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# Seeing Encke!

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### #26 David Knisely

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Posted 03 February 2005 - 02:18 PM

Scratching some quick equations on the back of the proverbial envelope...

According to Sidgwick (p.50) the Cassini division has a width of 0.5". If this corresponds to a width of 4,500 km as mentioned above, then the Enke division at 350 km has an apparent width of only about 0.04". Sidgwick (pg. 50) also states that the visibility of a "single dark line on light ground" (with Cassini as an example of such), is on average 1/5 of the Raleigh limit (R'), with values quoted up to 1/15 of R' in the case of exceptional observers and/or equipment.

At 1/5 R', a scope in excess of 20 inches aperture would be required to see the division.

At 1/15 R', a scope of nearly 8 inches would be required to see the division.

Interesting discussion.

This would, of course, be valid *if* the Encke Division was sitting all by itself in the middle of a white expanse with no other detail near it. It isn't. Again, the problem is that the Encke division sits in a brightness falloff (a more greyish portion of the A-ring) and is simply too close to the outer edge of the A-ring to be easily resolved in the diffraction sense. At *best*, it lies only 0.5 arc seconds from the edge (and much of the time, it isn't even that far in angle from the edge). At minimum one needs an aperture that can do that (ie: one larger than about 9.12 inches), and certainly a larger aperture would have a better chance of detecting this feature. In fact, the minimum aperture to see the Cassini Division is also constrained more by this resolution limit (2.4 arc seconds from the outer edge of the A-ring for the case of the Cassini Division) than by the width of the division itself. Clear skies to you.

### #27 EdZ

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Posted 03 February 2005 - 02:24 PM

Ed,
I have seen Saturn through a 7" f/12 refractor on a night of sub 0.1 arc-second seeing. Magnification was over 1000X,
Yet no one could see the Encke Minima for sure, and CERTAINLY no one could see the Encke Division.
So I will empirically verify your supposition about conditions required to see the Minima, let alone the Gap/Division, which I think requires a lot more scope.
Nice analysis, but VERY optimisitic.

Don,
You are correct to point out this analysis is VERY optimistic. It originally was intended to show the most optimistic of all events would be required to support some past claims and even some current claims of small scope/low power observations.

As you read down through this thread, you will see additional information, for instance, NASA data on the size of the division and others comments on diffraction affects and verification of historical observations, that go hand in hand with a more stringent approach to analysis and come much closer to being in line with your experiences.

edz

### #28 cildarith

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Posted 03 February 2005 - 02:46 PM

Thanks for the clarification edz and David.

How reliable to you feel the value R/5 is for this sort of analysis. The study quoted by Sidgwick is 90 years old, has anything been done on this subject since?

### #29 EdZ

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Posted 03 February 2005 - 03:20 PM

Thanks for the clarification edz and David.
How reliable to you feel the value R/5 is for this sort of analysis. The study quoted by Sidgwick is 90 years old, has anything been done on this subject since?
--------------------
Eric

I have no reason to dispute it. Consider it an average guidline, not an exact number.

The values and formula referred to by Sidgwick that were determined by Lord Rayleigh for the size of the Airy disk are older than that and they are indisputable.

edz

### #30 lighttrap

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Posted 03 February 2005 - 07:16 PM

I'm (in)famous for "not doing math". So, instead, I did something that I enjoy. I surfed up what telescope Encke would've used to discover the (in)famous division that bears his name. It was a 9" Fraunhofer refractor. You can read a minimal amount about it here in A brief History of Astronomy in Berlin and here, more history of astronomy in Berlin

I guess we can toss the concept of being able to use 1/2 the discoverer's aperture???

### #31 Ron B[ee]

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Posted 03 February 2005 - 07:31 PM

There are posts by people who claimed to have seen Encke with a 6" or 7" scope at 250x to 300x! I have doubted this, but the standard answer is usually "I know what I saw." The real explanation might lie in just exactly what it is that was seen. You just can't say what it is that someone else sees, but you can say what they should be able to see.

