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Striking linear shadow feature

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#1 Gert

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Posted 14 October 2018 - 01:43 PM

Hello All,

 

The crescent Moon on Saturday was tempting and I wanted to spend some time watching it and maybe take a few pictures of the earthshine. Visually scanning the terminator with the 130mm lens a striking linear shadow caught my attention. It was jumping-in-your-face obvious in the eyepiece and later I found it on the DSLR images as well. The lack of dynamic range in the photos doesn't show it as distinctly as the visual impression. Looking up the Virtual Moon Atlas the shadow originates in the crater rims of Metius and Fabricius. Has anybody seen or documented this shadow-effect yet? Would be interested to hear from other observations.

 

Mond_20181013_Shadow_Fabricius_Metius_tx

 

Full size image here : http://skywatcher.sp..._Metius_txt.jpg

 

Thanks & Clear Skies,

Gert

 

 

 


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#2 John_Moore

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Posted 14 October 2018 - 04:05 PM

Gert, you have an extremely good eye.

 

I wondered initially if it might have been a shadow from some of the high terrain between craters Metius and Fabricius, and so creating similar conditions, your striking shadow does indeed show up. Looking at the overhead view (bottom image), it appears to be created by one or two small, but old-looking craters with low relief terrain in-between. It might be an old valley-like, gouging-type feature due to some impact event hundreds of kilometres away in either direction (that is, north-westwards or south-eastwards), but it doesn't seem to 'point' to any obvious one nearby.

 

Still, it does appear to exist. I suppose it's incorrect to call it a shadow, given the below views, so, perhaps, 'The Gert Feature' or 'The Gert Valley' (Vallis Gert) might suit as a better title.

 

John Moore (ps. I put in a small inset image just for approximate reference and location).

Click for larger view

StrikingShadow

Edited by Jayem, 14 October 2018 - 04:29 PM.

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#3 Gert

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Posted 14 October 2018 - 05:21 PM

Hi John,

 

You found an excellent image of the area. Good point that the 'feature' is a combination of topological structure and shadow effect. Perhaps a combination of depression in the topology combining with incident illumination at a specific time.

 

Best,

Gert (discoverer of Vallis Gert) :-)


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#4 Tom Glenn

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Posted 14 October 2018 - 11:20 PM

Interesting Gert.  I took a look at the LRO Quickmap, and moved the map around at different angles to observe the terrain.  As John says, this appears to be a valley or channel of sorts, but it is extremely worn, and is not at all obvious, even when looking at satellite images.  I had to click on the option for "big shadows" on the LRO map in order to fully appreciate what is going on here.  There are a collection of several features, which all work together to cause this effect, and your image happens to be at the exact right time for all of these to be working in concert.  

 

The feature seems to begin with the shadowed region of the northern rim of Fabricius, on the border with Metius.  It then progresses to the west, where it is bounded on the southern side initially by a raised portion of the outer terraced rim of Fabricius, and then by less obvious rolling terrain including the two worn craters that John mentioned.  On the northern side, the feature appears to be bounded by the rampart separating Fabricius and Metius, and then by the worn craters, and finally by the southern rim of the crater Brenner A.  It then continues for a bit, as seen in your image, being outlined by the southern rim of Brenner.  None of these features is very impressive individually, but with the right lighting, they do indeed appear to make a valley of sorts.  

 

Here is a figure I made from a few of the LRO images I used to look at the region.  The white arrows point to the feature, although the regions that are apparent at any given time will depend on the sun angle.  Note that the northern boundary is much better defined than the southern one.

 

Vallis_Gert.jpg


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#5 Tom Glenn

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Posted 14 October 2018 - 11:35 PM

I also looked to see if I had any images that show the region, and although I have plenty of images, I don't have one with comparable illumination to yours.  The closest I really have is an image taken from first quarter, which does not show any shadows, but does show the region, and even under this more direct lighting, you can still appreciate the features that contribute to what you saw.  

 

Fabricius_crop_TG.jpg


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#6 John_Moore

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Posted 15 October 2018 - 07:06 AM

Tom, the East view, I think, shows Vallis Gert best, and in your own July image it does just about show it up...a very nice shot.

 

Gert's terminator view - around Waxing Crescent times - seems to be about the best option in planning to catch it, as terrain height at the north-eastern sectors of the valley are highest and so potential for shadows forming increased (Waning Gibbous times, I'd imagine, wouldn't be as productive).

