"Why is the sky blue?" is a wonderful question, and most answers you'll hear — it reflects the ocean, the air is just blue — are wrong. Here's the real one.

Sunlight is every color at once

Start with the light itself. Sunlight looks white, but it's really all the colors of the rainbow mixed together — the visible sliver of a far wider spectrum of light. Split it with a prism, or with a single raindrop, and they fan out. So when we ask why the sky is blue, we're really asking: out of all the colors pouring down from the Sun, why does blue end up smeared across the whole sky?

Tiny air molecules pick on blue

Air is mostly nitrogen and oxygen — molecules far, far smaller than a wavelength of light. When light meets specks that small it scatters, meaning it gets knocked sideways into new directions. The crucial fact is that the scattering is lopsided: shorter wavelengths get scattered much more strongly than longer ones. Blue much more than red. (The effect is called Rayleigh scattering, and it scales as one over the wavelength to the fourth power — a fancy way of saying "blue wins, and it's not close.")

So as sunlight crosses the sky:

• Reds and yellows mostly travel straight through to your eye from the Sun's direction.
• Blue gets bounced around, ricocheting off molecule after molecule until it's reaching you from every part of the sky.

That blue glow, arriving from all directions, is the sky. (Violet actually scatters even more, but the Sun sends less of it and our eyes are tuned more toward blue, so we read the mix as blue.)

You can stage this at the kitchen table: stir a few drops of milk into a glass of water and shine a flashlight through it. From the side the water glows faintly blue; looking straight down the beam, the light comes out warm orange. A sky in a glass.

So why are sunsets red?

Same physics, longer path. When the Sun sits low at dusk, its light has to skim sideways through far more atmosphere to reach you. Over that long slog essentially all the blue gets scattered away before it arrives — leaving the reds and oranges to come through.

It's the same Sun and the same air at both times — only the length of the journey changes. And the light starts out very slightly off-white to begin with, a faint signature of the Sun's surface temperature that you're reading every day without noticing. The same air plays a second, invisible trick: it lets that sunlight through but catches the infrared glow rising back off the warm ground, which is why the planet under all this blue is warm enough to live on.