Well history said 6.2" (by W. S. Jacob) seemed to have been the smallest. I have this book by Alexander that has much details about observation by Kater (6.25"), Encke, Lassell and Dawes (6.5" refractor), etc. In all cases, very high power and good condition was stated. And on top of this, no one had ever seen the division before!!!

Now I wonder if because these classical observers seemed to be able to keep at it night after night and years after years and for this reason they had better probability at "catching it at the right time"? Whereas, most of us might not have been able to spend as much time because we're all amateurs and have other life's demands ?

Ron B[ee]

### #32 lighttrap

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Posted 03 February 2005 - 07:46 PM

Now I wonder if because these classical observers seemed to be able to keep at it night after night and years after years and for this reason they had better probability at "catching it at the right time"? Whereas, most of us might not have been able to spend as much time because we're all amateurs and have other life's demands

That plus no light pollution, and a kind of dedication to discovering the new, not just relocating what we moderns can read about. Of course, it was that sense of discovery that presumably led to the "discovery" of the canals on Mars that have been debunked. But, I digress.

### #33 Starman1

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Posted 03 February 2005 - 07:57 PM

My assumption about size of scope needed was based on the idea that Keeler first observed the "Encke Division" while viewing with with the 36" refractor in 1888. Other observers may have seen it earlier (and, indeed, both Encke and Keeler may have seen it earlier).
Here is a web site telling the history of its having been observed:
Encke Minima and Division
It is unclear exactly what aperture can see it, based on these reports.
My 1987 observation at over 1100X magnification in a 7" f/12 refractor (Parks 3.8mm, 2X Barlow) could have seen it, but the rings may not have been at the correct angle. And seeing like that is a rare occurrence.
I guess I'd have to encourage everyone to look for himself.

### #34 David Knisely

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Posted 04 February 2005 - 01:21 AM

There are posts by people who claimed to have seen Encke with a 6" or 7" scope at 250x to 300x! I have doubted this, but the standard answer is usually "I know what I saw." The real explanation might lie in just exactly what it is that was seen. You just can't say what it is that someone else sees, but you can say what they should be able to see.

Well history said 6.2" (by W. S. Jacob) seemed to have been the smallest. I have this book by Alexander that has much details about observation by Kater (6.25"), Encke, Lassell and Dawes (6.5" refractor), etc. In all cases, very high power and good condition was stated. And on top of this, no one had ever seen the division before!!!

Now I wonder if because these classical observers seemed to be able to keep at it night after night and years after years and for this reason they had better probability at "catching it at the right time"? Whereas, most of us might not have been able to spend as much time because we're all amateurs and have other life's demands ?

Ron B[ee]

Again, the problem is that the early reports of the feature called "the Encke Division" early on usually put it in the wrong place (closer to the center of the A-ring). Some even put multiple divisions of high contrast in the A-ring where we know none really exist. This is the same problem as contemporary amateurs have, there is the "illusion" caused by the brightness profile of the rings which can trick the eye into seeing a division in the A-ring where none is really there (the so-called "Encke Minimum"). The Keeler drawing done at Lick Observatory is probably one of the best representation of the rings prior to the Voyager images, and it shows only the fine division near the outer edge of the A-ring in very nearly the same location as is shown in the Voyager images. This small but prominent division really should have been named "the Keeler Gap", but the IAU gave it to Encke instead (Keeler got a much finer gap shown in the Voyager images named after him). In any case, I have severe doubts about many of those early sightings of the Encke Division, especially in the light of recent spacecraft observations. Clear skies to you.

### #35 Dave Mitsky

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Posted 04 February 2005 - 08:55 AM

I observed Saturn on Tuesday night with the ASH 17 inch f/15 classical Cassegrain. The seeing was fairly good and 404x produced a pleasing image. The C Ring was obvious. I saw no hint of the Encke Division. Of course, I wasn't surprised since I've never seen a hint of it with telescopes as large as 20 inches.

Dave Mitsky

### #36 Astrosetz

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Posted 04 February 2005 - 09:16 AM

I have only seen the Encke Division twice in my life. Both times happened to be at the Great Plains Star Party in consecutive years, in a sort of a ground fog after midnight. Both times we had my 22" Starmaster up to 1,100x. The first time the seeing lasted for about 2.5 hours, and the second time (almost exactly a year later!) it lasted less than an hour. Rick Singmaster was observing with me both times, and the second time we also had a crowd of about 30 people who took their turns at the eyepiece.

