<p>Think about the patterns that you have seen on the bottom of a swimming pool when the sun shines through ripples on the surface.  These are a related phenomenon.  When the sun shines through a medium that is &quot;uneven&quot; it is refracted and a pattern of brighter and darker shapes appears. The particular appearance of particularly bright and dark lines is due to the formation of &quot;caustics&quot;.  Now the pattern that is created by ripples is much stronger as the refraction at the water's surface is very strong.  But the same occurs when sunlight shines through the atmosphere.  You won't notice it when the sun in un-obscured, since the size of the sun does have a &quot;smoothing&quot; effect. But it does become visible when the sun is reduced to a sliver that is a fraction of an arcminute thick.</p>
<p>The orientation in bands is due to the fact that the sliver of light is a band, extremely narrow, but quite long (relative to it's width) All shadows are affected by this change.</p>

CC BY-SA 4.0
2024-04-08T04:57:45.687Z
"typo" is a kind word...
James K
CC BY-SA 4.0
2024-04-10T20:14:20.527Z
Hi, I'm the OP (having problems with my 'New Editor' account). Thanks for explaining the shadow bands phenomenon, but I'm still unsure about the answer on my specific question. I guess that the atmosphere also have a wavy surface extending mainly in one dimension like the surface of water (not zero dimensional bumps like I thought before). And I guess that the shadow bands are only happening when the orientation of the sliver of the sun aligns with the orientation of the atmospheric surface waves, and not when they're perpendicular to each other. Are these statements correct?
George Lee
CC BY-SA 4.0
2024-04-07T19:27:59.983Z
2024-04-08T04:57:31.420Z
2024-04-08T04:57:31.420Z
James K
<blockquote>
<p>How can atmospheric turbulence obscure the sun-crescent immediately before a total solar eclipse? and why are they 'bands'?</p>
</blockquote>
<p><strong>tl;dr:</strong> Refraction by atmospheric turbulence <strong>redirects</strong> the almost-parallel light just before eclipse making it more concentrated in some areas and less in others rather than by &quot;obscuring&quot; it. Stars twinkle for the same reason. If a twinkling star was really bright, it would create similarly visible patterns.</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/27226/7982">Would an extremely bright star produce same kind of shadow band effect as seen in solar eclipses just before totality?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/27221/7982">What is causing this strange shimmering pattern of light during a total solar eclipse?</a></li>
<li>Smarter Every Day video <a href="https://youtu.be/lepQoU4oek4?t=264" rel="nofollow noreferrer">Space Station Transiting 2017 ECLIPSE, My Brain Stopped Working - Smarter Every Day 175 at about 04:30</a></li>
</ul>
<p><a href="https://astronomy.stackexchange.com/a/57381/7982">JamesK's answer</a> is correct but lacks supporting sources.</p>
<p>We cause these wavy alternating lighter and darker patterns that aren't really band-like <a href="https://en.wikipedia.org/wiki/Caustic_(optics)" rel="nofollow noreferrer">caustics</a>. Random refraction from inhomogeneities in the Earth's atmosphere happens all the time, but only when the light becomes collimated (almost parallel) can we see it. Random deflection of light causes more light in some places and less light in others. There's not really any loss of light or blockage or obscuring, it's just uneven distribution.</p>
<p>The image below shows a plate of normal, transparent glass with a &quot;wiggly&quot; uneven surface generate a portrait of Alan Turing using caustics. For every dark area, there's a nearby light area where the light has been re-directed, not blocked.</p>
<p>From Wikipedia's <a href="https://en.wikipedia.org/wiki/Shadow_bands" rel="nofollow noreferrer">Shadow bands</a></p>
<blockquote>
<p>Shadow bands are thin, wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-coloured surfaces immediately before and after a total solar eclipse. They are caused by the refraction by Earth's atmospheric turbulence of the solar crescent as it thins to a narrow slit, which increasingly collimates the light reaching Earth in the minute just before and after totality.</p>
<p>The shadows' detailed structure is due to random patterns of fine air turbulence that refract the collimated sunlight arriving from the narrow eclipse crescent.</p>
<p>The bands' rapid sliding motion is due to shifting air currents combined with the angular motion of the Sun projecting through higher altitudes. The degree of collimation in the light gradually increases as the crescent thins, until the solar disk is completely covered and the eclipse is total.</p>
<p><strong>Stars twinkle for the same reason.</strong> They are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence which acts like lenses and prisms diverting the light's path. Viewed toward the collimated light of a star, the shadows bands from atmospheric refraction pass over the eye.</p>
</blockquote>
<p>Also, from Physics SE's <a href="https://physics.stackexchange.com/q/637536/83380">What is this wavy light coming through my blinds?</a></p>
<p><a href="https://i.sstatic.net/i0bhm.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/i0bhm.gif" alt="enter image description here" /></a></p>
<p>and from <a href="https://physics.stackexchange.com/a/637662/83380">my answer therein</a></p>
<blockquote>
<p>The video <a href="https://youtu.be/0NXNAIqU8KM" rel="nofollow noreferrer">Taming light reflection to create images</a> discusses engineered caustics (refractive rather than reflective in this case). See also <a href="https://lgg.epfl.ch/caustics_project_smallturing.php" rel="nofollow noreferrer">this post</a> from the EPFL Geometric Computing Laboratory:</p>
<p><a href="https://i.sstatic.net/O6KMr.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/O6KMr.jpg" alt="from the EPFL Geometric Computing Laboratory" /></a></p>
<p>[...]</p>
<p>From James Gurney's blogpost <a href="http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html" rel="nofollow noreferrer">Caustic Reflections</a></p>
<p><a href="https://i.sstatic.net/uIetm.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/uIetm.jpg" alt="From James Gurney's blogpost Caustic Reflections http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html" /></a> <a href="https://i.sstatic.net/cj5g9.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/cj5g9.jpg" alt="From James Gurney's blogpost Caustic Reflections http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html" /></a></p>
<p>From forums.sketchup.com's <a href="https://forums.sketchup.com/t/water-reflection-on-surfaces/79291" rel="nofollow noreferrer">Water Reflection on Surfaces</a>:</p>
<p><a href="https://i.sstatic.net/gIyRp.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/gIyRp.jpg" alt="from forums.sketchup.com's Water Reflection on Surfaces https://forums.sketchup.com/t/water-reflection-on-surfaces/79291" /></a></p>
</blockquote>

