Starship Asterisk*

Saturn at Equinox

Explanation: How would Saturn look if its ring plane pointed right at the Sun? Before August 2009, nobody knew. Every 15 years, as seen from Earth, Saturn's rings point toward the Earth and appear to disappear. The disappearing rings are no longer a mystery -- Saturn's rings are known to be so thin and the Earth is so near the Sun that when the rings point toward the Sun, they also point nearly edge-on at the Earth. Fortunately, in this third millennium, humanity is advanced enough to have a spacecraft that can see the rings during equinox from the side. In August 2009, that Saturn-orbiting spacecraft, Cassini, was able to snap a series of unprecedented pictures of Saturn's rings during equinox. A digital composite of 75 such images is shown above. The rings appear unusually dark, and a very thin ring shadow line can be made out on Saturn's cloud-tops. Objects sticking out of the ring plane are brightly illuminated and cast long shadows. Inspection of these images is helping humanity to understand the specific sizes of Saturn's ring particles and the general dynamics of orbital motion. This week, Earth undergoes an equinox.

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I'm wondering -
1. what happened to "unusually illuminated ring plane" from the photo shown at - http://apod.nasa.gov/apod/ap090825.html ?
2. where are these "Objects sticking out of the ring plane are brightly illuminated and cast long shadows" in today's picture ? They seem to be clear in the photo shown at - http://apod.nasa.gov/apod/ap090415.html

Can someone please shed some light ?

Thanks in advance.

APOD Robot wrote: ... A digital composite of 75 such images is shown above.

FYI:

Photojournal wrote:The moon Janus (179 kilometers, 111 miles across) is on the lower left of this image. Epimetheus (113 kilometers, 70 miles across) appears near the middle bottom. Pandora (81 kilometers, 50 miles across) orbits outside the rings on the right of the image. The small moon Atlas (30 kilometers, 19 miles across) orbits inside the thin F ring on the right of the image. The brightnesses of all the moons, relative to the planet, have been enhanced between 30 and 60 times to make them more easily visible. Other bright specks are background stars. Spokes -- ghostly radial markings on the B ring -- are visible on the right of the image.

ShaileshS wrote:I'm wondering -
1. what happened to "unusually illuminated ring plane" from the photo shown at - http://apod.nasa.gov/apod/ap090825.html ?
2. where are these "Objects sticking out of the ring plane are brightly illuminated and cast long shadows" in today's picture ? They seem to be clear in the photo shown at - http://apod.nasa.gov/apod/ap090415.html

Can someone please shed some light ?

Thanks in advance.

That is confusing. I believe the image references apply to the linked 2009 APODs. Todays image does not show those features.

A great shot of Saturn. A side portrait, if you will...

I am surprised the APOD was not the Higg's Boson Destroying us... as that seems to be the rage....not that I am worried...

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alter-ego wrote:

ShaileshS wrote:I'm wondering -
1. what happened to "unusually illuminated ring plane" from the photo shown at - http://apod.nasa.gov/apod/ap090825.html ?
2. where are these "Objects sticking out of the ring plane are brightly illuminated and cast long shadows" in today's picture ? They seem to be clear in the photo shown at - http://apod.nasa.gov/apod/ap090415.html

Can someone please shed some light ?

Thanks in advance.

That is confusing. I believe the image references apply to the linked 2009 APODs. Todays image does not show those features.

I don't see that it is confusing....today's APOD is a LONG SHOT....those are close up views of particular areas....they would not be seen from today's perspective...too far away....

:---[===] *

Boomer12k wrote:

alter-ego wrote:

ShaileshS wrote:I'm wondering -
1. what happened to "unusually illuminated ring plane" from the photo shown at - http://apod.nasa.gov/apod/ap090825.html ?
2. where are these "Objects sticking out of the ring plane are brightly illuminated and cast long shadows" in today's picture ? They seem to be clear in the photo shown at - http://apod.nasa.gov/apod/ap090415.html

Can someone please shed some light ?

Thanks in advance.

That is confusing. I believe the image references apply to the linked 2009 APODs. Todays image does not show those features.

I don't see that it is confusing....today's APOD is a LONG SHOT....those are close up views of particular areas....they would not be seen from today's perspective...too far away....

:---[===] *

That's the case alright, but the description isn't clear in that regard.

Interesting. I appreciate seeing another picture of the rather unilluminated rings of Saturn.

When Voyager 1 reached Jupiter in 1979 and beamed back pictures, I was absolutely delighted. The closeups of Jupiter were stunning, of course, but the NASA people also generally enhanced the saturation of the colorful cloud tops of the planet to make them look like a color freak's fantasy. Never mind that the dominant color was red (or rather orange), I was in seventh heaven. (Or as Mae West would have said, Too much of a good thing is just wonderful.)

So my expectations for the Voyager 1 pictures of Saturn were sky-high. Alas, they were not to be fulfilled. Some of the Saturn pictures were so colorful that they looked totally artificial, while others were dull and drab. And the rings were dark.

