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Earth's 'shadow bands'

They’re a lot easier to see than those from a solar eclipse.
OMearaStephen
With the focus on the Moon’s shadow on the wane after the August 2017 total solar eclipse, I’d like to bring us back to Earth and its shadow. Some subtle effects of light and color accompany its projection onto our planet’s atmosphere during twilight, which can be overlooked from the ground but appear obvious from the air.

A physiological wonder
Earth’s shadow is a relatively brief twilight phenomenon. Seen against a flat horizon, it gradually rises in the east shortly after sunset. Before sunrise, you’ll find it setting in the west. At a glance, the dark sapphire blue band is capped by a rose-petal pink arch known as the Belt of Venus — a roughly 10°-wide wedge of reddened sunlight backscattered to our eyes like alpine glow. I find it impossible to discuss the shadow without the Belt because they meld together to form a single visual physiological wonder whose colors and intensities play off one another like two thespians improvising on stage.
ASYOM0218_01copy
If the sky is clear just after sunset, you’ll see Earth’s shadow rising low in the east. The pinkish region above it is known as the Belt of Venus.
Stephen James O'Meara
Our planet’s shadow stretches 90° to each side of the antisolar point (the point in the sky directly opposite the Sun’s position) and tapers northward and southward. When highest, it lifts only 6° above the horizon before it dissolves into the gathering darkness about 35 minutes after sunset. If only a segment of sky is visible, the bands appear deceptively parallel.

Its ill-defined upper boundary transitions into the Belt of Venus, which is most distinct when closest to the horizon. This blending zone becomes less defined as the shadow rises. A strong color contrast occurs when the air is largely free of contaminants. Dusty conditions or poor air quality can erase the shadow from view. Instead, a muddy gray band with a dull amber cap will ring the horizon like a dusky fog, confusing the view.

Subtleties
Under excellent conditions, the striking color contrast between the bluish shadow and the pinkish Belt of Venus is satisfaction enough. But if you’d like to increase your perception, take time to search for subtle gradations within the boundary between the two regions. The near 180° breadth of Earth’s shadow tends to draw the eye’s focus along the horizontal with a peripheral sweep. But the slight differences you’ll be searching for — in both intensity and color — are best noticed with direct vision and scanning vertically with your eyes.

From the ground under excellent conditions, Earth’s shadow generally has a colorful blue middle with a dull, washed-out bottom as if someone tried to scrub the color away. This is where our eyes look through the densest and dustiest layer of our atmosphere. The shadow’s upper boundary (where it mixes with the Belt) presents a grape tone, which brightens into lavender, followed by cotton candy pink in the middle, crowned by a lavender pink hue that diffuses into the pale sky above.

ASYOM0218_02copy

Top to bottom: When Earth’s blue shadow is visible, the landscape is also in shadow and appears blue. About 10 minutes before sunrise, the yellow sky of the east is mirrored in the west, and the terrestrial landscape is awash with yellow light. At sunrise, the western horizon appears more peach — a color reflected in the air above Earth.

Stephen James O'Meara
Up high
As seen from a jet some 35,000 feet (10,670 meters) up, Earth’s shadow stands out much more boldly, appearing as a deep purple wedge at its lowest to a royal blue arch when highest. Its upper boundary has a crisp edge. And, because we can detect our planet’s curvature from that altitude, the shadow’s slope is dramatically enhanced.

From altitude, the shadow’s color gradations are deeper and clearly defined. Moisture in the upper atmosphere makes the shadow appear washed out. But unlike at ground level, it doesn’t turn dull but bright, making that part of the shadow appear whitewashed. The sharp edge is a deep purple. Above it is a “raw meat” reddish tone, followed by orange, yellow, and a pale salmon that washes into the blue-white sky above.

Equally fascinating is the changing color of the terrestrial landscape below. At times, the shadow is so intense that detecting the horizon line takes effort. The landscape’s color, which we see through the densest layers of the atmosphere, mirrors the color of the sky immediately above it. (See the three-part sequence below.)

As always, keep your eye on the sky (day and night) and let me know your thoughts at sjomeara31@gmail.com.

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