CC BY-SA 4.0
2024-04-10T20:13:41.877Z
Hi, I'm the OP (having problems with my 'New Editor' account). Thanks for explaining the shadow bands phenomenon, but I'm still unsure about the answer on my specific question. I guess that the atmosphere also have a wavy surface extending mainly in one dimension like the surface of water (not zero dimensional bumps like I thought before). And I guess that the shadow bands are only happening when the orientation of the sliver of the sun aligns with the orientation of the atmospheric surface waves, and not when they're perpendicular to each other. Are these statements correct?
George Lee
CC BY-SA 4.0
2024-04-10T20:38:44.907Z
@GeorgeLee it's more that the *density* of the air fluctuates, rather than the "surface*. This causes the refraction direction of different parts of the air to differ. There's no alignment required, just a nearly parallel (collimated) beam of light. We get that from stars (point sources) and the slivers and dots of light around the edge of the moon moments before totality.
uhoh
CC BY-SA 4.0
2024-04-11T15:43:22.630Z
It doesn't really matter whether it's the density or surface which fluctuates, what matters is whether they (whatever causes the shadows) are present at random spots or are they forming lines. The former is more intuitive, but the shadow bands hints towards the latter.
George Lee
CC BY-SA 4.0
2024-04-11T18:42:19.387Z
@GeorgeLee I have not seen a good, clear set of images proving that the phenomenon really presents as roughly parallel bands, but I won't be surprised. While a star's light is collimated in all directions, moments before totality the sunlight is coming from a long "slit-shaped" source - collimated in only one direction. So if there were really "bands" with a predominant orientation, I would expect them to be oriented parallel to the crescent's brightest edge.
uhoh
CC BY-SA 4.0
2024-04-11T20:26:32.483Z
*parallel to the crescent's brightest edge* ...**and** to the orientation of the different parts (different densities) of the atmosphere. Meaning that they (the atmospheric parts) must form bands, and they must be parallel to the crescent.
George Lee
CC BY-SA 4.0
2024-04-11T23:33:57.633Z
@GeorgeLee No it does not mean that. The inhomogeneities will not have directionality. Visibility comes from the collimation of the light, and that happens preferentially in one direction. Just like the GIF in my answer - the shades are slits in one direction and long in the other, so the isotropic puddle waves, viewed through the  narrow shade slits, show up as "band-like"  patterns. Better start by showing what the eclipse "bands" actually look like, they may not be as distinctly "band-like" as you imagine.
uhoh
CC BY-SA 4.0
2024-04-15T03:09:33.860Z
The shadow bands look roughly parallel, with a fairly well-defined direction of motion. See https://youtu.be/rjLZGOBQ4zA especially around 40s.
PM 2Ring
CC BY-SA 4.0
2024-04-15T03:45:34.530Z
@George See the "Caustic Projections" in the left part of https://i.sstatic.net/cj5g9.jpg . All the rays still reach the ground, but some patches get more rays and some patches get fewer rays.
PM 2Ring
CC BY-SA 4.0
2024-04-15T03:52:27.993Z
Here's the classic shadow bands lithograph by Müller from the 1870 eclipse https://www.strickling.net/shadowbands1870large.png from Dr Wolfgang Strickling's page https://www.strickling.net/shadowbands.htm That site has shadow band photos & videos, but they're all very vague.
PM 2Ring
CC BY-SA 4.0
2024-04-15T04:40:33.933Z
@PM2Ring oh that's great! There are several links to videos in YouTube. The 2021 [2nd](https://youtu.be/Egh6aNu6rZE) and [3rd](https://youtu.be/Ma1CTLoBQ0g) contact videos from 2001-June-21 sort-of show some wiggliness - I wish there was a definitive video using the best equipment and a flat screen to finally characterize a pattern to see if "parallel bands" is really what they look like, or what lithographers reconstruct. **update:** OK [here are some](https://youtu.be/-NBoOxzumfs)! The "snakes" or bands seem to go from lower-left to upper-right? [Here are more](https://youtu.be/1I4YNa5-nuk).
uhoh
CC BY-SA 4.0
2024-04-15T23:25:19.040Z
@PM2Ring The drawing from 1870 is very exaggerated, and we can't really learn from it. Anyway, my question was not **if** the band are parallel, but rather **why**.
George Lee
CC BY-SA 4.0
2024-04-15T23:32:03.280Z
@GeorgeLee OK great! I was about to add a bounty to your question (I love adding bounties!) but I see that you've done it already. If you still don't get a satisfactory answer, I'll add another one once yours expires. In the mean time, why don't you edit your question a bit to emphasize your **why parallel?** aspect. Your question's title certainly doesn't make that very clear at the moment. You had a 2nd   chance to emphasize that in the bounty message but you left that blank as well. I have a hunch the bands will be parallel to the slit-like crescent but it's just a guess, not an answer.
uhoh
CC BY-SA 4.0
2024-04-15T23:34:08.323Z
@GeorgeLee fyi you can also click the "flag" button under your comment, ask for moderator assistance, and ask for your bounty to be refunded to your reputation score, explaining that uhoh has offered to add a bigger one instead. They already know that I bounty like crazy.
uhoh
