit around the Sun. So there are seasons on Saturn just like here on Earth, although each of Saturn's seasons lasts about seven earth years. Because of Saturn's seasonal wobble, we here on Earth see the rings edge-on around Saturn's equinoxes, we see the south side of the rings during Saturn's southern summer, and the north side of the rings during Saturn's northern summer.

...
[*] If Earth, Venus, and Mars were visible as small light disks from this location, does that imply that they were all farther away from Saturn at the time than the Sun? So that light could travel to them and then reflect back to Saturn? Or is it possible to see a planet that is between the Sun and Saturn? Perhaps at a great enough angle, even an object that is closer to Saturn than the Sun would be able to reflect and scatter light, and so be visible against the darkness of space.

Here's a nice online orrery. Mars was on the far side of the Sun from Saturn on July 19, 2013, but Venus and Earth were on the inside parts of their orbits around the Sun as seen from Saturn. Inner planets are bright when they're in their crescent phases as seen from an outer planet, if they're at a great enough elongation from the Sun. Look at Venus in the evening sky these days. She's bright! And if you look through a telescope, she's a crescent!

[Nitpicker]

The black crescent is Saturn's shadow on the half of the rings closest to Cassini. The shadow shows up only above Saturn because the rings below Saturn are on the other side of Saturn from Cassini's point of view, closer to the Sun, where Saturn can't cast a shadow.

The thin white ring that encircles Saturn is sunlight shining through Saturn's upper atmosphere. The black crescent shadow appears on the outside of the thin ring of atmosphere because the crescent shadow is the shadow of Saturn on the rings closest to Cassini. Parts of the thin ring are less bright because it's blocked by shadows cast by the rings, on the lower half of Saturn, or by the rings themselves, on the upper half.

I must confess to not understanding how that is possible. Does this mean that wherever the rings are unilluminated by direct sunlight, we cannot see them, but can effectively see through them onto the planet, where we can see ringshine from the illuminated parts of the rings?

I think I am finally starting to understand this image. Thanks MTD and others. It would have been more correct for me to say that the denser portions of the unilluminated rings, obscure parts of the ringshine from the planet, from Cassini's point of view (rather than that we can see through the rings). And if the planet did not reflect ringshine, then everything inside the thin white circle outlining the planet would appear black, as well as the narrow crescent above the white circle. Got it. It is not unlike looking at an M.C. Escher print.

[owlice]

Absolutely amazing image, and in natural colors. Is there a way to get a print of it, preferably in its highest resolution? I'd love to have it up on my office wall to stare and marvel at.

All NASA images are in the public domain. You could download a high resolution image and take it to your local print shop or use an online printing service to make yourself a poster-sized print. You could even add an inspiring quote, and sell prints online. I might buy one.

But this image isn't solely a NASA image.

[saturno2]

Beautiful image, indeed.

[Ann]

Is that Jupiter at the lower left, visible as a disc at about 8 o'clock?

No one has commented on Patrick60's question so far. I, too would like to know what it is. To me it really looks a bit like Jupiter. And if it isn't Jupiter, then what is it? Titan?

I note that Venus, Earth and Mars appear to be "perfectly colored". Venus is brilliantly white, Mars is faintly reddish, and the Earth-Moon system is slightly blue. "Jupiter" (or Titan?) is sand-colored, which seems like a reasonable integrated color for a white and brownish striped body at five astronomical units from the Sun. So this is a perfect color picture, then?

I note that some of the point sources that in most cases are background stars are quite differently colored. There is a very reddish point source to the left of "Jupiter", and a quite blue point source still further to the left.

Ann

[owlice]

This was answered; it's Tethys.

[Nitpicker]

Is that Jupiter at the lower left, visible as a disc at about 8 o'clock?

No one has commented on Patrick60's question so far. I, too would like to know what it is. To me it really looks a bit like Jupiter. And if it isn't Jupiter, then what is it? Titan?

I note that Venus, Earth and Mars appear to be "perfectly colored". Venus is brilliantly white, Mars is faintly reddish, and the Earth-Moon system is slightly blue. "Jupiter" (or Titan?) is sand-colored, which seems like a reasonable integrated color for a white and brownish striped body at five astronomical units from the Sun. So this is a perfect color picture, then?