I have never quite gotten over my initial delight at the first Voyager Jupiter pictures and my disappointment at the Saturn ones. Now, of course, I have seen so many great pictures of Saturn that I have long ago gotten over my suspicion that Saturn only looks good from afar. But I still prefer Jupiter as a planet over Saturn as a planet, even though I think that Saturn's moons are a fantastically varied and fascinating lot.

I have also realized that Saturn's rings are often quite well lit and bright. But I know from that first Voyager 1 encounter with Saturn that the rings don't always look so shiny and bright. That's why I'm glad to see today's APOD, which gives a good explanation and a fine portrait of the darkness of the poorly lit rings.

Ann

Ann wrote:Some of the Saturn pictures were so colorful that they looked totally artificial, while others were dull and drab. And the rings were dark.

The colorful one is definitely not an RGB picture, though. You have to judge it by whatever filters it used and what was accomplished by using them rather than purely by aesthetics and what you expect to see. Having no information available (and I can't seem to easily find it) makes this hard.

alter-ego wrote: That's the case alright, but the description isn't clear in that regard.

If you keep in mind the title "Saturn at Equinox," it's clear. At equinox, "Objects sticking out of the ring plane are brightly illuminated and cast long shadows" and this is clearly illustrated by following the links provided (and this link is definitely a wowsers even by Cassini standards! Boy howdy, do I love this mission!).

geckzilla wrote:

Ann wrote:Some of the Saturn pictures were so colorful that they looked totally artificial, while others were dull and drab. And the rings were dark.

The colorful one is definitely not an RGB picture, though. You have to judge it by whatever filters it used and what was accomplished by using them rather than purely by aesthetics and what you expect to see. Having no information available (and I can't seem to easily find it) makes this hard.

I'm sure you are right, Geck.

I remember that the hues of those original Voyager 1 Jupiter pictures were discussed by some people, including, I think, Carl Sagan. I'm sure someone pointed out that the pictures were "prettified" and made extra colorful compared with what Jupiter would look like up close to the human eye.

What you say about RGB filters is crucially important, but back in 1979 I knew nothing about such things. I thought that Jupiter "really" looked the way it did in those Voyager 1 pictures. But the pictures of Saturn where it was striped in purple, lime and turquoise never looked realistic to me.

Ann

Ann, IOW, you like Jupiter better. Okay, but what does that have to do with this APOD, please?

Heh, back in 1979 I'm sure that the people processing the images were still feeling around for what people liked to look at, too. Bright colors have always been popular, but back in the 80s, some of those colors went beyond simply being appealing. They were also very fashionable and all these neon, gaudy patterns saw popular use in all sorts of objects other than clothes. Since image releases have always been about reaching out to the public and popularizing the latest mission, it could have been desirable at that point. Or maybe people were never keen on representative colors. Who knows. I wonder who worked on those images? It would probably be an interesting story.

owlice wrote:Ann, IOW, you like Jupiter better. Okay, but what does that have to do with this APOD, please?

I was just so powerfully reminded of my keen disappointment at those Voyager 1 pictures of Saturn, and I wouldn't have been disappointed in them if I hadn't first seen those colorful Jupiter photos by the same probe. Oh well. There were two things that disappointed me about Saturn, one of them the color of the planet, and the other the darkness of its rings.

What I like so much about today's APOD is that is really shows me that darkness of Saturn's rings all over again. I appreciate that, because the rings of Saturn hardly ever look dark in any modern picture. But in today's APOD, the rings look graceful and beautiful even though they are dark.

Today's APOD is a beautiful picture which both explains why the Voyager 1 Saturn pictures disappointed me and at the same time shows me that Saturn can look stunningly beautiful even when its rings are in eclipse.

Ann

Okay, thanks.

Beautiful.

alter-ego wrote:

APOD Robot wrote: ... A digital composite of 75 such images is shown above.

FYI:

Photojournal wrote:The moon Janus (179 kilometers, 111 miles across) is on the lower left of this image. Epimetheus (113 kilometers, 70 miles across) appears near the middle bottom. Pandora (81 kilometers, 50 miles across) orbits outside the rings on the right of the image. The small moon Atlas (30 kilometers, 19 miles across) orbits inside the thin F ring on the right of the image. The brightnesses of all the moons, relative to the planet, have been enhanced between 30 and 60 times to make them more easily visible. Other bright specks are background stars. Spokes -- ghostly radial markings on the B ring -- are visible on the right of the image.

Also mentioned in that photojournal (thanks alter-ego) is the fact that this APOD was made from images taken over 8 hours, within a day and a half of equinox, yet long shadows across the rings tend to be visible during the few months before and after equinox. So, I might guess that this particular mosaic was perhaps too close to exact equinox to show off any long shadows at this scale. But the long and impossibly thin shadow of the rings around the planet's equator is just sublime. Thank you.