CC BY-SA 4.0
2024-04-16T00:21:21.860Z
Why parallel: In the title: *and why are they bands?* and in the body: *Especially I don't understand why they should be bands/lines/snakes, I know that the shape of the sun at that moment is similar to a line, but the shape of the obscuration shouldn't be related to that, but to the orientation of the air particles which causes it*
George Lee
CC BY-SA 4.0
2024-04-16T02:44:09.713Z
@George Yes, the Müller lithograph is very exaggerated and stylised. But it's been the standard illustration of shadow band for over 150 years. It shows us what *some people believe* the shadow bands look like, and it's probably been a major influence on why they are called bands.
PM 2Ring
CC BY-SA 4.0
2024-04-16T02:53:35.327Z
@George *"(the atmospheric parts) must form bands, and they must be parallel to the crescent."* That's an interesting hypothesis, but at this stage we don't know if it's correct. It's hard enough to photograph or video the shadow bands. I suspect that it's very difficult to get direct data of the shape of the density variations in the atmosphere.
PM 2Ring
CC BY-SA 4.0
2024-04-16T02:54:21.413Z
(cont) [Adaptive optics](https://en.wikipedia.org/wiki/Adaptive_optics) (AO) systems can respond to the effects of atmospheric refraction, but they don't directly produce a 3D map of atmospheric density, and I don't know if AO techniques have been tried during a solar eclipse.
PM 2Ring
CC BY-SA 4.0
2024-04-16T13:11:44.033Z
@PM2Ring By *"(the atmospheric parts) must form bands* I didn't mean to state that it is so (i.e. hypothesize), but to say that it's the only way I can understand it, and if it's not true then I need an explanation.
George Lee
CC BY-SA 4.0
2024-04-16T20:25:52.503Z
@uhoh I would be glad if you suggest an edit to my question to make it clearer. But I cannot promise that I will accept it, because maybe you don't really get what my question is (due to my bad explanatory skills).
George Lee
CC BY-SA 4.0
2024-04-16T22:15:07.783Z
Here's a title that isn't smooth or pretty, but I think it's the underlying question: "Why are solar eclipse "shadow bands" (also called "snakes") elongated with a specific orientation, and what's the relationship between their direction and the geometry of the eclipse?" Or just "Why do shadow bands look roughly like parallel bands, instead of being randomly oriented?" Or some combination of those.
uhoh
CC BY-SA 4.0
2024-04-07T23:29:57.383Z
2024-04-07T23:40:01.257Z
2024-04-07T23:40:01.257Z
uhoh
<p>The most known explanation for the <a href="https://en.wikipedia.org/wiki/Shadow_bands" rel="nofollow noreferrer">Shadow Bands</a>, which is visible right before and after a total solar eclipse, is that some air particles refract the light of the thin sun-crescent, preventing them from reaching the ground. And it's for the same reason which cause stars to twinkle.</p>
<p>And if the following:</p>
<blockquote>
<ol start="2">
<li>Good seeing is unfavourable, since no turbulence cells develop. (&quot;bad seeing is good for shadow bands&quot;). Therefore observation places at sea level are more favourable than higher locations.</li>
<li>Moderate wind velocities in middle atmospheric heights let the shadow bands become well visible. Very strong wind results in fast movements so that the eye cannot follow any longer. With zero wind speed the shadows are nearly without motion and therefore hardly remarkable.</li>
</ol>
</blockquote>
<p>...(from <a href="https://www.strickling.net/shadowbands.htm" rel="nofollow noreferrer">https://www.strickling.net/shadowbands.htm</a>) is proven, it most probably means that this explanation is the right one.</p>
<p>For the star-twinkling part, I understand it perfectly, because there's just one point of light out there which can easily be obscured. But for a crescent, I don't understand it. Because no matter how thin the crescent is in the East-West direction, it is still composed of many points of light, which extends in the North-South direction, which shouldn't be so easily obscured.</p>
<p>Especially I don't understand why they should be bands/lines/snakes, I know that the shape of the sun at that moment is similar to a line, but the shape of the <strong>obscuration</strong> shouldn't be related to that, but to the orientation of the air particles which causes it. And it's hard to believe that they are oriented in bands (but if they are, it may solve the first part of my question too).</p>
<hr />
<p><strong>Edit:</strong> I perfectly understand the shadow bands visible on the ground of a water body (like a swimming pool): The water waves are big enough that the entire sun is relatively small enough to produce those bands. But if the sun would be like an oval where the long side is ten times bigger than the sun is in reality (5 degrees), and the longer dimension would be perpendicular to the orientation of the waves, it would probably not produce shadow bands, no matter how small it is in the other dimension.</p>
<p>And if the water wouldn't have band-like waves, but zero-dimensional bumps, it wouldn't produce shadow <em>bands</em> at all, just shadow <em>spots</em>.</p>
<p>So does the atmospheric turbulence have to match these criteria in order to  produce shadow bands? If not, how do they still work despite that?</p>

