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Stephen James O'Meara's Secret Sky: Twilight musings

January 2011: If you wait until the dark of night to do your observing, you're missing a lot.
omeara
In July 2010, Sverrir Gudmundsson of Reykjavík, Iceland, decided to search the twilight for naked-eye stars about one and a half hours after sunset. He noticed that the sky surrounding 1st-magnitude Deneb (Alpha [α] Cygni), at an altitude of about 70°, appeared a much darker shade of blue than the sky around similarly bright Altair (Alpha Aquilae), at an altitude of 35°. What Gudmundsson observed is a little-known aspect of the twilight sky — one that artists and nature observers alike have long appreciated.
 
Victorian artist and writer John Ruskin urged readers in his 1857 book, The Elements of Drawing, to watch the sky, “the largest and the most beautiful” of nature’s spaces, especially at twilight. Look through a paned window of 1 or 2 square feet, he said, and try to consider each pane of glass in the window a separate piece of paper colored blue, gray, or purple. He told us to observe how “quietly and continuously” the gradation extends over the space in the window.
 
Ruskin also encouraged us to observe the shades of twilight on the outside and inside of a simple white cup or bowl, and then on folds of white drapery. “And thus gradually you will be led to observe the more subtle transitions of the light as it increases or declines on flat surfaces,” he wrote. “At last, when your eye gets keen and true, you will see gradation on everything in Nature.”
Earth's shadow appears projected against our atmosphere every transparent night
We commonly wait to view Earth’s shadow during a lunar eclipse, but it also appears projected against our atmosphere every transparent night. All you need is a clear view of a low eastern horizon shortly after sunset (and a bright Moon rising against the colors doesn’t hurt). Photos by Stephen James O'Meara
Bit by bit
On the clearest of days, the sky around sunset and deep into twilight darkens by degrees away from the horizon, with the deepest shades appearing straight up. You may have difficulty seeing the gradations at first because they blend almost imperceptibly with a casual sweep of the eye. It’s a soft palette that requires time, patience, and a careful eye to discern. “The perception of gradation is very deficient in all beginners (not to say, in many artists),” Ruskin wrote.

In my studies, I’ve found the subtle differences easiest to see when comparing small vertical segments of sky framed between tree branches or patches of leaves. You can also try scanning a vertical section of sky between two tall, close buildings or any other landmarks. The following exercise should help.
Anticrepuscular-rays
Anticrepuscular rays seem to diverge from the antisolar point, or the spot directly opposite the Sun, in Earth's shadow — in this case, in the west before sunrise. Each ray of light and shadow actually travels a parallel path, but, because of perspective, they appear to converge on the horizon, just as parallel train rails do when seen from a flat expanse.
First, frame a vertical section of sky in the north or south, then study the color in the highest segment. The difference should be stark. Now look at a section of sky about 10° below the top segment and compare it again to the bottom segment. If you continue this progression, setting your sights lower and lower from the top, you should begin to appreciate the subtle shades of twilight.

Ruskin suggested similar methods of framing the sky to detect the gradations. He also cautioned us against being hasty in the task and offered a spiritual reward. “When you look at the beauty of the sky,” he extolled, “the sense you will have gained of that beauty is something to be thankful for. But you ought not to be impatient.”
Earthsshadow
Earth’s shadow appears sharpest just after sunset and at high elevations. This view from atop the nearly 14,000-foot-high (4,300 meters) Mauna Kea in Hawaii reveals yet another fantastic twilight phenomenon — the shadow the mountain projects against the atmosphere.
The twilight wedge
The sky turns deep blue overhead around sunset and during twilight for two reasons: sunlight scattering in the atmosphere high above you, and ozone in Earth’s stratosphere absorbing the long wavelengths of light (reds, oranges, and yellows) and radiating the shorter (blue and violet) ones. You can enhance the deep-blue hole overhead by looking at it through a red filter, which prevents blue light from reaching your eye, turning the sky black.

But there’s another dark segment in the twilight sky that’s much easier to detect. You’ll need a low horizon and air free of haze, clouds, or contaminants (natural and artificial) to best notice the effect. Winter in the Northern Hemisphere can offer these crisp atmospheric conditions.

After the Sun has set, focus your attention on the twilight sky along the horizon in the antisolar direction (directly opposite the Sun). There you should see the twilight wedge, a band of dusky sky caused by Earth’s shadow. The band appears sharpest just after sunset because we’re looking directly along the boundary line between the Earth’s shadow and the illuminated sky.
 
As Earth turns, its shadow rises in the eastern sky. Notice how the band appears higher in the sky at the exact antisolar point than it does to the north and south, where it tapers into the horizon, lending it the appearance of a roughly 180°-long wedge of dusky light. As the shadow rises, it also mingles with air still illuminated by sunlight, causing it to appear more diffuse.
 
Perhaps the most beautiful aspect of the shadow is the fiery-rose pallor of counter-twilight mantling it. Called the Belt of Venus (after the Roman goddess), this roughly 15°-wide band of warm light reflects highly reddened sunlight still passing through the dense layers of Earth’s atmosphere.
Earth's shadow will appear muted
If smog, dust, or other atmospheric hazes contaminate the air during twilight, Earth’s shadow will appear muted. Here, local dust from the North African coast partly obscures Earth’s shadow and the Belt of Venus at the antisolar point (directly opposite the Sun) along the horizon.
Sometimes, shadows from clouds in the west can stretch fully across the sky — slicing through the Belt of Venus, visually fraying it into light and dark stripes — before they converge on the antisolar point in Earth’s shadow. These anticrepuscular rays (crepuscular literally means twilight) are a dim extension of the “rosy fingers of dawn” described in classic poetry.
 
The leading edge of Earth’s shadow gradually blends with the coming of night and fades from view about 40 minutes after sunset. And if you’re more of a morning person than a night person, everything I’ve described here repeats itself in reverse before sunrise.
A weird illusion?
I’ll end with another interesting twilight observation Gudmundsson made in May 2009. He found it more difficult to see the stars of the Big Dipper in the dark-blue region of sky high overhead than the stars in the horizontal beam of the Northern Cross (some of which are 1 magnitude fainter than the Dipper’s stars), even though they were lower in the sky in a  significantly brighter area.
 
Although Gudmundsson is uncertain of the effect, he’s interested to know if you’ve had a similar experience or if you have a possible explanation. Send your comments and observations on any of these twilight phenomena to someara@interpac.net.
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