Seeing the Earth's Shadow

By Dennis Mammana

April 30, 2020 5 min read

Week of May 3-9, 2020

During spring and summer months, it's fun to sit outdoors and gaze at the stars, but beginning stargazers might be surprised to learn that some stars seem to move.

Suppose you see one that is drifting slowly across the sky, accompanied by red and green blinking lights. Well, that's called an airplane, and it's not terribly exciting.

Maybe you'll spot a star that flashes across the heavens, sometimes leaving a glowing trail in its wake; in that case, you've just seen a shooting star or falling star. Astronomers know these as meteors — specks of interplanetary dust that fall into our atmosphere and glow brightly as they disintegrate.

Now, suppose you spot what appears to be a single star drifting slowly across the sky. This may be an Earth-orbiting satellite. With thousands of these in orbit, it's not uncommon to see a few during early evening hours. You can predict which satellites will pass over you — and even print out a star map showing their paths — by visiting http://heavens-above.com.

While watching satellites, you may occasionally notice that some appear to unexpectedly fade from view. It's not that unusual; satellites are visible because they are high enough to be illuminated by sunlight, but when one appears to fade from view, we know it has drifted into the Earth's shadow.

It's not only after dark that one can see the Earth's shadow. I'm pretty sure that just about everyone reading these words has seen it but didn't know what it was.

The next time you have a cloudless sky with a fairly low eastern horizon (such as the ocean or flat plains), face eastward as the sun is setting. Low over the eastern horizon, you'll notice an immense purple arc stretching from south to north, bordered on top by a beautiful fringe of pink.

Many people see this every evening but dismiss it as simply haze or pollution. Not true. This is the shadow of our Earth.

Early risers can see it, too. Just before sunrise, face west, and you'll see the arc as it sets behind the terrain. Your best chance to spot it is when you've got a cloudless sky with a low horizon.

This phenomenon occurs because our planet is a solid body that casts a shadow away from the sun. When the sun sets, for example, we find ourselves on the boundary between daytime and nighttime.

Sunlight continues to illuminate the atmosphere in the west, producing the sky's light blue color, but our solid planet blocks the sunlight from reaching the air to the east, so that part of the sky appears a darker blue or purple color. Between the darker and brighter parts of the atmosphere lies a fringe of pink — also known as the "Belt of Venus" or the "anti-twilight arc" — illuminated by the reddened sunset light that's passing through the atmosphere itself.

Depending on the clarity of the evening air, the Earth's shadow will appear most prominent for a few minutes after sunset. Eventually, this shadow becomes more diffuse as it rises high enough to engulf us.

And that is what we call — you guessed it — nighttime!

 Satellites are visible because they are high enough to be illuminated by sunlight, but when one appears to fade from view, we know it has drifted into the Earth's shadow.
Satellites are visible because they are high enough to be illuminated by sunlight, but when one appears to fade from view, we know it has drifted into the Earth's shadow.

Visit and follow Dennis Mammana at facebook.com/DennisMammana. To read features by other Creators Syndicate writers and cartoonists, visit the Creators Syndicate website at www.creators.com.

Like it? Share it!

  • 0

Stargazers
About Dennis Mammana
Read More | RSS | Subscribe

YOU MAY ALSO LIKE...