 

John Moore  


Edited by Jayem, 15 October 2018 - 11:09 PM.


#7 Gert

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Posted 15 October 2018 - 10:12 AM

Hi John, hi Tom,

 

Thanks for the excellent research on LRO Quickmap. Tom, esp. the 3D view is very interesting. Is there a tutorial on LRO Quickmap? I tried calling it in the browser and it seems very powerful, but I was unable to do an angled view.

 

Thanks,

Gert



#8 John_Moore

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Posted 15 October 2018 - 11:23 AM

Apologies: 3D explanation will leave it to you Tom...as he inquires/asks.

 

John Moore


Edited by Jayem, 15 October 2018 - 11:38 PM.


#9 doolsduck

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Posted 07 August 2019 - 02:39 AM

I got it too a couple of nights ago in an afocal image with an iPhoneSE and 100mm f/10 reflector.  The Moon was 23.6% illuminated.

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  • clairobscurfabricius - 1 (1).jpg

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#10 ed_turco

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Posted 10 October 2019 - 02:11 PM

With all due respect, there is something about the geometry of the shadow that doesn't ring true.  I'm not claiming that anyone didn't see this but take a look at the other shadows in the images.  For a given the same sun height. a craters' features causes shadows of similar length.  For higher sun angles the shadows are shortened.

 

It is this aspect that troubles me.  The length of that shadow is a surprise.  It has to be a high feature to cause that shadow's length, or the sun has to be very low to cause such a long shadow, and there are no other shadows nearby that approach that length of that fascinating line.

 

If there were a feature high enough to cause that feature, it ought to be high enough to stand out  when not causing a shadow, and yet it isn't.  I truly wonder what it is.


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#11 Tom Glenn

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Posted 10 October 2019 - 02:43 PM

With all due respect, there is something about the geometry of the shadow that doesn't ring true.  I'm not claiming that anyone didn't see this but take a look at the other shadows in the images.  For a given the same sun height. a craters' features causes shadows of similar length.  For higher sun angles the shadows are shortened.

 

It is this aspect that troubles me.  The length of that shadow is a surprise.  It has to be a high feature to cause that shadow's length, or the sun has to be very low to cause such a long shadow, and there are no other shadows nearby that approach that length of that fascinating line.

 

If there were a feature high enough to cause that feature, it ought to be high enough to stand out  when not causing a shadow, and yet it isn't.  I truly wonder what it is.

The visual effect is quite real, and is confirmed by multiple photographs from Earth.  As was described above, however, the feature is not a single shadow cast by a single object.  You are quite correct that if it were a single shadow, then the object creating it would have to be obvious.  But as was described above in multiple posts, the feature is actually a collection of many smaller shadows, created by complex terrain, that give the impression of a single shadow when viewed at low resolution from Earth.  In particular, look at the images in John's post #2 above.  It is actually quite common for apparent features observed from Earth to look completely different when viewed in high resolution spacecraft imagery.  In particular, many shadows that appear as complete when viewed from Earth are actually quite complex, with a mix of shadow and sunlight throughout, but this is completely out of the range of Earthbound resolution, and so the entire structure appears dark when viewed with amateur scopes.  


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#12 John_Moore

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Posted 10 October 2019 - 04:51 PM

Just a snippet of 'doolsduck''s image (below): I'll know he'll forgive me for editing; showing the directions of light and shadow effects.
 
So, descriptive if not explanatory, sunlight rays striking our Earth and Moon at such distances are 'really' parallel-like (slight curvature of the Moon noted)
 
John Moore
 
LinearFeature

 


Edited by John_Moore, 11 October 2019 - 03:20 AM.

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#13 doolsduck

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Posted 11 October 2019 - 12:28 AM

It really does look weird on the camera display/phone too if you are lucky enough to be looking at the right time.  I remember wondering at the time whether some portion of my phone display had died and caused a black line.  It really doesn't gel with what the brain is interpreting from the surrounding shadows.  It is quite real though.


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#14 ed_turco

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Posted 12 October 2019 - 10:14 AM

I never said that it wasn't real.



#15 frank5817

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Posted 12 October 2019 - 12:31 PM

Here is another long dark narrow shadow that was visible last night between Grimaldi and Hevelius pointing at smaller Lohrmann between them.