I note that some of the point sources that in most cases are background stars are quite differently colored. There is a very reddish point source to the left of "Jupiter", and a quite blue point source still further to the left.

Ann

I think that if you were to fit a straight line on the image, between Earth, Venus and Mars, you'd have a pretty good approximation to the Ecliptic plane. Based on that and the date of the image, I estimate Jupiter would be out of frame, above the top edge, somewhat to the left hand side.

Regardless, Ann, you have remarkably colour-perceptive eyes. You frequently make me feel like I see in monochrome.

[Nitpicker]

I note that some of the point sources that in most cases are background stars are quite differently colored. There is a very reddish point source to the left of "Jupiter" [Tethys], and a quite blue point source still further to the left.

I think the red and blue point sources are:

Red: HIP 13495, mag 6.5, class M3.

Blue: Lambda Ceti, mag 4.7, class B6III.

(It also looks like the stars far beyond the ring system, around the perimeter of the image, were not captured.)

[Nitpicker]

Absolutely amazing image, and in natural colors. Is there a way to get a print of it, preferably in its highest resolution? I'd love to have it up on my office wall to stare and marvel at.

All NASA images are in the public domain. You could download a high resolution image and take it to your local print shop or use an online printing service to make yourself a poster-sized print. You could even add an inspiring quote, and sell prints online. I might buy one.

But this image isn't solely a NASA image.

Well, if you do run into any legal troubles, you are most welcome to instead use the following image of mine, taken from the other side a month earlier:

Mine is a minimalist variant, appealing to a certain aesthetic, achieved in a single exposure of only 5 seconds, and with a glass of wine in one hand.

[geckzilla]

I like it. You could make a stencil out of it and emphasize the moons a bit and spray paint it all over as a secret astronomical society logo. Name all the pieces and you are in the club.

[Chris Peterson]

I note that Venus, Earth and Mars appear to be "perfectly colored". Venus is brilliantly white, Mars is faintly reddish, and the Earth-Moon system is slightly blue. "Jupiter" (or Titan?) is sand-colored, which seems like a reasonable integrated color for a white and brownish striped body at five astronomical units from the Sun. So this is a perfect color picture, then?

There's no such thing. But this image was made using three broadband filters covering red, green, and blue. The overlap between the green and blue filters is suboptimal, but it's still possible to generate a final image with a gamut similar to our eyes, and a reasonable approximation of natural colors... that is, an image similar to what we would see visually. And unlike most astronomical images, which show objects too dim to stimulate our color vision, this scene is actually a bright one.

[alter-ego]

I note that some of the point sources that in most cases are background stars are quite differently colored. There is a very reddish point source to the left of "Jupiter" [Tethys], and a quite blue point source still further to the left.

I think the red and blue point sources are:

Red: HIP 13495, mag 6.5, class M3.

Blue: Lambda Ceti, mag 4.7, class B6III.

(It also looks like the stars far beyond the ring system, around the perimeter of the image, were not captured.)

Very good! We agree once again!
Last night I spent some time inspecting the star field across the image and was impressed with faintness reached. The limiting magnitude reached 10⁺ magnitude. It's odd to see such faint stars next to such a normally dominantly bright planet, but being on the back side and in the Sun's shadow permits longer exposures to pick up both planet details and fainter stars.

[Nitpicker]

Very good! We agree once again!
Last night I spent some time inspecting the star field across the image and was impressed with faintness reached. The limiting magnitude reached 10⁺ magnitude. It's odd to see such faint stars next to such a normally dominantly bright planet, but being on the back side and in the Sun's shadow permits longer exposures to pick up both planet details and fainter stars.

And this time we're both right, too! (Don't mention the non-instantaneous black crescent. I mentioned it once, but I think I got away with it!)

[Anthony Barreiro]

I think that if you were to fit a straight line on the image, between Earth, Venus and Mars, you'd have a pretty good approximation to the Ecliptic plane. Based on that and the date of the image, I estimate Jupiter would be out of frame, above the top edge, somewhat to the left hand side.

Yup.

[Ann]

Nitpicker and alter-ego, I don't doubt for a moment that you are right about Lambda Ceti and HIP 13495!