On that second occasion we also saw a y-shaped marking on Ganymede. I don't know if I'll ever get seeing like that again, but it certainly motivates me to precisely collimate every time I set up, because you just never know...

### #37 Redfish

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Posted 04 February 2005 - 07:10 PM

Tonight I decided to try my shorty plus on saturn. It gave me about 500 magnification with my 6mm eyepiece. Seeing wasn't very great, but 360x magnification wasn't a problem and gave about 4 out of 5 seconds clear steady views of saturn.
With 500 magnification the image blurred a bit and about 1 out of 5 seconds was clear image, but i had trouble focusing, the limits of my focuser were reached. I only once thought i saw a hint of encke, cassini was very clear, as were bands on the planet and also the shadow on the ring itself. I'll just have to wait for better seeing.

### #38 matt

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Posted 07 February 2005 - 07:55 AM

I think I should also throw into the ring (pun intended) that a lot of observers are also influenced by the very nice CCD pictures taken by amateurs these last years; and some among the best and most honest of astroimagers admit that some of the features of the outer A ring their pictures show are actually artifacts of the image processing.

### #39 Guest_**DONOTDELETE**_*

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Posted 07 February 2005 - 08:18 AM

The so-called "Ghost of Encke":
http://www.astro-ima...t_of_encke.html

### #40 EdZ

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Posted 07 February 2005 - 10:24 AM

Very interesting Link. And much to my pleasant surprise, guess where his article points to for a discussion on "Seeing Encke." Right back to this original thread post.

edz

### #41 Guest_**DONOTDELETE**_*

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Posted 07 February 2005 - 10:29 AM

I saw that too Ed, kind of like a revolving discussion on the Encke in cyberspace if you will.

### #42 photonovore

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Posted 15 February 2005 - 04:20 PM

Did anyone consider the fact that the ring structure of saturn is not fixed but dynamic over time?

For example, what we now know to be a result of the varying density of the particulate matter *within* the cassini division was observationally documented over the last 150 years as varying transparencies of the cassini division itself (photometric measurements during star occultations proved handy in this reagrd). That other major divisions of the ring system--like encke-- may likewise vary in transparency (and visability) at different timeframes seems more reasonable than assuming them to be steady state features --as is unavoidably indicated by the short period nature of the observational timeframes of the cassini and voyager probes. But over timeframes of years the density of particulates within, and thus visability, of ring features may vary considerably--such is in fact rather firmly indicated in the long-term observation record documented in Price's book, the Planetary Observer's Handbook, Saturn chapter.

Food for thought when considering the veracity of observing reports concerning this feature from the past....I'm not implying any changes night to night or even month to month but more on a year to year, decade to decade basis.

Also something else to consider-- concerning not the transparency of the divisions but their actual width- is that the shephard moons of saturn define the rings themselves and the divisions within them. For example the moon mimas is considered to be in resonance with the cassini division... and another shepherding satellite *within* the encke division is the responsible party for that gap's existance. (see further details here) Now, since Saturn is a rather fluid planet and thus subject to more physical deformations in it's structure than solid planets, and since these deformations would likely have gravatational ramifications affecting the orbits of the various near satellites, it's reasonable to consider that the gaps themselves may not be of a static width over time, just as saturn herself is not a static body over time. Again these would be discreet variations, not gross variations--but perhaps enough to affect visability of these features from our point-of-view?

All considered encke division visability may not be so simple to answer as an ordinary arc second/resolution calculation would indicate.

### #43 David Knisely

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Posted 15 February 2005 - 05:26 PM

Did anyone consider the fact that the ring structure of saturn is not fixed but dynamic over time?

