Why is the Sun Yellow and the Sky Blue?

The sky is not blue – it’s mostly transparent air which is, at best, the color of whatever

light it scatters. Sure, it does scatter blue light more than red light; in fact, the higher

the frequency of light, the more it gets scattered by the atmosphere, so ultraviolet scatters

more than blue, which scatters more than green, more than yellow, more than red, more than

infrared – but still, only a tiny bit of light scatters while most of it goes straight

through, which is, you know, how the sun can light up the ground, why we can see the moon

and stars, etc.

The sun itself actually emits a wide range of frequencies of light, and our human eyes

perceive this particular combination as "white” or neutral in color. The vast majority of

the sky appears blueish because sunlight that was trying to go somewhere else got scattered

by the air and instead ended up in your eye. Bummer. It still has a wide range of frequencies

in it, but with slightly more blue than in white light, roughly the same amount of green,

and less red.

You can see a simple demonstration of this on a computer if I take a white background,

add a bit of deep blue and subtract a tiny bit of pure red: I get a nice sky color!

And the reverse effect happens when you look near the sun – light that was trying to

get to your eyes gets scattered away, and so the remaining light has a lot less blue

and slightly more red compared with white light, which is why the sun and the sky directly

around it appear yellowish during the day! At sunset, there’s even more air for the

light to scatter off of before it reaches you, hence the even richer oranges and reds.

And of course you can do the computer demo again, this time subtracting pure blue and

adding a little red: voila! Noontime sun. Subtract more blue and add even more red?

Sunset orange.

So, the sky is not blue; it’s a stage upon which all colors dance. Red colors tend to

dance in straighter lines, green colors more randomly, and deep blue colors dance the most

frenetically of all. Yet at some point, they ultimately dance their way into your eyes,

and my eyes, and to the earth and into space, so that everyone everywhere can appreciate

the grand ballet

of light.