CC BY-SA 4.0
2024-04-13T00:34:32.277Z
*"some air particles refract the light of the thin sun-crescent, preventing them from reaching the ground."* Do you have a reference for that claim? Why would refraction prevent light rays from reaching the ground? Refraction deflects light, it doesn't block it.
PM 2Ring
CC BY-SA 4.0
2024-04-14T14:46:11.653Z
By the time of writing I wasn't sure where the refracted light goes. But in any case it's true that it prevents light from reaching the ground *at some locations* (the shadow bands).
George Lee
CC BY-SA 4.0
2024-04-14T20:12:20.920Z
Oh, I forgot to write @PM2Ring
George Lee
CC BY-SA 4.0
2024-04-15T03:37:18.387Z
@George All the light reaches the ground, but the distribution of the light is uneven because of the refraction. Say we have two adjacent patches of ground, A and B. Without refraction, the light intensity at A equals the intensity at B. With refraction, some of the rays that would've hit A instead hit B, so the intensity at A is reduced and the intensity at B is increased.
PM 2Ring
<solar-eclipse><visible-light>
Shadow Bands: How can atmospheric turbulence obscure the sun-crescent immediately before a total solar eclipse? and why are they 'bands'?
George Lee
# Shadow Bands: How can atmospheric turbulence obscure the sun-crescent immediately before a total solar eclipse? and why are they 'bands'?