For example, what we now know to be a result of the varying density of the particulate matter *within* the cassini division was observationally documented over the last 150 years as varying transparencies of the cassini division itself (photometric measurements during star occultations proved handy in this reagrd). That other major divisions of the ring system--like encke-- may likewise vary in transparency (and visability) at different timeframes seems more reasonable than assuming them to be steady state features --as is unavoidably indicated by the short period nature of the observational timeframes of the cassini and voyager probes. But over timeframes of years the density of particulates within, and thus visability, of ring features may vary considerably--such is in fact rather firmly indicated in the long-term observation record documented in Price's book, the Planetary Observer's Handbook, Saturn chapter.

Food for thought when considering the veracity of observing reports concerning this feature from the past....I'm not implying any changes night to night or even month to month but more on a year to year, decade to decade basis.

Also something else to consider-- concerning not the transparency of the divisions but their actual width- is that the shephard moons of saturn define the rings themselves and the divisions within them. For example the moon mimas is considered to be in resonance with the cassini division... and another shepherding satellite *within* the encke division is the responsible party for that gap's existance. (see further details here) Now, since Saturn is a rather fluid planet and thus subject to more physical deformations in it's structure than solid planets, and since these deformations would likely have gravatational ramifications affecting the orbits of the various near satellites, it's reasonable to consider that the gaps themselves may not be of a static width over time, just as saturn herself is not a static body over time. Again these would be discreet variations, not gross variations--but perhaps enough to affect visability of these features from our point-of-view?

All considered encke division visability may not be so simple to answer as an ordinary arc second/resolution calculation would indicate.

Well, the Encke division exists due to the presence of the tiny moon Pan which orbits within the division. Its affect is fairly uniform and stable (ie: it either kicks stuff out of the division or physically sweeps it up). If its orbit got wild or changed, Pan would probably disrupt a large portion of the ring structure or get itself pummeled into non-existance eventually, resulting in no division at all. Again, Keeler's fine drawing back in 1888 (done with the big Lick refractor) has the detail in pretty much the same location and intensity as that shown in the Voyager, HST, and Cassini images. Thus, it seems unlikely that the 'true' Encke division has moved significantly or changed width over the past 117 years or so. It still remains a challenge feature for telescopes 10 inches and larger in aperture. Clear skies to you.

### #44 peterg

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Posted 15 February 2005 - 08:49 PM

When my wife & I re-read our journals in 30 years we want to make sure we have the right terminology.

In our previous journals we first said that we see the Encke Division--that's wrong!

After reading previous posts we now say we see the Encke Illusion.

After tonight's post, however, should we start writing the Encke Ghost?

### #45 Guest_**DONOTDELETE**_*

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Posted 16 February 2005 - 02:53 AM

When in doubt, and if you are seeing some type of darkening within the ring itself, just call it the Encke minima.

### #46 photonovore

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Posted 16 February 2005 - 06:20 AM

Again, Keeler's fine drawing back in 1888 (done with the big Lick refractor) has the detail in pretty much the same location and intensity as that shown in the Voyager, HST, and Cassini images. Thus, it seems unlikely that the 'true' Encke division has moved significantly or changed width over the past 117 years or so. [/quote]

Hmm..you don't seem too familiar with the observational record-- do take a look at Price's chapter on Saturn--I think you will be suprised at the variations documented in Encke's visability over the years. Here's a few tidbits to whet your apetite...

The instruments used in the Encke division's discovery were Encke's 9" refractor at the Berlin Observatory (1837), Dawes' 9 inch reflector (1843) and Kater's 6.25" and 6.75" reflectors(1825). It was confirmed by Keeler in 1888. It's position is *mostly* stable, but Holden, using the 26 and 36 inch refractors noted it's positional shift from 1/3 to 2/5ths the distance from the edge of the A ring during a period of just 3 months in 1880. (Interestingly, the -oft seen by us- Crepe ring was not discovered until 1850 by G. Bond at Harvard College Observatory using their 15 inch refractor.)

