Total eclipse 2017

Antje and I pose with our eclipse glasses during the early partial phase of the eclipse.

Yesterday, my wife Antje, older daughter Annika, and I had the once-in-a-lifetime experience of being at the center of the path of the August 21 total eclipse. Our location: the Crab Orchard marina just east of Carbondale, Illinois.

The partial eclipse began at 11:52 AM and progressed slowly. Wearing our eclipse glasses, we could see the growing bite taken out of the sun’s disk, but there were no remarkable changes in the ambient lighting until the degree of coverage was well past 50%.  Only at that point did it start to become noticeable that the light was unnaturally muted relative to normal noon sunlight – but without either the warmer colors or the long shadows of late afternoon. 

(Interestingly, the effect of the reduced light during a partial eclipse is completely undetectable in photographs using automatic exposure metering, because the camera compensates by increasing the shutter time or opening the aperture by a stop or two, and there are no other visual clues in the light that the sun is partially blocked.)

Now, I have to admit, I have seen a number of partial eclipses in my lifetime, and I don’t find them terribly exciting. Unless you view the sun’s disk through a filter, it could easily escape your notice that an eclipse is happening at all.  Indeed, the only real clue might be the crescent shaped shadows that commonly occur where sunlight is filtered through a tree or other small openings.   I would not drive hundreds of miles to catch a glimpse of a partial eclipse.

That said, the progression of the partial eclipse was not without its excitement. We warily monitored the increasingly widespread cumulus congestus clouds that had begun developing throughout our area starting around 11 AM.  These were large and slow-moving, and I worried that one might block out the sun during the critical 2 minutes and 38 seconds of totality. Indeed, as we got to within 10 minutes of the start of the total eclipse, our tension was heightened as one towering cloud edged ever closer to the sun’s disk.  At that point, I sadly reckoned that we had at best a 50/50 chance of directly observing the total eclipse.  Would the 950+ mile round trip have been worth it? But fortune was with us, and the clouds remained more or less clear of the Sun’s disk.

As we got to within a couple of minutes of totality, the reduction in light accelerated rapidly.  Recall that the Moon slides over the Sun’s disk in linear fashion with time, while the eye responds logarithmically to light and thus to each factor-of-two change in the amount of uncovered area of the Sun’s disk.  The first factor-of-two took around 45 minutes, the next factor-of-two required maybe 20 minutes, and each successive factor-of-two required about half as long as the one before (these are crude estimates; the exact math is complicated).  Only on the brink of total eclipse were we able to perceive the rapid change of  ambient light as if someone was working the slider on a dimmer switch.

Through our eclipse filters, we watched as the uncovered portion of the Sun’s disk became an impossibly thin sliver and then, briefly, a residual bright spot known as the “diamond ring” (more on that below).

Seconds later, even that was gone.

As we removed our eclipse glasses, a collective gasp of appreciation went out from all those in our vicinity as we all beheld a sight that I have  never seen adequately capture in either print or photograph. This blog post will be no exception. 

The sky was dark, though not the dark of the midnight sky; more like pre-dawn.  The corona of the sun was suddenly visible in all its other-worldly glory.  This photograph shows the exact same corona we saw, but without capturing the fine fibrous texture or subtle coloration we saw with the naked eye. I think no photograph can.

Looking around us, we saw that the towering cumulus clouds and sky low on the horizon were illuminated with an orange glow – in all directions. It was as if the sun had just set to the north, east, south, and west, and all points in between, all at the same time.

At our location at the center of the path, we had a full two minutes and 38 seconds to appreciate the spectacle of a total eclipse, and I was grateful for every second. For a brief period, it partly disappeared behind a shred of cloud, only to reappear through a gap.  If anything, the proximity of clouds only enhanced the beauty of the scene.

I allowed the sun to begin reemerging from behind the moon without putting on my eclipse glasses again.  I did not look at the sun itself during this time but instead took in the environment around me during this critical transition back to a partial eclipse.  And that’s when the next surprise hit me.

When the “diamond ring” phase occurs, the intense light of the sun comes from an exceedingly small, almost point-like source rather than from the  half-degree diameter disk that we’re used to. Why is this significant?  It’s because the “large” unobscured disk causes shadows to have blurry edges. Look at the shadow of  the leaves of a tree on an ordinary sunny day, and you’ll have a hard time making out the individual leaves; the shadows are all smeared by that half-degree variation in the direction from which the light is coming. 

During that “diamond ring” phase, however, that blurring effect is almost entirely eliminated – shadows are exceptionally sharp and detailed.  A co-worker who also traveled to see the eclipse told me that he was able to discern the shadows of the hairs on his own head.
In fact, the lighting during the “diamond ring” phase reminded me of that cast by someone operating an arc welder during my shipboard days – the light is unnaturally white and the shadows are unnaturally sharp.

But what really drove that observation home for me was the shadows cast by the cloud wisps that passed in front of this point-like source of intense light: I could see astonishingly fine-scale texture in the clouds’ shadows as they moved across the ground before me, even though the clouds casting the shadows were some 2,000–3,000 feet above ground level!  I don’t know why I had never previously seen mention of this truly striking phenomenon.  I did retroactively find this page, which describes “shadow bands,”  which I believe are closely related to what I saw, but the latter apparently result from turbulent inhomogeneities in the cloud-free upper atmosphere.  The thin sliver-like source of sunlight is central to their occurrence as well.

You’ll notice that I have not posted any eclipse photos of my own.  I did not believe I could capture the experience in photographs well enough to compensate for the distraction. If you want to see images professionally captured by NASA from approximately the same vantage point, you can check out this YouTube video.

Rainbow photographed from the car as we headed back north through Illinois on the way to Madison.

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