My 9 year old son and I were running through the park. I was helping him train towards running his first 5kms, and he was doing quite well so far.
The weather was amazing. The air was crisp and cool. The sky was bright and blue. There was not a cloud in the sky.
All was going fine until we took a 20 second break.
Between puffs, my son asked me, “Mum, why is the sky BLUE? If the earth is in space, and space is black, shouldn’t the sky look BLACK?”
I rolled my eyes. Is this kid for real? Deep, complicated questions while we are sweating?
I rustled through a rolodex of “Quick Answers” in my brain… but nope, I found nothing.
I had no idea WHY the sky was blue.
His question was a good one. Why isn’t the sky a depressingly black colour?
I blurted, “I dunno, let’s look it up when we get home.” And sprinted off the last 2kms.
So here it is, after an hour or so on the Internet… the answer (as I understand it) to the very good question: “Why is the sky BLUE, not black?”
The earth’s atmosphere is made up of zillions of tiny molecules, mainly nitrogen and oxygen. These have no colour of their own. But when sunlight goes through the atmosphere, the oxygen and nitrogen molecules get very excited (lucky them!),which makes them turn into a kind of one-molecule antenna that radiates and scatters the sunlight in all different directions. But sunlight looks white, and so the sky should look white, right? And that would just be too plain!
But NO. White sunlight is actually a combination of all the colours of the rainbow. If you look at the colours of the rainbow on a spectrum, red light has a slow frequency, green has a fast frequency and blue and violet have very fast frequencies. ALL the colours of the rainbow are scattered by the atmospheric molecules. But the size of the molecules produces an antenna that is BETTER at scattering high frequency colours – blue and purple light – not red, yellow or green light.
Therefore the sky should actually look blueish purple! Now, that would be quite garish and trippy if you ask me.
BUT (and this is my favorite bit) because human eyes are more sensitive and receptive to blue light (our eye receptors are made up of red cones, green cones and blue cones)… the sky LOOKS BLUE TO US , but not necessarily to other animals.
And THAT is why the sky is blue, not black, or white, or a funky rainbow, or disco lurid purple.
I really liked that answer. It was simple and elegant… much like most of the mysteries of life.
My son was rather unimpressed. He wanted an answer involving aliens and laser beams.
But I was actually pretty impressed.
The sky looks so beautifully and cheerfully blue because, after however many millions of years of evolution… that’s how we like to see it.
Beauty IS in the eye of the beholder.