But it is Encke's division that proved to be the most elusive feature---in 1880 only occassional glimpses of Encke's division were seen by aforementioned Holden, using 26" and 36" refractors. Hall, the discoverer of the satellites of Mars, however, was unable to see Encke's at all during this same timeperiod with a 26" refractor alone. By 1898, Barnard, using the Yerkes refractor (40") could discern "only a dusky shading where Encke's division is usually seen." By 1900, no one reported being able to observe the Encke division in any instrument of the day. Brut by 1917 Encke's division was once again being reported as a clear line in the same telescopes in which it was invisable a few years prior. So, yes, we must assume the Encke division's visability varies over time, lest we discount the sum observational record of amateurs and professionals alike. The best info on it's current visability would come from ALPO or the BAA i suspect. Both maintain formal observing programs on saturn.

Also, if you reread my post you will notice that i wrote not of "movement" but of discreet width and transparency variations in the two major divisions--tho as i related above movement *is* documented as well. You did mention something which could enter into this equation that i neglected to: ring matter collision events (internally caused as well as externally caused) and the resultant ejecta's possible effects upon transparency. In looking into this i found a reference from the University of Illinois which contends that there are a multitude of particle collisions during every rotation of the rings. Moonlet impacts are also not uncommon (according to NASA & ESA scientists) and according to Cassini researchers there are a multitude of undiscovered (due to resolution limits of the orbiter's imagers) moonlets whose presence is nonetheless evidenced by effects upon the ring's internal structures. What these moons are up to over the longer term of years and decades, and what their transient effects upon the ring's structures are, if any, no one knows.

There is a lot going on up there---our solar system is no more "steady state" than the universe is--the rings of Saturn included.

### #47 BillFerris

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Posted 16 February 2005 - 12:38 PM

Again, Keeler's fine drawing back in 1888 (done with the big Lick refractor) has the detail in pretty much the same location and intensity as that shown in the Voyager, HST, and Cassini images. Thus, it seems unlikely that the 'true' Encke division has moved significantly or changed width over the past 117 years or so.

Agreed. In the absence of persuasive evidence that the visual appearance of Saturn's ring divisions have changed, it's illogical to conclude they have.

I consider many of the Saturn observations from the 1800s to be of questionable accuracy. Seeing divisions in Saturn's rings grew to be as popular as observing canals on Mars. Yet, prior to 1850, nobody had reported seeing the C (Crepe) ring. The Crepe is a trivial detection in any good quality moderate aperture scope. If 19th century observers weren't able to detect the Crepe ring, I find it difficult to believe that they were observing extremely narrow ring divisions (such as Encke) that push the resolution limits of the apertures used.

One advantage contemporary observers have, is that reported observations of Encke can often be checked against images of Saturn taken within days--sometimes hours--of the observation. If observing conditions allowed detection of Encke, they also allowed much fine detail in Saturn's atmosphere to be seen. If those details, recorded in a sketch or carefully described in an observing report, match details captured in an image, then one has a reasonable case for claiming detection.

Regards,

Bill in Flagstaff

### #48 EdZ

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Posted 16 February 2005 - 12:58 PM

The instruments used in the Encke division's discovery were Encke's 9" refractor at the Berlin Observatory (1837), Dawes' 9 inch reflector (1843) and Kater's 6.25" and 6.75" reflectors(1825). It was confirmed by Keeler in 1888. It's position is *mostly* stable, but Holden, using the 26 and 36 inch refractors noted it's positional shift from 1/3 to 2/5ths the distance from the edge of the A ring during a period of just 3 months in 1880. (Interestingly, the -oft seen by us- Crepe ring was not discovered until 1850 by G. Bond at Harvard College Observatory using their 15 inch refractor.)

I believe if you read this entire thread you will find a great deal of discussion already took place which may cast doubt on some of those earlier findings. And very reasonable doubt at that.

And FWIW, I think David's knowledge of the observational record far exceeds most people's. That's kind of like telling Pedro he doesn't know what a curve ball is.

edz

### #49 David Knisely

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Posted 16 February 2005 - 04:51 PM

photonovore wrote:

Hmm..you don't seem too familiar with the observational record-- do take a look at Price's chapter on Saturn--I think you will be suprised at the variations documented in Encke's visability over the years.

