Driftless Dark Skies

John Heasley

One summer at science camp, a tween asked me why the sky was blue. They said they asked their parent who told them it was because God liked it that way. I sympathize with that parent. And also with the middle schooler. As we get older, we sometimes get discouraged and stop asking the really interesting questions.

So, it’s because of Rayleigh scattering. Visible light comes in lots of colors. Remember ROY G. BIV? Red Orange Yellow Green Blue Indigo Violet? Earth is enveloped by miles of atmosphere, and the molecules in our sky scatter the light differently depending on its color/energy level. Blue light gets scattered more and gives the sky its color. It’s also why sunsets are orange and red. When our day star is close to the horizon, sunlight travels through more of our atmosphere. Even more of the blue light gets scattered and more of the red light makes it to our eyes. Check it out for yourself next twilight. After the sun slips below the horizon, face the other direction and watch the eastern horizon. You’ll see a dark band. That’s the shadow of Earth cast by the sun as night begins to rise. Above that is a pinkish band called the Belt of Venus. If you were that high above the horizon, you could still see the setting sun and all the red light coming through. And if you were on Mars, it would be very different. The thin Martian atmosphere scatters more of the red light, so you would see a pinkish sky and a blue sunset!

Then there’s all the light we cannot see such as the more energetic ultraviolet rays. I’m grateful for a sheltering sky that filters a lot of that out. When the sun is high in the sky, such as around noon or in summer, the light travels through less of the atmosphere and more of the damaging UV gets to our eyes and skin increasing the risk of cataracts and skin cancer. But it’s easily blocked by sunglasses and sunscreen. In the winter, the sun travels lower in the sky and we get that wonderful golden light. Often there are ice crystals in the sky that diffract the sunlight creating pillars and halos and sundogs.

I like to think of our atmosphere as if we are lying at the bottom of a pool looking through shimmering water. Looking straight up, we see through less water and more clearly than if we are looking sideways. Which is why stars twinkle more when they are lower on the horizon than when they are overhead. The starlight passes through more shimmering layers with different temperatures and gets distorted. Keep looking east this month as the sky darkens after sunset.  The brightest and first star you see is Sirius. Just watch how its light dances and changes color. And that’s why we have blue skies and twinkling stars and the warm light of winter to enjoy. Stay curious.

John Heasley is an astronomy educator and stargazer who enjoys connecting people with the cosmos. For more information about stargazing in southwestern Wisconsin, like Driftless Stargazing LLC on Facebook and find out whenever there’s something awesome happening in the skies above.