A beautiful sunny day might prompt you to wonder why the sky is blue. It's a good question. And it's related to why sunsets and a totally eclipsed Moon are red. The answer has to do with sunlight, Earth's atmosphere, and our eyes.

Sunlight and the visible spectrum
Our eyes respond only to the very small part of the electromagnetic spectrum known as visible light. [Image: Biro Emoke Shutterstock]

The visible sunlight is white light. It's a mixture of the colors we see separated in rainbows or prisms. Seven colors make up the visible spectrum. Each color has its own wavelength and frequency. The wavelength is the distance from the top of one wave to the next, and the frequency is the number of waves that pass per second. Waves with a short wavelength and high frequency, such as blue light, are more energetic than long waves like red light. [Image: cyberphysicsco.uk]

Rayleigh scattering
Our atmosphere is made up of gas molecules, mostly nitrogen and oxygen. They have little effect on the longer waves of red, orange and yellow light, and these tend to carry on through the atmosphere without disruption.

However, the air particles interact strongly with the shorter wavelengths such as blue and violet, scattering the light. This means that the light is briefly absorbed by the particles and then thrown out in random directions. It happens many, many times, and the blue light fills the sky to give it its blue color. This selective scattering is called Rayleigh scattering, after 19th-century English physicist Lord Rayleigh who discovered the phenomenon.

The more air the blue light passes through, the more often it gets scattered. If you look at the sky straight overhead you can see that it's a darker blue than elsewhere. The light has traveled to your eyes by the shortest path, and undergone the least scattering.

Why isn't the sky violet?
Since violet light has an even shorter wavelength than blue, it seems that it ought to be scattered more and make the sky violet instead of blue. In fact, violet is scattered more than blue. However, our eyes are much more sensitive to blue light than to violet. They have three types of color receptor: one is most sensitive to red, one to green, and one to blue. Each of the receptors also has some sensitivity to a few other colors. Our greatest sensitivity is to green, and this receptor also responds to green-blue wavelengths. Indigo and violet stimulate both the red and the blue receptors. Overall the blue is stimulated much more strongly than the rather weak violet.

Sunrise, sunset
Interestingly, the same processes that make the sky blue also produce the red color of sunset. The Sun is very low in the sky, so sunlight is traveling the longest possible path. All the while the blue is being scattered away. This means that we get the reddish colors full strength, but the blue has been diluted along this path. The Sun itself appears a pinkish red. But if you look away from the setting or rising Sun, elsewhere in the sky you should be able to see the scattered blue light.

Total lunar eclipses
Rayleigh scattering can also gives us a red-orange Moon during a total lunar eclipse. Even though the Moon can't receive direct sunlight at totality, it's not completely in the dark. There is indirect lighting from sunlight that travels through the Earth's atmosphere. Since the blue light gets scattered, it leaves the redder colors to travel through the atmosphere. Some of this light shines on the Moon, often making it look as though it's covered in blood.

Reflection nebulae
Although nebulae are clouds of gas and dust with no light of their own, the Witch Head Nebula (IC 2118) in the constellation Eridanus glows a beautiful blue. [Image credit & copyright: Jeff Signorelli] It's reflecting the light from a nearby bright star. The light is blue because of Rayleigh scattering by extremely small grains of carbon dust in the nebula. Because of their size they scatter blue light more efficiently than red light. The dust does the job that air molecules do on Earth.