The thin nature of the rings is indeed enhanced by this equinox image. In the full-sized image, you can see three (or more) moons, mere specks compared to the large lunar APOD of a couple of days ago. The rings are indeed quite wispy, a fact usually hidden in Earth-source images. How something that thin can appear so bright and colorful leaves me amazed. Gluing together 75 images is quite a task, well worth the work! Thank you for producing this image.

I don't believe that is Saturn at Equinox. Let me clarify: I DO believe it is Saturn; it just ain't Equinox. The shadows of the rings are "below" the plane of the rings, implying the Sun is "above" that plane. Since it is a composite of lots of images, perhaps ONE of those was truly during Equinox?

bill.starguy wrote:I don't believe that is Saturn at Equinox. Let me clarify: I DO believe it is Saturn; it just ain't Equinox. The shadows of the rings are "below" the plane of the rings, implying the Sun is "above" that plane. Since it is a composite of lots of images, perhaps ONE of those was truly during Equinox?

Photojournal wrote:...The images were taken on Aug. 12, 2009, beginning about 1.25 days after exact equinox

Your statement is true about the illumination direction; it's from above the ring plane. However, all of the pictures are after the exact equinox. The opening angle to the sun at the start of the image sequence = +0.019°, and increased to +0.024° at the end of 8 hours, corresponding to a rate ≈2.3 arcseconds/hour.

When you look at the large image, zoom into the shadow of Saturn on the rings to the right and look near the centre of the shadow upon the rings, there is a faint pinprick of light in there, could that be one of the moons and if so, does anyone know which one it may be?
Or could that be some sort of artifact..which I don't think it could be really, it doesn't look like any artifact I've ever seen before.

Thanx in advance!

Chappy wrote:When you look at the large image, zoom into the shadow of Saturn on the rings to the right and look near the centre of the shadow upon the rings, there is a faint pinprick of light in there, could that be one of the moons and if so, does anyone know which one it may be?
Or could that be some sort of artifact..which I don't think it could be really, it doesn't look like any artifact I've ever seen before.

Thanx in advance!

It could easily be either a hot pixel or a tiny moon. It is easier to tell when looking at the raw data but I am currently unwilling to put forth the effort to make the determination... call it laziness or just wanting to do something else with my time. Either one works.

Didn't think of a hot pixel, definitely could be one.
I won't be the one to judge you geck so I'll just believe you're busy doing other things

Chappy wrote:Didn't think of a hot pixel, definitely could be one.
I won't be the one to judge you geck so I'll just believe you're busy doing other things

Within the shadow of Saturn, I (just) see a small clump of about 10 dim pixels, which appear to be between the planet and the innermost part of the rings. I don't think there are any moons in between the planet and the rings, so it would have to rule out a moon. There was quite a lot of processing done to this image, so my best guess would be a processing artefact, but a longshot guess might also be a star shining through the gap between the planet and rings.

Nitpicker wrote:

Chappy wrote:Didn't think of a hot pixel, definitely could be one.
I won't be the one to judge you geck so I'll just believe you're busy doing other things

Within the shadow of Saturn, I (just) see a small clump of about 10 dim pixels, which appear to be between the planet and the innermost part of the rings. I don't think there are any moons in between the planet and the rings, so it would have to rule out a moon. There was quite a lot of processing done to this image, so my best guess would be a processing artefact, but a longshot guess might also be a star shining through the gap between the planet and rings.

If you open up the 83Mb tif file the dots are the clearest and on the right (most enhanced) side I found 13 dots of varying brightness: 4 are outside of the planet shadow (2 in the rings), and 9 in the planet shadow (4 between the planet and rings, 5 within the rings). Given that

Photojournal wrote:...Other bright specks are background stars.

I'd say most, if not all, are all stars. Moons don't show in the planets shadow, and they have the appearance of real sources.

alter-ego wrote:

Nitpicker wrote:

Chappy wrote:Didn't think of a hot pixel, definitely could be one.
I won't be the one to judge you geck so I'll just believe you're busy doing other things

Within the shadow of Saturn, I (just) see a small clump of about 10 dim pixels, which appear to be between the planet and the innermost part of the rings. I don't think there are any moons in between the planet and the rings, so it would have to rule out a moon. There was quite a lot of processing done to this image, so my best guess would be a processing artefact, but a longshot guess might also be a star shining through the gap between the planet and rings.

If you open up the 83Mb tif file the dots are the clearest and on the right (most enhanced) side I found 13 dots of varying brightness: 4 are outside of the planet shadow (2 in the rings), and 9 in the planet shadow (4 between the planet and rings, 5 within the rings). Given that

Photojournal wrote:...Other bright specks are background stars.

I'd say most, if not all, are all stars. Moons don't show in the planets shadow, and they have the appearance of real sources.

I was originally looking at the ~630 kB APOD:
http://apod.nasa.gov/apod/image/1409/sa ... i_7227.jpg

But your comment made me go and download the 83 MB tif ... and I can still only see a single clump of dim pixels in the planet's shadow, and nothing else in the image apart from the obvious bright moons (and Saturn!). I didn't bother to dim the office lights or clean my monitor, but you must have some pretty keen eyesight there, alter!