I have had Price's book for many years and I have read it. It has some useful information but it also has a few inaccuracies which are troubling (it badly needs revision). In any case, all it does is basically repeat some portions of the reports of the early observers, and draws a few incorrect conclusions based on these accounts. The early purely visual accounts of Saturn's rings are clearly subject to some question here, as sometimes the "Encke" division was alleged to be seen and sometimes it wasn't. The variability of the accounts and, more importantly, exactly *what* they describe is fairly well known, as is the contrast effect which can cause some observer's to report a division where none really exists (the so-called "Encke Minimum"). Some showed a division near the middle of the A-ring (Encke himself tended to report it near there), while others showed it in different places (and some even showed three or more prominent divisions). However, a number of competent observers saw no divisions at all in the A-ring (Asaph Hall, using the 25 inch Washington Refractor never saw the Encke Division as a gap, and in 1898, E.E. Barnard didn't see it with the 40 inch for two examples). Even in the same apparition, some reported a division and some did not, even under good seeing conditions (but perhaps not good enough to pick out the fine gap near the ring edge). It was never really clear here exactly what was being seen, and it was this inconsistency which puts many of these early observations in doubt. Again, Keeler's drawing was probably the most detailed and consistent one at the time (1888). Voyager 1 in 1980 imaged only the very fine division near the outer edge of the A-ring (nearly 80% from the Cassini Division to the outer edge of the A-ring), which is *now* known as the Encke Division. It is thus somewhat farther out (and narrower) than most observers reported it to be (except perhaps Keeler, who drew it in about the correct position). Since then, images by HST and Cassini have shown it to be in nearly the very same position with about the same width as it had been imaged in 1980, so at least over a 25 year period, this very narrow division has not changed significantly, nor have other prominent ones tended to appear. In any case, the facts clearly state that there is only one prominent division in the A-ring, it has been in the same location for at least 25 years (and possibly as long as 117 years), and it is probably not visible in apertures significantly smaller than 10 inches. Clear skies to you.

### #50 photonovore

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Posted 16 February 2005 - 05:41 PM

I read the discussion.

I guess i am not comfortable with the casual dismissal of 150+ years of observational record created by observers as dedicated and skilled as any alive today in favor of an utterly unsupported long term steady state version of planetary ring structure as (uniquely) proposed here. That such a position runs contrary to present scientific knowledge about the dynamics of ring structure generally is a vast understatement. That it runs contrary to the long term observational record of Saturn, and specifically it's visible ring divisions, is a fact. But what most perplexes me is that even a casual reading of the literature on the subject indicates this most unambiguously? Contrary statements coming from someone here associated with an astronomical educational facility is therefore rather numbing, frankly. I can understand a contention that elements of the ring structure vary slowly--many decidedly do--but none observable at all over many decades!? I'm sorry-- but that's simply an absurdly unsupportable contention. Expecting one to defer to the opinion of posters on this forum over the professionals in the field, past & present, would be simply irrational-- yet that is the push i feel here. Although my past professional experience in science was limited to the field of veterinary bacteriology, I did manage to acquire the tools with which to properly evaluate scientific evidence along the way, therefore i feel no need to display any false modesty in my ability to do so here, "newby" or otherwise. I don't know what else to say except that my part in this discussion seems to have come to a rather disconcerting end.

PS to David Knisely; your response appeared after i posted the above. Thanks for the further explanation. I would only say in response that the probes, until the Cassini mission, have provided only observations which were severely constrained in their duration. Same for Hubble. Therefore, to date, the only long term evidence extant remains that of earth based observations. Based upon this evidence, structural variations of significant magnitude within the rings are indicated, generally. That the Encke division, alone among the features of Saturn's rings, would be exempt from such variations is not reasonable. Until a much longer term observational record from space is established than we presently possess, the earth based observational record must be given precedence over space based observations concerning speculation about the nature of long term variations in the ring structure simply due to the much longer span of time they document, if for no other reason. The size of telescope with which one could hope to glimpse the Encke division is irrelevant to me, frankly; I'm not into 'scoring' objects or obscure features. On the contrary, the information i presented was entirely about evidencing the presence of long term variations in the ring structure as seen from this planet as they relate to Encke division visibility, the subject of this discussion, and nothing more.

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