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18 ASTRONOMY • FEBRUARY 2018

W

ith 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 f lat 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.

SECRETSKY

Earth’s

‘shadow bands’

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 inten-

sity 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.

Up high

As seen from a jet some 35,

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

BY 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 posi-

tion) and tapers northward and

southward. When highest, it

lifts only 6° above the horizon

before it dissolves into the gath-

ering darkness about 35 minutes

after sunset. If only a segment of

sky is visible, the bands appear

deceptively parallel.

Its ill-defined upper bound-

ary 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 satis-

faction enough. But if you’d

like to increase your perception,

take time to search for subtle

gradations within the bound-

ary between the two regions.

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 terres-

trial 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 atmo-

sphere, 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

[email protected].

They’re a lot easier to see than

those from a solar eclipse.

BROWSE THE “SECRET SKY” ARCHIVE AT http://www.Astronomy.com/OMeara.

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. ALL IMAGES: STEPHEN JAMES O’MEARA

Left to right: 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

is a globe-trotting observer

who is always looking for the

next great celestial event.