Why is the sky blue?
Actually, the sky was orange until about 2.5 billion years ago, but if you jumped back in time to see it, you’d double over in a coughing fit. Way back then, the air was a toxic fog of vicious vapors: carbon monoxide, carbon dioxide, nitrogen, cyanide, and methane. This last gas gave the sky an orange tint and the land a strange glow. But then something happened that would change the sky—and the planet—forever.
Blue-green microbes called cyanobacteria formed in the ocean that were capable of a special trick that transformed the planet: photosynthesis. Later used by plants, this natural process converts sunlight and carbon dioxide into energy, creating oxygen as a by-product. Fed by nutrients in the sea and powered by the sun, cyanobacteria exploded across the ocean, pumping more and more oxygen into Earth’s atmosphere. Slowly, over the next two billion years, oxygen in the atmosphere rose to its present levels, and the sky took on the blue hue on view today.
So why is the sky blue?
The light shining from the sun is made of all the colors of the rainbow, and each color travels on its own special type of wave, called its wavelength. When light hits the air molecules in our atmosphere, its colors are scattered in all directions. Blue light is scattered more because of its short, choppy wavelength, making it the color we see the most. At sunrise and sunset, when the sunlight must travel though a thicker chunk of atmosphere to reach our eyes, blue light is scattered completely out of our field of vision, and we’re typically left seeing brilliant red and orange colors.
Text adapted from the Nat Geo Kids book Why Not?, by Chrispin Boyer, boy running photograph by Juice Images Ltd / Getty