(Asked by: username_0 on 2024-04-07)

The most known explanation for the [Shadow Bands](https://en.wikipedia.org/wiki/Shadow_bands), which is visible right before and after a total solar eclipse, is that some air particles refract the light of the thin sun-crescent, preventing them from reaching the ground. And it's for the same reason which cause stars to twinkle.

And if the following:

> 2. Good seeing is unfavourable, since no turbulence cells develop. ("bad seeing is good for shadow bands"). Therefore observation places at sea level are more favourable than higher locations.
> 3. Moderate wind velocities in middle atmospheric heights let the shadow bands become well visible. Very strong wind results in fast movements so that the eye cannot follow any longer. With zero wind speed the shadows are nearly without motion and therefore hardly remarkable.

...(from <https://www.strickling.net/shadowbands.htm>) is proven, it most probably means that this explanation is the right one.

For the star-twinkling part, I understand it perfectly, because there's just one point of light out there which can easily be obscured. But for a crescent, I don't understand it. Because no matter how thin the crescent is in the East-West direction, it is still composed of many points of light, which extends in the North-South direction, which shouldn't be so easily obscured.

Especially I don't understand why they should be bands/lines/snakes, I know that the shape of the sun at that moment is similar to a line, but the shape of the **obscuration** shouldn't be related to that, but to the orientation of the air particles which causes it. And it's hard to believe that they are oriented in bands (but if they are, it may solve the first part of my question too).

---

**Edit:** I perfectly understand the shadow bands visible on the ground of a water body (like a swimming pool): The water waves are big enough that the entire sun is relatively small enough to produce those bands. But if the sun would be like an oval where the long side is ten times bigger than the sun is in reality (5 degrees), and the longer dimension would be perpendicular to the orientation of the waves, it would probably not produce shadow bands, no matter how small it is in the other dimension.

And if the water wouldn't have band-like waves, but zero-dimensional bumps, it wouldn't produce shadow *bands* at all, just shadow *spots*.

So does the atmospheric turbulence have to match these criteria in order to produce shadow bands? If not, how do they still work despite that?

**Tags:** solar-eclipse, visible-light

> **Comments:**
> **username_1**: *"some air particles refract the light of the thin sun-crescent, preventing them from reaching the ground."* Do you have a reference for that claim? Why would refraction prevent light rays from reaching the ground? Refraction deflects light, it doesn't block it.  
> **username_0**: By the time of writing I wasn't sure where the refracted light goes. But in any case it's true that it prevents light from reaching the ground *at some locations* (the shadow bands).  
> **username_0**: Oh, I forgot to write @username_1  
> **username_1**: @username_0 All the light reaches the ground, but the distribution of the light is uneven because of the refraction. Say we have two adjacent patches of ground, A and B. Without refraction, the light intensity at A equals the intensity at B. With refraction, some of the rays that would've hit A instead hit B, so the intensity at A is reduced and the intensity at B is increased.  

