Why is our sky not green?

The elementary school going son, like many children his age, pulls a full why-wagon with him wherever he goes. The questions tumble out with ease, and can be anywhere on the spectrum : 

They are all fair game.

Sometimes, of course, his questions chip away at the stoutest of theories. For instance, a few years ago, as we mooned about the hills overlooking the bay at sunset and taking in the shades of pinks, oranges, blues, grays, purples and reds, he said, “Why is the sunset never green?

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Now, that is a perfectly valid question with a perfectly scientific answer. However, it had me stumped, for it never occurred to me to ask that particular question.  I remember being awed a few years ago, when the children had drawn rust and pink colored skies when asked to imagine a sky for their imaginary world. 

How often do we take the time to question things that just are? 

This is why when I read the Pale Blue Dot by Carl Sagan, and he comes up with a marvelous chapter based on determining the planetary world one is in simply based on the color of the sky, I shone with girlish delight. Here, was the kind of leap in imagination where only deep thought and research can take you, and here he was, simply giving it away in a book. All his marvelous thought processes, his wonder of the world, his eternal curiosity and scientific rigor just laid out on a page so we could embrace it in one simple reading. 

“The color of the sky characterizes the world. Plop me down on any planet in the Solar System, without seeing the gravity, without glimpsing the ground, let me take a look at the sun and the sky, and I can, I think, pretty well tell you where I am, That familiar shade of blue, interrupted here and there by fleecy white clouds, is a signature of our world. “ – Carl Sagan, Pale Blue Dot

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The whimsical side of me wants to ask whether he will recognize Earth at sunrise, sunset, during wildfires and what-not. 

The essay, Sacred Black , in the book, Pale Blue Dot is well worth reading. He explains the reasoning behind the colors of the planets as we see them. He deduces the color of the sky based on the elements found in their atmospheres. 

  1. Venus, he says, probably has a red sky.
  2. Mars has a sky that is between ochre and pink much like the colors of the desert.
  3. Jupiter, Saturn – worlds with such giant atmospheres such that sunlight hardly penetrates it, have black skies. He talks about this bleak expanse of a sky being interrupted here and there by strokes of lightning in the thick mop of clouds surrounding the planets. This image does make for a sober shiver for someone who loves the sky and its myriad attractions. Imagine, not being able to the stars, the sun, or anything beyond the clouds – brrrr.
  4. Uranus & Neptune – uncanny, austere blue color. The distant sunlight reaches a comparatively clean atmosphere of hydrogen, helium and methane in these planets. The skies may be blue or green at a certain depth resulting in an aquamarine or an ‘unearthly blue’.

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He shows us how in the absence of an atmosphere, an inky deep purple is all there is. How, our planet is only a pale blue dot floating in an inky void illumined by a ray of light from the sun. Our eyes may not show us green colors in the sky at sunset, but it does detect plenty of green in the flora around us.

What would you like to see in a sky?

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