Why is the Sky Blue?

Why is the sky blue? Usually the books start going on about wavelengths of light, Tyndall effect, and Rayleigh scattering. It's a bit much for young children. First they try to teach some correct but complicated physics. Then they use it to explain blue sky and sunsets. But what happens when kids don't understand the physics? Doesn't this make their explanation useless? And do the kids just give up?

They're wrong: you don't need complicated physics to understand this. The sky is blue for a very simple reason:

The Earth's atmosphere is not a perfectly transparent material. Instead it is blue!

The sky is blue for much the same reason that a cloud of powder is white. Powder isn't invisible. Throw some dust into the air on a sunny day and you'll see a visible white cloud. But what happens if you could throw some AIR? You might think that a cloud of air would be invisible. You'd be wrong. Air isn't invisible, instead its molecules scatter light in the same way that any small particles do. Air is a powdery-blue substance. (But then... shouldn't air be a white substance. Yes! And explaining why it's blue rather than white is where the complicated physics comes in.)

The color of air can be confusing because air seems transparent. It's true that small amounts of air are almost perfectly transparent. So are small amounts of water. Go to an opaque muddy river or pond and use a cup to dip out some water. The water looks fairly clear, no? Yet the deep river is opaque brown. Whenever you try to look through ten cups of water, or a hundred cups, the water seems to turn into brown paint. Yet a single cup of river water almost looks clean.

Air behaves like this too. A mile of air looks clear, but ten miles of air looks misty blue, and a thousand miles of air looks opaque white. The air is acting like the dirty river water, where a thin layer looks colorless but a thick layer does not.

The sky is blue because air is a powdery blue material, and when the sun shines upon it, we can see this blue color. Each molecule of air behaves like a bluish-looking mote of dust. Stare upwards on a sunny day, and you're looking into a thick cloud of brightly-lit air. (Note: there really is no "sky" up there at all. The sky is an illusory surface. You're not really looking at a blue surface. Instead you're just seeing the Earth's layer of blue air against the blackness of outer space. )

Suppose you could go far out into space away from the Earth, then build yourself a thin hollow glass bubble a thousand miles wide. Viewed from the Earth, your empty glass bubble would be almost invisible. OK, now fill your bubble with air. It won't be invisible any more. It will look like a giant droplet of bright blue paint. It might even look whitish in the middle, since very thick layers of air seem as white as milk. What if you let your giant glass bubble crash into the moon? The air inside would pour out over the moon's surface and form a thick temporary layer of atmosphere. The moon wouldn't look white anymore. It would turn blue.