---

## 2 answers

### Answer by username_2 on 2024-04-07. Score: 4

Think about the patterns that you have seen on the bottom of a swimming pool when the sun shines through ripples on the surface. These are a related phenomenon. When the sun shines through a medium that is "uneven" it is refracted and a pattern of brighter and darker shapes appears. The particular appearance of particularly bright and dark lines is due to the formation of "caustics". Now the pattern that is created by ripples is much stronger as the refraction at the water's surface is very strong. But the same occurs when sunlight shines through the atmosphere. You won't notice it when the sun in un-obscured, since the size of the sun does have a "smoothing" effect. But it does become visible when the sun is reduced to a sliver that is a fraction of an arcminute thick.

The orientation in bands is due to the fact that the sliver of light is a band, extremely narrow, but quite long (relative to it's width) All shadows are affected by this change.

> **Comments:**
> **username_2**: "typo" is a kind word...  
> **username_0**: Hi, I'm the OP (having problems with my 'New Editor' account). Thanks for explaining the shadow bands phenomenon, but I'm still unsure about the answer on my specific question. I guess that the atmosphere also have a wavy surface extending mainly in one dimension like the surface of water (not zero dimensional bumps like I thought before). And I guess that the shadow bands are only happening when the orientation of the sliver of the sun aligns with the orientation of the atmospheric surface waves, and not when they're perpendicular to each other. Are these statements correct?  

---

### Answer by username_3 on 2024-04-07. Score: 3

> How can atmospheric turbulence obscure the sun-crescent immediately before a total solar eclipse? and why are they 'bands'?

**tl;dr:** Refraction by atmospheric turbulence **redirects** the almost-parallel light just before eclipse making it more concentrated in some areas and less in others rather than by "obscuring" it. Stars twinkle for the same reason. If a twinkling star was really bright, it would create similarly visible patterns.

* [Would an extremely bright star produce same kind of shadow band effect as seen in solar eclipses just before totality?](https://astronomy.stackexchange.com/q/27226/7982)
* [What is causing this strange shimmering pattern of light during a total solar eclipse?](https://astronomy.stackexchange.com/q/27221/7982)
* Smarter Every Day video [Space Station Transiting 2017 ECLIPSE, My Brain Stopped Working - Smarter Every Day 175 at about 04:30](https://youtu.be/lepQoU4oek4?t=264)

[username_2's answer](https://astronomy.stackexchange.com/a/57381/7982) is correct but lacks supporting sources.

We cause these wavy alternating lighter and darker patterns that aren't really band-like [caustics](https://en.wikipedia.org/wiki/Caustic_(optics)). Random refraction from inhomogeneities in the Earth's atmosphere happens all the time, but only when the light becomes collimated (almost parallel) can we see it. Random deflection of light causes more light in some places and less light in others. There's not really any loss of light or blockage or obscuring, it's just uneven distribution.

The image below shows a plate of normal, transparent glass with a "wiggly" uneven surface generate a portrait of Alan Turing using caustics. For every dark area, there's a nearby light area where the light has been re-directed, not blocked.

From Wikipedia's [Shadow bands](https://en.wikipedia.org/wiki/Shadow_bands)

> Shadow bands are thin, wavy lines of alternating light and dark that can be seen moving and undulating in parallel on plain-coloured surfaces immediately before and after a total solar eclipse. They are caused by the refraction by Earth's atmospheric turbulence of the solar crescent as it thins to a narrow slit, which increasingly collimates the light reaching Earth in the minute just before and after totality.
>
> The shadows' detailed structure is due to random patterns of fine air turbulence that refract the collimated sunlight arriving from the narrow eclipse crescent.
>
> The bands' rapid sliding motion is due to shifting air currents combined with the angular motion of the Sun projecting through higher altitudes. The degree of collimation in the light gradually increases as the crescent thins, until the solar disk is completely covered and the eclipse is total.
>
> **Stars twinkle for the same reason.** They are so far from Earth that they appear as point sources of light easily disturbed by Earth's atmospheric turbulence which acts like lenses and prisms diverting the light's path. Viewed toward the collimated light of a star, the shadows bands from atmospheric refraction pass over the eye.

