Colors of the Sky

The oft admired beauty of sunsets is a common theme in photography, movies, and visual arts in general.  Their vibrant colors lend inspiration to people around the world.  However, not all sunsets are so colorful.  How often do you see a picture-worthy sunset?  Usually you won't see more than the sun disappearing below the horizon taking the daylight along with it.  So what is the reason behind the magnificent sunsets?

Let's start with a seemingly more simple question: is the sun actually yellow? (Warning: Never look directly at the sun!)  The answer to that is no.  Even though the sun appears to be yellow, it is actually pinkish-white in color.  It may seem odd at first, but the explanation for how we see the sun is tied to why the sky is blue.  Yes, the answer to the question that every kid asks is also responsible for the yellow daytime sun.

People can only see the light that enters their eye.  Light can get to your eye in one of two ways.  The first way applies to objects that are light sources.  These objects emit light that can travel directly to your eye.  Fires and the sun both fit into this category, but most things that you come across do not.  However, a light emitter is needed in order to see them, which should be obvious to anyone who has tried to walk through a dark room.  Let's say you want to walk through your dining room at night.  You first turn on the lamp.  The light-emitting lamp sends light rays around the room.  Some of these light rays will be absorbed.  The ones that are not absorbed bounce off and travel through the air to your eyes, allowing you to see the object.  An object appears white if all visible colors of light are reflected and very little is absorbed.  When the opposite happens and only a little light is reflected, the object will appear black.  The color of an object depends on the colors of light rays that it reflects, since those are what you see.  The reflected colors are determined by properties of the molecules which make up the object.

Back to the sky: The Earth's atmosphere is made of many types of molecules.  Nitrogen is by far the most common gas in the atmosphere.  If you were to take a sample of 1000 air molecules from a desert (dry air - no water), about 780 of them would be nitrogen gas.  Oxygen would come in second with about 210 molecules.  Third place would fall to argon with a measly 9 molecules.  That adds up to 999 molecules.  The last molecule in your sample would likely be carbon dioxide or possibly another low-quantity molecule, such as helium.  Can you guess which color of light nitrogen reflects?  Because it is the most numerous molecule type in the atmosphere, nitrogen has the largest effect on the color of the sky.  Since the sky is blue, you should have concluded that nitrogen reflects blue light.

Now that we've solved why the sky is blue, we need to figure out the connected problem which is the color of the sun.  After the sunlight leaves the sun, it must first travel through space (which is mostly empty) and then the Earth's atmosphere.  As the blue sunlight is scattered (or reflected) by the air molecules, most of the light left to continue on the path to our eyes is red and yellow  Thus, the sun appears to be yellow (the remaining blues balance the reds).

When the sun is low in the sky at dawn and dusk, the sunlight has to travel through the atmosphere at an angle.  This makes the path through the air molecules longer.  Since the light is moving past more molecules on a longer path, the chances of its being reflected increases.  This results in a larger amount of blue light being reflected.  By the time the light reaches your eyes, all of the blues will have scattered and only the reds and yellows remain. 

Dust particles, smog, and clouds can enhance the color of sunsets.  Dust particles and smog tend to filter out the blues and greens creating a more reddened sunset.  Clouds reflect all colors of light, which is why they generally appear white.  Therefore, they will reflect whichever colors of sunlight reach them and can add to the colorful effect.  They can appear dark in the foreground and add texture and variety, as seen in the photo above.

The above explanation is also why full moons can appear bright orange when seen at dusk.  The moon is visible to us because it reflects sunlight.  The initial moonlight contains many of the same colors as sunlight for this reason.  As with sunsets, the blue light is scattered while traveling a longer path through the atmosphere leaving only the red colors for us to see. 

References
Earth Fact Sheet by NASA
What colour is the Sun? by Prof Hamilton, Univ of Colorado