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AstroBob: How dark will it get during total eclipse?

A student looks at the partially eclipsed sun in northern England on March 20, 2015 using a pair of eclipse glasses. Credit: Reuters1 / 6
There are lots of ways to view the eclipsed sun indirectly. One of the most creative is simply to take along a colander. The individual openings in the mesh act like pinhole lenses and will project tiny images of the sun. Credit: Reuters2 / 6
Small gaps along the length of this palm leaf acted as “pinhole lenses” to project a series of solar crescents on the ground during the July 1991 eclipse. Credit: Bob King3 / 6
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 4 / 6
At maximum eclipse on August 21 in Duluth, 76% of the sun will be covered. Credit: Vox5 / 6
When the moon completely covers the sun in total eclipse, lucky observers get to see one of the rarest sights in nature, the sun's atmosphere called the corona. It glows as brightly as a full moon, but it's invisible in daylight because of the overwhelming brilliance of the sun. Credit: Reuters6 / 6

If you're planning to be on the centerline for the Aug. 21 total solar eclipse, you're in for a light show like no other. 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 two 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, S.C. 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.

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.

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.

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."

If you can't make it to the total eclipse and plan on staying where you are, you'll see a partial eclipse with varying amounts of the sun covered. For the Duluth area, the eclipse begins at 11:46 a.m., reaches maximum at 1:07 p.m., when the moon will block 76% of the sun, and ends at 2:27 p.m.

To view the sun safely during partial eclipse, you'll need approved eclipse glasses or handheld filters from a reputable dealer such as or check your local public library. Libraries across the U.S. will be handing out 2 million pairs of free eclipse glasses. Walmarts and other major retailers are selling them, too. All of these are safe. You can also purchase a #14 welder's glass from a welding supply company.

Never look directly at the sun or use smoked glass, exposed film or mylar balloons. Approved filters only! You can also view the sun indirectly under a leafy tree — the tiny gaps between the leaves act as pinhole lens projectors and will beam a blizzard of tiny solar crescents on the ground below. Or you can project a crescent sun through the little hole in your almost-clenched fist onto a piece of paper. Another favorite is to use a colander to project multiple little solar crescents on the ground. Try doing-it-yourself by punching your name or a pattern in a poster board with a sewing needle and projecting that on the ground. Use your cellphone to take a picture of the result.

There are lots of ways to enjoy an eclipse, whether total or partial.