A blue sky feels like a plain fact, but it is partly something your eyes and brain build. Our eyes have three kinds of color receptors, called cones, and our brains mix the spectrum of skylight into the sky blue we see. A typical human eye responds to wavelengths from about 380 to about 750 nanometers, though the edges are not sharp and vary from person to person.

The story starts at the Sun. The Sun's light looks white, but it is made up of all the colors of the rainbow, and each color in a rainbow corresponds to a different wavelength of electromagnetic radiation.

Then the light meets the atmosphere. The air has tiny bits in it that are too small to see, and they bounce sunlight around the sky. The air molecules that scatter sunlight are mostly nitrogen and oxygen. Rayleigh scattering is the scattering of light by particles much smaller than the wavelength of the light, such as individual air molecules. The amount of light scattered is inversely proportional to the fourth power of its wavelength, so shorter waves scatter far more. Blue light, at about 400 nanometers, is scattered about ten times more than red light, at about 700 nanometers.

Scattered light goes everywhere. Light that has been scattered this way travels in all directions, so a patch of sky far from the Sun still shines with blue light. The sky is blue because of Rayleigh scattering, where gas molecules in the air send short blue waves of sunlight in all directions. About 78 percent of the air is nitrogen and about 21 percent is oxygen, so those two gases are most of what scatters the light. Blue light is scattered much more than red light because blue has shorter waves.

If scattering were the whole story, the sky would be violet. Violet is scattered even more strongly than blue, yet the sky does not look violet because sunlight contains less violet than blue. Human vision is less sensitive to violet wavelengths than to blue ones, which is another reason. A blue rather than violet sky needs several effects together: the scattering, how little violet sunlight holds, and how our eyes respond.

Now turn toward the setting Sun. At sunset the sky turns orange and red, and blue skies and red sunsets come from the same scattering process. At sunrise and sunset the low Sun's light passes through a thicker layer of atmosphere, so more of the blue is scattered out of the beam. On Earth sunsets are red because the molecules in the air scatter or filter out the blue light, leaving the longer wavelengths. Clouds low on the horizon at sunset can be lit red and orange by the sunlight that has been reddened on its long path. Never look straight at the Sun, even when it is low. Even dark sunglasses do not make it safe for your eyes.

The sky also changes between overhead and the horizon. Looking toward the horizon the sky is lighter and whiter than overhead, because the longer path scatters and re-scatters the light. Closer to the horizon, light travels through more air, mixing in white light and making the blue paler. The sky fades to a lighter blue or white there. When the Sun is low in the sky, its light has to go through more air to reach your eyes.

High above, the air has another job. The ozone layer, in the stratosphere, lies somewhere between about 9 and 30 miles above the ground. So the air around Earth does more than one job, because it scatters sunlight into every corner of the sky, while the ozone layer absorbs most of the Sun's ultraviolet radiation.

Mars is a useful test. It has a thin atmosphere, with a surface pressure about 0.6 percent of Earth's, made up mostly of carbon dioxide, nitrogen and argon. Dust storms on Mars are not rare, and the dust changes the color of the Martian sky with the weather. To our eyes, the sky on Mars would look hazy and red because of suspended dust. Dust storms there can cover much of the planet.

Two other skies make a comparison. On the Moon, with only a very thin exosphere and no air to scatter sunlight, the sky looks black even in the daytime. Mars's daytime sky is darker than Earth's because its atmosphere is thinner. Yet a NASA rover took a color picture of a sunset on Mars, and the sky around the setting Sun looked blue. Each sky is a different answer to the same question about light and air, and in every case the answer depends on how much air there is and what the air is made of.

Words to know

• cones: color receptors
• spectrum: band of colors
• atmosphere: layer of air
• horizon: distant skyline
• suspended: floating