How Dark Will It Get During The August Total Eclipse?

Total solar eclipse on the island of Svalbard in 2015. The planet Venus is visible off to the left. Shadow bands flicker across the foreground seconds before totality. They form when the last tiny bit of sun refracts through air pockets of different densities and temperatures, creating a series of undulating bands of light. Toss a white sheet on the ground and watch for them starting about 20 seconds before totality and up to about 20 seconds after.

If you’re planning to be on the centerline for the August 21 total solar eclipse, you’re in for a light show like no other. We touched on this a couple days ago, but let’s go darker. Once the moon completely covers the sun, depending on where you’re standing in the eclipse path, you’ll have from a few seconds to a maximum of 2 minutes 41.6 seconds of totality.  During the last seconds, when all that remains is a narrow sliver of sun, it’s still a “sunny” day but only by definition. It feels more the end of the world riding in on a fast horse. During these last moments of light, we can really connect with our distant ancestors, for whom a total eclipse evoked more trauma than wonder.

Be sure to look at  your shadow. Unlike the fuzzy-edged one you see on a typical day, it will look much sharper as the sun is now very close to becoming a “point” source rather than an extended disk. Off in the northwest, the sky will appear dramatically darker in the final seconds before totality as the moon’s shadow comes racing in your direction at more than 1,000 mph. Its speed depends on your location along the path: 2,410 mph in western Oregon; 1,747 mph in central Nebraska, 1,462 in western Kentucky and 1,502 mph in Charleston, SC. It would move even faster if the Earth didn’t rotate, but since the planet rotates in the same direction as the moon moves (to the east), we deduct our speed from the moon’s. Good thing! Earth rotation slows down the shadow’s pace and helps to lengthen totality a little.

Total eclipse on November 2012 from Australia. This the best video I could find showing the beads of sunlight along the moon’s edge just before and after totality. They’re called Baily’s Beads. When a single one shines all by itself, the eclipsed sun looks like a diamond ring. Eclipse watchers call it the “Diamond Ring Effect.”

When the final bits of sunlight shining between peaks along the moon’s edge wink out, it finally happens. Darkness! Just kidding. It’s not really dark during totality. While it sort of resembles moonlight, it’s brighter than that and more colorful. You might compare it to what it feels like to walk around in twilight a half-hour after sunset or before sunrise. Light still pours in from a couple sources. First, the corona or sun’s atmosphere. It’s about as bright as the full moon, though its light is more spread out. The other main source of illumination comes from where the sun is still shining — albeit only as a thin crescent — just outside the path of totality.

The moon’s shadow on Earth is oval-shaped and 65-70 miles wide. Standing on the centerline, you’re only about 35 miles away from weak sunshine outside the eclipse band. This light reflects and refracts back to you to brighten the scene. If the awe of it all causes you to drop your mobile phone, you’ll easily see it on the ground. Even after seeing three totalities, I’m still at a loss for words in describing the scene. I remember it best in 1991 in Baja California: a cool breeze came with the disappearing sun and a sense of relaxation, the way you feel after sunset at the end of a steamy day.

When the sky darkens during totality in the western U.S. you’ll catch a glimpse of a couple planets and the brightest stars of winter. I didn’t include Mercury because it will be too faint to bother with. You’ll have better things to see. Created with Stellarium

As you saw in the video, the shadow slowly moves away as the seconds tick by. During the August eclipse, it will slide off to the southeast as daylight grows higher and brighter in the west. Finally, the moon and the curtain of sunlight sweeps overhead as the shadow retreats to the southeast, ready to touch more lives on down the line until it departs the U.S. at the South Carolina coast.

The main feature of a total eclipse is the sun’s corona or pearly atmosphere that surrounds the blackened moon. You’ll also notice little nubs of pink flame around the moon’s circumference, the prominences. These are best viewed in binoculars or low power telescope. The atmospheric experience of the changing light, falling temperatures and the reactions of those around you form another great part of the experience. But if you have a few seconds, take a quick, wide-ranging look around the sky for the simple pleasure of viewing planets and stars in the daytime.

The view from Columbia, South Carolina during totality. Created with Stellarium

Venus should appear to the right of the sun about 10 minutes before totality and remain visible for 10 minutes after. The sun-moon combo will lie only 1.2° to the right or west of the first magnitude star, Regulus, in Leo. You may not notice it with the naked eye but it’ll stand out in binoculars. Mars is off to the right of the sun and Mercury to its lower left. Both aren’t particularly bright, so you may not see them. No worries.

For western observers, Jupiter will be too low in the southeast but for you in the east, it’ll be easily visible in that direction. Western observers are compensated by better views of Sirius and Orion, below and to the right of the sun. Grab a look and return to the eclipsed sun. Soak in the seconds left of totality and feel Euripedes’ famous words in your bones: “All is change; all yields its place and goes.”

6 Responses

  1. Don Cwiklowski

    Hi, thanks for the info. I live outside of St. Louis and they’re predicting it’ll be crazy here. I plan on playing golf under it, and was checking to see how dark it will get. I just had a golf course tell me it will be pitch black and dangerous to be out. Crazy.

    1. astrobob

      Hi Don,
      If the path of totality crosses your golf course, you’ll experience a total eclipse, but it will hardly be as dark as night. It resembles early twilight much like the light about 30 minutes after sunset or 30 minutes before sunrise. If the golf course isn’t on the path, it will get weirdly dim but remain sunny. The dim and totality parts only last about 15 minutes, otherwise it will seem like a regular sunny day. If it were me, I’d forego all golfing, get a safe filter and watch the total eclipse. Just sayin’ 🙂

  2. Gary

    Thank you. Your writings and videos are very illuminating. I have forwarded them on to folks who are planning to get in the 100% path.

    As for me, I am a few degrees off.


  3. Lori C.

    Very interesting! I am reading now that there is no safe way to view the eclipse which is disappointing if true. Your thoughts?

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