At some point in every life, each of us has looked up at that huge blue umbrella of sky and simply marvelled. Sometimes it's painted with clouds, with the golden ball of the Sun smiling down at us. But setting the daydream aside, there's a curious question hiding up there: why is the sky the colour it is? Why blue most of the time, and why does it take on a tint of orange and red in the early mornings and late evenings?
The short answer comes down to something called the scattering of light.
Light is made of many wavelengths
That golden ball, the Sun, sends out light as rays that travel a huge distance to reach Earth — roughly ninety-three million miles, or about one hundred and fifty million kilometres. The distance isn't constant, since Earth is always moving along its orbit while spinning on its own axis, which changes how far the light has to travel to any particular point.
What is light, though? Basically, it's a form of energy in waves. Part of that energy is visible to the human eye, and even that visible portion moves in waves. If you imagine a big wavy line and cut it in the middle, you're left with a smaller one that's still a wave. Different points on that wave, measured by height, give us what we call wavelength.
Those different wavelengths reach our eyes as different colours. Remember VIBGYOR — violet, indigo, blue, green, yellow, orange, red — from school? Going from the shortest wavelength to the longest, we have violet, blue, green, yellow, orange and red.
How scattering paints the sky blue
When sunlight enters Earth's atmosphere, it gets scattered by the atoms and molecules of the gases in the air, bouncing and spreading in all directions. Scattering happens when the light waves hit those gas molecules instead of being absorbed. The key detail is that air molecules don't scatter every wavelength equally. The shorter wavelengths, like violet, blue and green, scatter far more than the longer ones, like yellow, orange and red.
According to Rayleigh's scattering theory, the higher the wavelength, the less it spreads. And that tells you which colours fan out across the sky the most.
Why not violet, then?
If shorter wavelengths scatter more, why don't we see a violet or violet-blue sky? Half the reason is that violet light does scatter, but the other half is about us: our eyes are simply more sensitive to the colour blue. Together, that's why blue wins.
Why sunsets turn red
So why does the sky go red at sunset? It comes down to distance. By evening, the sunlight has to travel a longer path through the atmosphere to reach your spot on Earth. Passing through so much more air causes the shorter blue wavelengths to scatter away heavily before they ever reach your eyes, leaving the yellow-orange and red tones you finally see.
That's the yellowish-orange-red tint we admire as the day ends.
What about other worlds?
Does every planet get a blue day and a red sunset? It depends entirely on the atmosphere there. On Mars, which has a thin atmosphere full of carbon dioxide and dust particles, the sky takes on an orangish-red tint, while sunsets can appear greyish blue.
And on the Moon, where there is no atmosphere, light travels without scattering. Look toward the Sun from the lunar surface and you'd see a big ball of white light; look away, and you'd meet only dark, empty space.
It is remarkable how a little gas and sunlight can draw such a different picture in each sky. Next time you look up, you'll know the wave of colour above you is a story about scattering and how far the light has travelled to reach you.