 

Frank

 

DSC00693.JPG



#16 John_Moore

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Posted 14 October 2019 - 06:11 AM

Nice catch, Frank.

 

It looks like a gouging, gash-like effect from ejecta coming from some source, however, what that source is, is anyone's guess (Riccioli, Orientale off to the southwest?). In the LROC link here (the gash lies approximately near and between Riccioli G and Lohrmann B); after when one zooms out, it doesn't seem to point exactly towards Orientale's approximate centre. That said, there are, literally, hundreds of these small, gash-like features in the general area that do. PS. The nomenclature can be turned off by going to the 'three-layer' icon at top-left, and un-tick Nomenclature.

 

John Moore  


Edited by John_Moore, 14 October 2019 - 06:12 AM.


#17 frank5817

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Posted 14 October 2019 - 08:24 PM

John,

 

Thank you, yes the LPOC feature is a very useful tool for helping with details.

 

Frank


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#18 ed_turco

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Posted 18 October 2019 - 09:50 AM

The visual effect is quite real, and is confirmed by multiple photographs from Earth.  As was described above, however, the feature is not a single shadow cast by a single object.  You are quite correct that if it were a single shadow, then the object creating it would have to be obvious.  But as was described above in multiple posts, the feature is actually a collection of many smaller shadows, created by complex terrain, that give the impression of a single shadow when viewed at low resolution from Earth.  In particular, look at the images in John's post #2 above.  It is actually quite common for apparent features observed from Earth to look completely different when viewed in high resolution spacecraft imagery.  In particular, many shadows that appear as complete when viewed from Earth are actually quite complex, with a mix of shadow and sunlight throughout, but this is completely out of the range of Earthbound resolution, and so the entire structure appears dark when viewed with amateur scopes.  

I agree with your appraisal.  But among those multiple objects, doesn't at least one have to make the longest shadow?  If so, we're back to the geometry problems again.



#19 John_Moore

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Posted 18 October 2019 - 12:13 PM

Below impossible lit views (from North and South).

 

John Moore

 

NorthSouth


#20 ed_turco

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Posted 18 October 2019 - 02:36 PM

I'm only trying to understand.  Physical laws dictate how things happen.   I still have to ask ,where is that one object causing that long shadow.  By the way,I do love your book!



#21 Tom Glenn

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Posted 18 October 2019 - 04:11 PM

I agree with your appraisal.  But among those multiple objects, doesn't at least one have to make the longest shadow?  If so, we're back to the geometry problems again.

Ed, the issue here is one of perspective and resolution.  What appears as a solid black line in the photos (a shadow), is not a solid black line, and hence, not a long shadow.  It may help to think about what happens if you view a series of closely spaced lines from far away.  Unless you have extraordinary vision like an eagle, you will perceive the closely spaced lines as a single solid line if you stand far enough away.  If you come closer to the chart, you would see that the solid line isn't actually solid.

 

Look at the simulations below, taken form LTVT using the time of the original observation by the OP.  In the second image, I have overlaid a white line corresponding to the perceived "shadow", and yellow lines pointing towards the Sun.  The perceived feature is not a single cast shadow, and is not pointing towards the Sun.  The effect is created by many small shadows created by local terrain.  These small shadows appear to merge together if viewed from far away.  If you zoom in on the image, it's not very impressive.  But if you reduce the size until it looks as small as it might appear in a telescope, you see a solid black line that gives the false impression of a single cast shadow.  I suppose you could call it an illusion, although there really are shadows there, just small ones.  

 

feature1.jpg

 

feature2.jpg


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#22 John_Moore

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Posted 19 October 2019 - 03:26 PM

Thanks, Ed (ed-turco), for the nice comment re: book waytogo.gif and also enjoyed reading your S&T articles.

 

John



#23 JoeInMN

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Posted 21 October 2019 - 11:37 AM

Ed, the issue here is one of perspective and resolution.  What appears as a solid black line in the photos (a shadow), is not a solid black line, and hence, not a long shadow.  It may help to think about what happens if you view a series of closely spaced lines from far away.  Unless you have extraordinary vision like an eagle, you will perceive the closely spaced lines as a single solid line if you stand far enough away.  If you come closer to the chart, you would see that the solid line isn't actually solid.

Interesting how this somewhat recalls the once widespread but completely illusory perception of Martian canals. I guess it would be kind of hard for even Lowell to explain why the Lunarians would be building a canal on a moon without any water, though...




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