With the passing of the "Great American Eclipse of 2017," a large swathe of the American public has now been introduced to the rare conditions associated with these unique celestial events. Arguably, there have been no previous eclipses that have seen quite as much public attention and promotion; in equal measure, retailers selling eclipse eyewear have probably never made quite as much money as they did with this event.
And for those who missed out due to weather conditions or other circumstances, don't worry. On April 8, 2024, skywatchers in North America will be treated to a full solar eclipse, which might even rival this week's event.
That said, there are a variety of unusual phenomena associated with eclipses over the years. As far back as 1715, Edmund Halley described the presence of a “chill and damp which attended the darkness [that caused] some sense of horror” during an eclipse he and others observed that year. This, of course, referred to what is known as the "eclipse wind," which is known today to be caused by a cooling of the ground, and a reduction of heat rising off of it, which results in a shift in wind direction due to surface temperature changes.
More interestingly, as I noted in an article earlier this year, had been an anomalous shadow pattern coinciding with eclipses, noted by researcher J.L. Codona who wrote about them in a 1991 article in Sky and Telescope. Codona described these shadows as, "mysterious gray ripples... sometimes seen flitting over the ground within a minute or two of totality. The bands are initially faint and jumbled; but as totality approaches, they become more organized, their spacing decreases to a few centimeters, and their visibility improves. After totality ends the bands can reappear and become progressively fainter and more disorganized until they disappear."
"Shadow bands seem to move perpendicularly to their length, but this is only an illusion. It stems from a lack of features that allow the eye to track motion along the length of the bands.”
With all this in mind, during the eclipse on August 21, 2017, I decided to see if I could capture this phenomenon on film myself. Despite having to change my location at the last minute (due to overcast conditions), my team and I did manage to relocate to a secondary location where visible conditions were ideal, and still managed to successfully film the "eclipse shadow band" phenomenon.
In the short video below, you'll see the "bands" moving in a way reminiscent to loose sand blowing across a surface on a windy day, moving quickly in a diagonal orientation from the upper right, to lower left of the frame:
[video width="1280" height="720" mp4="https://mustorage.blob.core.windows.net/images/2017/08/Shadow-Bands.mp4"][/video]
An associate of mine, located near Columbia, South Carolina, also managed to see this during the eclipse totality. As he described in a subsequent email:
"[We were] staring at our white sidewalk and driveway during totality, the concrete seemed to shimmer. I even pointed it out to my wife and sister-in-law, it looked almost like the kind of shimmering you get as heat rises off really hot pavement but it was the white concrete itself shimmering. So I just chalked it up to the light and shadow cast by the corona light (not the beer)... maybe called the 'coronal light'."
Indeed, it sounds like what my friend described had likely been the same shadow band phenomenon, and though it is indeed unusual, it may not be all that difficult to explain. The likeliest source of the light anomaly here is probably a result of atmospheric turbulence as little as a few hundred feet above ground, which is shadowed below due to the unique daylight conditions presented around the time of eclipse totality.
Whether or not it is indeed anything "mysterious," it was a real treat to be able to see the famous eclipse shadow-bands ourselves, which for this observer, may have been a bit more exciting than the actual eclipse, in truth!