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.
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.
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.
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.
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.