Why is the Sky Blue?

We love science because, often, the most child-like questions are most profound. You’ve probably heard that the sky is blue because the atmosphere “scatters” more blue light than any other color (wavelength)—which is true. But, to fully understand this simple answer invites us into the deeper worlds of electromagnetism, astrophysics, evolution, and biology.

Try an even more child-like question first: what is color? In a sense, color doesn’t exist; it’s a hallucination. We say that the colors of the spectrum correspond to certain wavelengths of light from about 200 violet to 700 red nanometers (whereby, a nanometer is 1 billionth of a meter). But, actual colors are manufactured entirely within our brains; you can measure a wavelength, but you can never know what it looks like to another eye and brain, especially since there are different kinds of eyes on our planet, translating light into nerve signals in different ways. Every species sees a different range of light and therefore has different color experiences as well.

Moreover, we don’t simply see the color of the light that comes into our eyes: colors appear differently to us depending on how they’re mixed with other colors or next to other colors and how bright they are.

Our brains present us with color experiences that should be useful, as determined by evolution. For example, the blue-detecting “cone” cells in our retinas are adapted to respond most strongly to sky-blue. If our atmosphere were different, say more violet, we might have violet cones instead, which would make us see more of the violet light coming through our atmosphere and less of the blue. In fact, the light scattered by Earth’s atmosphere includes enough violet to see, but our blue cones don’t respond strongly to it, so the sky manufactured by our brains is bluer and less violet than the “real” sky!

Every star, whether orange like our sun, or blue or red like some other stars, radiates in wavelengths all across the visible spectrum and beyond. Stars are so large and dense that the photons (particles of light) created by nuclear fusion within their cores take thousands to millions of years to reach the surface, bouncing off countless particles and molecules on the way. This is why we can tell the chemical composition of a star from its light; the light has a history that can be read into the various wavelengths coming out of (or not coming out of) the star.

These photons travel across space as electromagnetic waves, hit Earth’s atmosphere and scatter, get absorbed, or pass through. Photons don’t literally “bounce off” air molecules, but rather they are absorbed by the molecules’ electrons and re-emitted in different directions—or not re-emitted, or not absorbed to begin with, depending on their wavelength. The light from the sun includes many wavelengths, but the molecules of oxygen and nitrogen in our atmosphere scatter (absorb and re-emit) more blue light than other colors.

So, why is the sky blue? A “child’s question” calling on a lot of physics and a little philosophy of mind!

Quotes

“The sky is the daily bread of the eyes.” – Ralph Waldo Emerson

“Two things awe me most, the starry sky above me and the moral law within me.” – Immanuel Kant