If you’re as curious and inquisitive as we are, perhaps this is one of the marvels of nature you’ve been wondering about since you were a little kid, and certainly one of life’s age old questions… and today, we have the answer.
In a Nutshell
To quickly summarise: the sky looks as though it is blue when viewed by our human eyes, because the waves of blue light we see are shorter and therefore distributed across our horizon even more than the other colors in the spectrum.
If that’s not making any sense to you, don’t worry. It’s pretty complicated, especially if you’ve been out of school as long as us! To make things easier, let’s break it down. First things first, we’re going to have to go through a quick crash course about light.
When you draw a picture of the sun, you probably color it in a nice butter yellow. Up in the sky, the light it produces looks bright and white, but it’s actually made up of all of the colors you can imagine - which is what we see when a rainbow is produced.
In simple terms, light can be understood as being a wave of energy, with each individual color coming in a different size of wavelength, represented on what’s called a spectrum, which is essentially just a band of colors.
At one side of this band you’ll find red light, boasting the longest wavelength of any color, and on the opposite end of the spectrum are blue and violet lights, possessing especially small wavelengths.
For instance, the average red light wave measures at approximately 750 nanometers (a very very small unit of measurement) whilst blue or violet waves come in at around 400. A nanometer refers to one-billionth of a meter.
To put that into perspective, pull off a hair from your head (or just look at one if you’re squeamish!) That single hair is probably around 50,000 nanometers in thickness!
All light will traditionally travel in a straight line…unless another material intervenes and interrupts its movement in one of three ways. Light can either be reflected, as with a mirror, bent using a prism, or scattered by other molecules.
Now that we understand a little more about light and wavelengths, we can get down to the real nitty gritty. As the sun casts light down upon our Earth, it is then deflected by the minuscule molecules of nitrogen and oxygen in our atmosphere.
As such molecules are even tinier than the visible light being distributed, the amount that they are scattered across our sky is determined by the wavelength of each individual light color.
This complicated occurrence is known as Rayleigh Scattering, after the Lord who discovered and explained the process, and is the primary reason that the sky takes on such beautiful colors throughout the year.
The Sky Is Blue, Da Ba Dee Da Ba Di
So, as we’ve already established, blue and violet lights have the shortest wavelengths, and are therefore subject to Rayleigh Scattering more than any other color. But… why, then isn’t the sky some kind of beautiful purple?
Well, as humans, our eyes are actually more sensitive to blue light than violet, and can therefore pick it out much more easily; furthermore, there isn’t that much violet light produced by the sun to begin with, which is why it is overpowered by blue.
The other color wavelengths don’t appear in the color of the sky as they form their own unified group and are viewed as white light. Some of this excess brightness can be wholly absorbed by the Earth’s upper atmosphere, too.
So bright is the light we see during the day, coloring our humongous outdoor ceiling a beautiful blue, that it completely obscures all of the stars in the sky, as their lights are dimmer and easily concealed. They don’t just disappear during the day!
Eagle-eyed sky observers might notice that the blues we can see directly above us in the sky are the best and brightest, gradually fading out to a paler and paler color as it stretches out towards the horizon. Why is that?
Well, this scattering is repeated the further a wave of light has to travel through the atmosphere, as the Earth’s surface contributes to additional reflecting and distributing, decreasing the vibrancy of the blue the further it goes.
In such areas of increased scattering, the blue light’s dominance over other colors is diminished, so we instead see more white light. Think of it like adding white paint to blue paint - it doesn’t overpower the color entirely, just fades it out.
So…Why Is A Sunset Red?
Okay, we’ve covered blue and that was easy enough. What causes those beautiful bold blood red and orange colors when the sun is setting and the sky is fading towards night time, then?
As the sun, our source of light, lowers into the horizon, more of it must pass through the Earth’s atmosphere in order to reach us and be perceived by our light. This results in an increased scattering of those blue and violet wavelengths.
Because of this, some of the reds and yellows that are usually obscured by the scattered lights are able to pass through far more easily, creating some truly stunning scenes as day transitions into night time.
Unfortunately, an increased contribution of dust, aerosols and other pollutants in our atmosphere are responsible for further scattering of those longer wavelength colors, which occasionally results in an entire horizon glowing ominously red.
Is The Sky Blue On Other Planets?
According to the space experts over at NASA, that is totally dependant on the atmosphere of the planet in question and what kinds of things it contains. Let’s take Mars, the Red planet, as an example.
Because Mars has an atmosphere that is especially thin, full of fine dust particles and consists mostly of carbon dioxide molecules, this has a different effect on the sun’s light than the atmosphere here on Earth.
As photos from the Mars Rovers indicate, this creates an almost opposing effect to our own sky, with an orange or red appearance during the day, and more of a bluish grey color once the sun has set. Fascinating!