Also, from Physics SE's [What is this wavy light coming through my blinds?](https://physics.stackexchange.com/q/637536/83380)

[![enter image description here](https://i.sstatic.net/i0bhm.gif)](https://i.sstatic.net/i0bhm.gif)

and from [my answer therein](https://physics.stackexchange.com/a/637662/83380)

> The video [Taming light reflection to create images](https://youtu.be/0NXNAIqU8KM) discusses engineered caustics (refractive rather than reflective in this case). See also [this post](https://lgg.epfl.ch/caustics_project_smallturing.php) from the EPFL Geometric Computing Laboratory:
>
> [![from the EPFL Geometric Computing Laboratory](https://i.sstatic.net/O6KMr.jpg)](https://i.sstatic.net/O6KMr.jpg)
>
> [...]
>
> From username_2 Gurney's blogpost [Caustic Reflections](http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html)
>
> [![From username_2 Gurney's blogpost Caustic Reflections http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html](https://i.sstatic.net/uIetm.jpg)](https://i.sstatic.net/uIetm.jpg) [![From username_2 Gurney's blogpost Caustic Reflections http://gurneyjourney.blogspot.com/2010/07/caustic-reflections.html](https://i.sstatic.net/cj5g9.jpg)](https://i.sstatic.net/cj5g9.jpg)
>
> From forums.sketchup.com's [Water Reflection on Surfaces](https://forums.sketchup.com/t/water-reflection-on-surfaces/79291):
>
> [![from forums.sketchup.com's Water Reflection on Surfaces https://forums.sketchup.com/t/water-reflection-on-surfaces/79291](https://i.sstatic.net/gIyRp.jpg)](https://i.sstatic.net/gIyRp.jpg)

