Astronomical and sky phenomena are part of everyday life. Sometimes they blend in so well we don't notice them. Little scenes unfold around us all the time like the Earth's shadow rising opposite the sun in the eastern sky after sunset. I saw this on the way to the store the other evening and took a quick photo while stopped at a stop sign. It shows a slice of the shadow cast by our planet on the atmosphere. The full arc is about 180° long, and like the Earth, it's curved.
After sunset, the shadow climbs up the eastern sky for about 20 minutes before blending into the growing darkness. I've seen the phenomenon a hundred times, but it never gets old. The setting sun paired with the rising shadow helps us sense our planet as a sphere spinning in space.
You'll also notice a glowing pink band called the Belt of Venus along the shadow's edge. When the sun has set for you it's still shining on the upper atmosphere. The Belt of Venus is caused by reddened sunlight from the setting sun scattered by fine dust in the atmosphere from the opposite end of the sky.
Both shadow and belt are visible anytime it's clear for up to 20 minutes after sunset in the eastern sky or before sunrise in the west. The shadow appears broad because we're standing on the planet, so it's literally in our backyard. But at the moon's distance of 239,000 miles (385,000 km) the Earth's shadow cone only spans about 2.6 full moons or less than 1.5° of sky.
The next day, Nov. 5, a classic 22° solar halo appeared around the sun, but the best views were reserved for that evening. As the moon topped the trees around 10 p.m. two halos appeared: the 22° halo and part of the much larger and rarer 46° halo. A time exposure revealed vivid colors in a short segment at the crown of the upper halo called the circumzenithal arc or CZA. Cirrus and cirrostratus clouds, made of ice crystals, are responsible for halo phenomena, and the study of their evanescent forms is called atmospheric optics.
In addition to the halos, a narrow Parry arc and an upper tangent arc touched the inner halo and resembled a soaring condor. Rings and arcs form when light is refracted or bent by tiny, six-sided ice crystals. Light is refracted when it enters one facet of the crystal and again when it exits through another facet. The two refractions bend the light by 22° from its original direction and make a halo of light 22° from the sun or moon. When you see one you're witnessing the combined glint of billions of bits of ice.
Longer, pencil-shaped crystals called columns are responsible for both the 22° and 46° halos as well as the arcs I saw that night. The form of each differs depending on the orientation of the crystals (they can be floating horizontally or randomly oriented, even spinning!) and what face the light enters and exits. The colorful CZA is a bit different. It forms when light is refracted by six-sided plate-shaped crystals.
If we drill down to the molecular level, water is composed of two hydrogen atoms bonded to one oxygen. At freezing temperatures, individual water molecules slow down enough to bond together into a layered crystal lattice of solid water (ice) made of countless six-sided rings. The same happens with snowflakes.
From beautiful sky displays to inviting ski trails you'll find hexagons all the way down.
Our final subject requires a small telescope and safe solar filter. This past week a brand new sunspot group — the largest of the current solar cycle — appeared on the sun. It's been fun to watch its changing appearance from day to day.
Today (Nov. 8) the group, known as AR2781, produced a moderately strong flare that caused a shortwave radio blackout over Australia and the Indian Ocean. Because it's facing Earth for the next few days any additional flares will be directed our way and could spark a northern lights show. None is in the forecast yet, but should one occur, it would be yet another demonstration of how "the cosmic" asserts itself in our lives.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob.