The incredible truth behind why the sky isn’t really blue

Shutterstock.com/ Iakov Kalinin

One of the most common questions a small child asks is “Why is the sky blue?”. It is such a simple question that very often only the mind of a child is able to come up with it. In adult life, we rarely question such things that we take for granted. However, if you are worried that someday a child will ask you this very question, we will help you to explain.

The easiest way to explain the color of the sky is to talk about the scattering of light. When the sun shines light down on our planet it interacts with some of the gases that we have. It hits this light and gets scattered around. The blue light is the one that scattered most and so we see a blue sky. That should be enough of an answer to satisfy an easily distracted child but if like many children, they just keep persisting, you will need to know a little more.

Picture a prism that you learned about in science class or from a Pink Floyd album. When light hits a prism it is separated into all the colors of the rainbow. However, when light enters our Earth, there is no prism. There are many molecules of oxygen and nitrogen and all those other tiny bits that make up our atmosphere. When the white light hits these molecules the light is scattered in all different directions by these molecules. Each color is separated from white and travels in different waves. Blue travels as the shortest, smallest waves. Red and orange are pretty long and flat. As blue is the shortest and smallest it is the most widely scattered. Thus, everywhere we look in the sky, we see blue. 

At sunset and sunrise, the light has to travel through a lot more of the molecules in the atmosphere to reach you. This means that as the blue light scatters, less of that blue reaches you by sunset. Instead, the long and fat reds and oranges are far more visible as they don’t scatter that much and so are still visible after passing through the atmosphere. 

While you may be satisfied with this answer. The curious child will ask another question. What about other planets? Is the sky on other planets blue? A tough question, as we know so little about so many planets. One planet that we have seen a sunset on, is Mars

Mars acts very differently to Earth. During the day the sky is very orange or reddish. While at night as the sun starts to set the sky starts to get a blueish gray tone. This all comes down to the different atmosphere. Mars has an incredibly small atmosphere that only has carbon dioxide and tiny dust inside. These particles essentially scatter light very differently to those on Earth. 

The child will still ask one more question. It will ask if the dog down the street sees the sky same way or the birds in the sky. The answer is no. Based on the number of cones a person or animal has, will decide what color we see. If you have ever used Paint on your computer you may know that every color in our world has an RBG score. That is a certain amount of Red, Blue and Green to get any other color. This is the basis of our whole ability to see color. There are some animals with nine cones and the colors they see must be truly fantastic. Imagine the sunset if you could see three times more colors than we do now (and likely many more as you extrapolate out the different multiples). Now that would be a sunset worthy of an Instagram post.