I note that Lambda Ceti really is fairly blue. In other words, it really has a relatively blue B-V index, about -0.11. HIP 13295 is rather red, too, with a B-V index of about +1.46.

Far to the left of Lambda Ceti is a rather faint, relatively whitish star. It could be HD 19525 (HIP 14607), mag about 6.3, spectral type G9III, B-V index about +1.03.

Much closer to Lambda Ceti, to the lower left of it, is a very faint star, which is relatively neutral in color. It could be SAO 110928, mag about +10.1, spectral class G(?), B-V index about +0.60. To me this star looks ever so slightly bluish, suggesting that the color balance here is just very slightly blue.

But fascinatingly, right "below" (to the south of) SAO 110928 is a considerably brighter star, HD 18909 (HIP 14176), mag about +8.2! The eighth magnitude star is quite red, too, with a B-V index of about +1.42, very nearly as red as HIP 13295. Why don't we see it? The declination of faint SAO 110928 is +08 29' 38.8", while the declination of HD 18909 is +08 28' 18.9". The right ascensions of both stars are very similar, too, at 03h 02m 48.64s and 03h 02m 49.47s, respectively. Why do we see the "upper", neutral-colored, very faint one, but not the "lower", brighter, quite reddish one?

Another star that "should" show up, but doesn't, is HD 18262 (HIP 13679). This is a sixth magnitude star (mag around 6.0), spectral class F7IV, B-V index about +0.48. It is located "between" Lambda Ceti and red HIP 13495, but "below" them, at declination +08 22 54.4. Is it hidden behind something, perhaps one of Saturn's rings?

Ann

[Nitpicker]

But fascinatingly, right "below" (to the south of) SAO 110928 is a considerably brighter star, HD 18909 (HIP 14176), mag about +8.2! The eighth magnitude star is quite red, too, with a B-V index of about +1.42, very nearly as red as HIP 13295. Why don't we see it? The declination of faint SAO 110928 is +08 29' 38.8", while the declination of HD 18909 is +08 28' 18.9". The right ascensions of both stars are very similar, too, at 03h 02m 48.64s and 03h 02m 49.47s, respectively. Why do we see the "upper", neutral-colored, very faint one, but not the "lower", brighter, quite reddish one?

Another star that "should" show up, but doesn't, is HD 18262 (HIP 13679). This is a sixth magnitude star (mag around 6.0), spectral class F7IV, B-V index about +0.48. It is located "between" Lambda Ceti and red HIP 13495, but "below" them, at declination +08 22 54.4. Is it hidden behind something, perhaps one of Saturn's rings?

I haven't looked for the missing stars you mention, but I did note earlier that the stars that would/should be apparent around the black periphery of this image, are not there. I assume that a black perimeter was blended into the image to fill in the gaps in the mosaic, and give the overall image a nice balance, by centring the main subject.

[alter-ego]

... But fascinatingly, right "below" (to the south of) SAO 110928 is a considerably brighter star, HD 18909 (HIP 14176), mag about +8.2! The eighth magnitude star is quite red, too, with a B-V index of about +1.42, very nearly as red as HIP 13295. Why don't we see it? The declination of faint SAO 110928 is +08 29' 38.8", while the declination of HD 18909 is +08 28' 18.9". The right ascensions of both stars are very similar, too, at 03h 02m 48.64s and 03h 02m 49.47s, respectively. Why do we see the "upper", neutral-colored, very faint one, but not the "lower", brighter, quite reddish one?

HIP 14176 is visible, and as you said, so is its neighbor SAO 110928.

Another star that "should" show up, but doesn't, is HD 18262 (HIP 13679). This is a sixth magnitude star (mag around 6.0), spectral class F7IV, B-V index about +0.48. It is located "between" Lambda Ceti and red HIP 13495, but "below" them, at declination +08 22 54.4. Is it hidden behind something, perhaps one of Saturn's rings?

This star does not show up, and as Nitpicker mentioned it is near the margin where many stars are not shown.
There is not a clear, straight-edge boundary that separates star visibility from no stars. This star is bright enough it should've shown up if it were in one of the component images. Maybe stars were selected for removal to camouflage a sharp-edge border yielding a blended image with a "soft" edge. I don't know. My inclination is to retrieve all the Cassini images to see the order in which they were taken and assembled. Nah, not tonight ...