> **Comments:**
> **username_0**: Hi, I'm the OP (having problems with my 'New Editor' account). Thanks for explaining the shadow bands phenomenon, but I'm still unsure about the answer on my specific question. I guess that the atmosphere also have a wavy surface extending mainly in one dimension like the surface of water (not zero dimensional bumps like I thought before). And I guess that the shadow bands are only happening when the orientation of the sliver of the sun aligns with the orientation of the atmospheric surface waves, and not when they're perpendicular to each other. Are these statements correct?  
> **username_3**: @username_0 it's more that the *density* of the air fluctuates, rather than the "surface*. This causes the refraction direction of different parts of the air to differ. There's no alignment required, just a nearly parallel (collimated) beam of light. We get that from stars (point sources) and the slivers and dots of light around the edge of the moon moments before totality.  
> **username_0**: It doesn't really matter whether it's the density or surface which fluctuates, what matters is whether they (whatever causes the shadows) are present at random spots or are they forming lines. The former is more intuitive, but the shadow bands hints towards the latter.  
> **username_3**: @username_0 I have not seen a good, clear set of images proving that the phenomenon really presents as roughly parallel bands, but I won't be surprised. While a star's light is collimated in all directions, moments before totality the sunlight is coming from a long "slit-shaped" source - collimated in only one direction. So if there were really "bands" with a predominant orientation, I would expect them to be oriented parallel to the crescent's brightest edge.  
> **username_0**: *parallel to the crescent's brightest edge* ...**and** to the orientation of the different parts (different densities) of the atmosphere. Meaning that they (the atmospheric parts) must form bands, and they must be parallel to the crescent.  
> **username_3**: @username_0 No it does not mean that. The inhomogeneities will not have directionality. Visibility comes from the collimation of the light, and that happens preferentially in one direction. Just like the GIF in my answer - the shades are slits in one direction and long in the other, so the isotropic puddle waves, viewed through the  narrow shade slits, show up as "band-like"  patterns. Better start by showing what the eclipse "bands" actually look like, they may not be as distinctly "band-like" as you imagine.  
> **username_1**: The shadow bands look roughly parallel, with a fairly well-defined direction of motion. See https://youtu.be/rjLZGOBQ4zA especially around 40s.  
> **username_1**: @username_0 See the "Caustic Projections" in the left part of https://i.sstatic.net/cj5g9.jpg . All the rays still reach the ground, but some patches get more rays and some patches get fewer rays.  
> **username_1**: Here's the classic shadow bands lithograph by Müller from the 1870 eclipse https://www.strickling.net/shadowbands1870large.png from Dr Wolfgang Strickling's page https://www.strickling.net/shadowbands.htm That site has shadow band photos & videos, but they're all very vague.  
> **username_3**: @username_1 oh that's great! There are several links to videos in YouTube. The 2021 [2nd](https://youtu.be/Egh6aNu6rZE) and [3rd](https://youtu.be/Ma1CTLoBQ0g) contact videos from 2001-June-21 sort-of show some wiggliness - I wish there was a definitive video using the best equipment and a flat screen to finally characterize a pattern to see if "parallel bands" is really what they look like, or what lithographers reconstruct. **update:** OK [here are some](https://youtu.be/-NBoOxzumfs)! The "snakes" or bands seem to go from lower-left to upper-right? [Here are more](https://youtu.be/1I4YNa5-nuk).  
> **username_0**: @username_1 The drawing from 1870 is very exaggerated, and we can't really learn from it. Anyway, my question was not **if** the band are parallel, but rather **why**.  
> **username_3**: @username_0 OK great! I was about to add a bounty to your question (I love adding bounties!) but I see that you've done it already. If you still don't get a satisfactory answer, I'll add another one once yours expires. In the mean time, why don't you edit your question a bit to emphasize your **why parallel?** aspect. Your question's title certainly doesn't make that very clear at the moment. You had a 2nd   chance to emphasize that in the bounty message but you left that blank as well. I have a hunch the bands will be parallel to the slit-like crescent but it's just a guess, not an answer.  
> **username_3**: @username_0 fyi you can also click the "flag" button under your comment, ask for moderator assistance, and ask for your bounty to be refunded to your reputation score, explaining that username_3 has offered to add a bigger one instead. They already know that I bounty like crazy.  
> **username_0**: Why parallel: In the title: *and why are they bands?* and in the body: *Especially I don't understand why they should be bands/lines/snakes, I know that the shape of the sun at that moment is similar to a line, but the shape of the obscuration shouldn't be related to that, but to the orientation of the air particles which causes it*  
> **username_1**: @username_0 Yes, the Müller lithograph is very exaggerated and stylised. But it's been the standard illustration of shadow band for over 150 years. It shows us what *some people believe* the shadow bands look like, and it's probably been a major influence on why they are called bands.  
> **username_1**: @username_0 *"(the atmospheric parts) must form bands, and they must be parallel to the crescent."* That's an interesting hypothesis, but at this stage we don't know if it's correct. It's hard enough to photograph or video the shadow bands. I suspect that it's very difficult to get direct data of the shape of the density variations in the atmosphere.  
> **username_1**: (cont) [Adaptive optics](https://en.wikipedia.org/wiki/Adaptive_optics) (AO) systems can respond to the effects of atmospheric refraction, but they don't directly produce a 3D map of atmospheric density, and I don't know if AO techniques have been tried during a solar eclipse.  
> **username_0**: @username_1 By *"(the atmospheric parts) must form bands* I didn't mean to state that it is so (i.e. hypothesize), but to say that it's the only way I can understand it, and if it's not true then I need an explanation.  
> **username_0**: @username_3 I would be glad if you suggest an edit to my question to make it clearer. But I cannot promise that I will accept it, because maybe you don't really get what my question is (due to my bad explanatory skills).  
> **username_3**: Here's a title that isn't smooth or pretty, but I think it's the underlying question: "Why are solar eclipse "shadow bands" (also called "snakes") elongated with a specific orientation, and what's the relationship between their direction and the geometry of the eclipse?" Or just "Why do shadow bands look roughly like parallel bands, instead of being randomly oriented?" Or some combination of those.  

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