Tungsten Valley - So, there’s this kid standing under a bright sky here in Nederland. They look up at you with wonder in their adorable little eyes and ask, innocently, “What makes the sky blue?”
Thoroughly annoyed, because you always knew this day would come and yet, somehow, you’re still completely unprepared, you fumblingly answer, “I don’t know. Now get out of the fresh air and go to your room and play a video game, please.”
Want to prep for that dreaded moment? I’m here to help.
First, physics. The sun’s rays hit our atmosphere with all visible colors, from red to blue. The longest-wave color, red, tends to penetrate straight-on through. So does green, to some extent. But the shortest-wave color, blue, gets bounced around when it runs into oxygen and nitrogen, the two gases that make up pretty much all the air we breathe. (Penetration of reds, and loss of blues, is what makes orangish sunsets and sunrises.)
Where does the blue go? Blue-color waves bouncing off oxygen and nitrogen molecules is called scattering. It’s like when a water balloon bursts, and the water goes every which way. Blue light goes every which way when it hits air molecules. Some of it randomly bounces toward wherever we happen to be standing.
Scattering occurs through all air, so we see it everywhere we look across the sky. This was all mathematically described in the 1800s by a British physicist, Lord Rayleigh (RAY-lee); it’s called Rayleigh scattering after him.
Noting that oxygen and nitrogen molecules are pairs of atoms (O2 and N2), which we call diatomic, you now know what to say when that child asks you the dreaded question:
“The sky is blue, [enter child’s name here, with a drippingly-sarcastic tone], because of Rayleigh scattering of short-wavelength blue light from the atmosphere’s diatomic oxygen and nitrogen. Now get out of the fresh air and go to your room and play a video game, please.”
There’s an aspect of Rayleigh scattering, for getting better photographs. Blue-sky light that bounces leftward or rightward of the sun gets its electromagnetic vibrations sorted into a single direction. This preferred vibrational orientation is called polarization.
The practical upshot being, if you put a polarizing filter (called circularly polarizing, or “CPL” in marketed filters) on your camera lens and rotate it correctly on sunny days, you can get better contrast; enriched colors; a more beautiful, darker-blue sky; brighter-white clouds, and less reflections.
You like your smart phone camera? That’s OK. Get a CPL filter that clamps onto, or magnetically sticks to, your phone body, and swings over your phone’s lens. (Again, rotate the filter until the sky darkens to where you like the effect.)
There is no digital-processing filter that replicates CPL glass. This one is all-analog.
Final note: Bees’ eyes are tiny polarizers; they see in polarized light, to help navigate by the sun. With a CPL filter, you see the world a little bit more like a bee does.
In October Skies:
The sun begins the month in Virgo, entering Libra on October 30. At mid-month, days and nights are 10.5 and 13.5 hours long, respectively.
The moon’s dates are: New October 2; First Quarter October 10; Full (Hunter’s Moon) October 17; Last Quarter October 24.
October Meteors: Camelopardalids October 6; Draconids, southern Taurids and delta Aurigids October 9-11; epsilon Geminids October 18; Orionids October 22; and Leonis Monorids October 25.
Best Sky Viewing Nights (Minimal Moon): October 1-9 and 25-31.
Sunset (Mid-Month): Pegasus is low in the east; the Navigator’s Triangle of Vega, Deneb and Altair is high overhead; and Arcturus is setting in the west.
Midnight (Mid-Month): Capella is high in the northeast in Auriga. Just above it is brilliant Perseus with Algol. Andromeda is directly overhead. Cetus, the Whale, is in the south. Orion is rising in the east.
Sunrise (Mid-Month): Leo is high in the southeast. Orion is in the southwest. Sirius, in Canis Major, is low in the south.
Mercury, in Libra, is lost in the sunset.
Venus, in Libra, is very low and hard-to-spot in the southwest at sunset.
Mars, in Gemini, rises at midnight. It is high in the southeast at sunrise to the left of, and below, Jupiter.
Jupiter, in Taurus, is super-bright as it approaches its December opposition. It rises at 9:20 p.m. and is high in the south at sunrise.
Saturn, in Aquarius, is low in the east-southeast sunset; high in the south at 10:30 p.m.; and sets at 4 a.m.
Notable Space Missions: The European Space Agency’s BepiColombo planet Mercury exploration probe is performing a series of new thruster firings to adjust its course after experiencing earlier problems with its solar-electric propulsion system. The orbital adjustments will bring it to Mercury later than originally planned, but a recent mid-trip Mercury flyby returned excellent images.
Frank Sanders, a spectrum scientist at the U.S. Department of Commerce in Boulder, takes astronomy-related inquiries at backyardastronomy1@gmail.com.
