Light-gulping Atmosphere And Solar Storms Keep Us On Our Toes

Monday evening’s sunset was all red and orange from heavy haze combined with the normal “thick air” the sun encounters when near the horizon. Rarely the sun can appear blue, when particles from volcanoes, forest fires and industrial pollutants are just the right size to scatter away the warmer hues of sunlight. Credit: Bob King

The persistent haze that’s made for lovely orange moons and ruddy sunsets has moved on, leaving a blue sky once again. I can’t say I miss the milky sky look, but I did enjoy those fireball sunsets.

Most of the air in our atmosphere is concentrated in the bottom 10 miles. Compared to our planet’s diameter, this airy shell is about as thin as the plastic wrap you use to cover an item in the microwave. The lower you go in the atmosphere, the thicker the air is and the greater the concentration of suspended dust particles, pollutants and water vapor.

The sunset sun takes the long path through the lower, thickest part of the atmosphere and gets heavily filtered. The noontime sun passes through a minimal amount of air. Illustration: Bob King

When the sun rides high in the sky, its light takes a very direct path to Earth, penetrating the extremely thin outer atmosphere, zipping through the lower 10 miles and arriving at the surface to light up the day. The air molecules on this short path only remove a sliver of the sun’s blue and violet light and scatter it to create a blue sky. To the eye the it appears pale yellow-white.

Light from the sunset or sunrise sun (left) and the noonday sun are a study in color contrasts. The sun’s white light is a combination of all the colors of the rainbow spectrum. Thicker air near the horizon removes most of the cool colors from the sun’s light, leaving red and orange. Credit: Bob King

Now take a look at the sunset sun (which also applies to sunrise). Its light comes in at such a low angle that it passes through hundreds of miles of dense and dusty air in the bottom 10 miles of atmosphere. All the colors are scattered or absorbed by the air except for the warm reds and oranges. These have the ability to penetrate where the others can’t. The same is true for the moon.

A recent August sunset photographed by astronauts on the space station. The nighttime Earth is in the foreground. The white band is the entire lower atmosphere;  above it are streaky octilucent clouds  about 50 miles (80 km) where the air is extremely rarified. Further up it’s airless outer space. Credit: NASA

Flying along at over 17,100 mph (27,600 km/hr), astronauts aboard the International Space Station astronauts experience some 16 sunsets and sunrises a day – one every 90 minutes. Their view of the atmosphere is more extensive than ours. They look low over the curvature of the Earth to see the deep red ball of the sun setting in what appears to be a very thin skin of air. Above it, the sky quickly transitions to blue and then black. Seen from 250 miles high, nearly all of Earth’s atmosphere lies far below the spacecraft.

An astronaut can see a sunset and stars (higher up) all at once, since her gaze quickly leaves the atmosphere for the vastness of airless space.

Here on the planet’s surface at the bottom of the atmosphere the transition from sunset to starry night takes time; the sun continues to make the air glow even when it’s well below the horizon. An hour an a half must pass before it’s night.

The prediction model for the coronal mass ejection leaving the sun on Aug. 21 that could affect Earth on the 24th. You’re watching for the crescent-shaped cloud headed outward toward the yellow dot (Earth) in the animation. Credit: Goddard Space Flight Center

While we’re on the topic of the sun, two coronal mass ejections earlier this week may increase our chances of seeing the aurora beginning late tonight through Saturday. The first involved an eruption of a large cloud of fiery hydrogen gas called a prominence or filament on Aug. 20. A second smaller filament came barrelling our way on the 21st. We’re not talking anything big, but skywatchers in southern Canada and in the northern border states should keep an eye out for northern lights.

7 Responses

  1. Edward M. Boll

    I have to be up early so I do not plan on going out late and looking for possible Northern Lights. To me tonight the ideal time to see the most in our Solar System would be about 9:30 PM. Mercury will not be visible. It sets 4 minutes after the Sun does. Venus sets at 9:41. Saturn is dimly visible. Oh, I will throw in Spica too. The Moon will be low in the East South East. 100 days from today is November 30. I am hopeful for a -1 magnitude comet to rise about an hour before sunrise, close to Mercury, I believe. At least Mercury is easily visible when it rises about an hour before sunrise at magnitude -1.

  2. Mike

    There was a short lived but at times dramatic aurora about 9:30 Ththat the bright moon diminished. Beautiful night with loons laughing and calling.

  3. Edward M. Boll

    Mercury is entering it’s evening phase. But this is one most will likely not see in this Hemisphere. The best for those of us in the north USA is a setting planet about 45 minutes after sunset during the first week in October.

  4. Hi Bob. As of 10 pm PDT (05 00 UTC aug 25) still awaiting the arrival of the small CME reported on the 21st. Lots of sky to look at now that the Moon is waning a bit and rising later.
    Any update on the calculations from the SDO and STEREO initial observations ?
    Thank you again

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