How to Explain Why the Sky Is Blue to a 5-Year-Old

Bad Astronomy
The entire universe in blog form
March 22 2014 7:30 AM

Meme, Myself, and Eyes

An interesting side effect of being a person who puts words together for a living is that you sometimes get quoted. Normally, I have some marginal control over my words, squirmy and slippery as words can be. But once they are unleashed into the greater world they are free to pursue their own meaning, or, at least, for others to interpret them and find meaning in them as they see fit.

That’s OK, and I accept that as the way of things. But it does sometimes cause me great amusement.

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For example, a little while back a quote-meme featuring something I wrote started making its way around the interwebz. It’s been around a while, but I saw that Elise Andrew put it on her IFLS feed, and it caught my eye again. I don’t know who originally created it, but here it is:

why the sky is blue
Don't try this at home.

Photo from the depths of the Internet

The quote says, “If a little kid ever asks you why the sky is blue, you look him or her right in the eye and say, ‘It’s because of quantum effects involving Rayleigh scattering combined with a lack of violet photon receptors in our retinae.’”

What amuses me is how people interpret it. A lot of folks think it’s smug, or stupid, or counterproductive, or that I’m being a jerk (the irony on that last one? The picture usually used is from my TAM 8 talk, commonly called the “Don’t Be a Dick” talk).

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

What never gets mentioned is that this quotation is out of context, and it was overly complicated on purpose as a joke!* It’s from my first book, Bad Astronomy, and it comes at the very end of the chapter explaining why the sky is blue. I open that chapter saying that this question is asked by every little kid, and it’s actually a bit complicated to answer. But in the end it has to do with molecules in the air scattering blue light more than red, diffusing blue photons from the Sun so they appear to come from all over the sky, and not just from the Sun. Plus, the Sun doesn’t emit nearly as many purple photons as blue ones, and our eyes aren’t very sensitive to purple light, so overall the sky winds up looking blue.

I’ll note that I start that chapter of my book by saying that after reading the chapter you’ll be able to explain the blue sky to any 5-year-old who asks you about it. At the end of that chapter I supply the above tongue-in-cheek quote.

What’s important, though, is what I wrote next [emphasis mine]:

Okay, that might not work. In reality, explain to them that the light coming from the Sun is like stuff falling from a tree. Lighter things like leaves get blown all around and fall everywhere, while heavier things like nuts fall straight down without getting scattered around. Blue light is like the leaves and gets spread out all over the sky. Red light is like the heavier material, falling straight down from the Sun into our eyes.

So obviously that quotation, when seen in context, was just a joke. I wrote it nearly 15 years ago, and I still think it’s funny, but it tickles me further that people still misinterpret it after all these years.

And the kicker is what I wrote to end that chapter in my book. Maybe you’ll like it:

Even if [the kids] still don’t get it, that’s OK. Tell them that once upon a time, not too long ago, nobody knew why the sky was blue. Some folks were brave enough to admit they didn’t understand and went on to figure it out for themselves.
Never stop asking why! Great discoveries about the simplest things are often made that way.

Hey, future meme generators! If you want to quote me, why not use that? I suspect it will always be true and worth letting others know.

*Back when I was originally writing the book, and I was fiddling with the wording of that line, I had to take out a lot of stuff so that it would be readable; for example, it omits the fact that the Sun doesn’t emit many violet photons compared to other colors. Also, I threw in the word “quantum” as something of a meta-joke, since Rayleigh scattering is not a quantum process. Looking back on it now, I would have phrased it differently, but either way I suspect it would still be misinterpreted the way it is today. Oh well!

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