Why Is the Sky Blue and Why Are Sunsets Red? The Science Explained
The sky is blue because the gases in Earth's atmosphere scatter blue light in all directions more than they scatter the other colours. Sunsets are red because, at that hour, sunlight has to pass through so much air that most of the blue has already been scattered away.
Sunlight is every colour at once
Light from the Sun looks white, but it is really made of all the colours of the rainbow. A prism shows this: when white light passes through it, the light separates into its colours.
Light travels in waves. Blue light has shorter waves than red light. That difference in wavelength is the key to the whole story.
What the atmosphere does to light
Light travels in a straight line unless something gets in the way. NASA lists three things that can happen to it:
- It can be reflected, like in a mirror.
- It can be bent, like in a prism.
- It can be scattered, like by the molecules of gas in the air.
When sunlight enters the atmosphere, it is scattered in all directions by the gases and tiny particles in the air. Blue light is scattered more than the other colours because it travels as shorter, smaller waves. That scattered blue light reaches your eyes from every part of the sky, so the sky looks blue.
Why the sky is paler near the horizon
Look toward the horizon and the sky fades to a lighter blue or white. Light from low in the sky has passed through much more air than light from directly overhead. On the way, the blue has been scattered and re-scattered many times in many directions. The surface of the Earth also reflects and scatters light. All this scattering mixes the colours back together, so we see more white and less blue.
Why sunsets are red and orange
As the Sun gets low, its light travels through even more of the atmosphere to reach you. More of the blue light is scattered out of the beam, which lets the reds and yellows pass straight through to your eyes.
Dust, pollution and other tiny particles called aerosols also scatter blue light. This can leave the sky glowing with purer red and yellow, which is why the western sky sometimes seems to burn after sunset.
Is the sky blue on other planets?
It depends on what is in the atmosphere. Mars has a very thin atmosphere made mostly of carbon dioxide and filled with fine dust. These fine particles scatter light differently from Earth's air.
Photos from NASA's rovers and landers show that Mars does the opposite of Earth. In the daytime the Martian sky looks orange or reddish. At sunset, the sky around the Sun takes on a blue-gray tone, according to images taken by NASA's Mars Pathfinder Lander.
Is the sky always blue?
No. NASA notes it fades toward white near the horizon and glows red or orange at sunset, depending on the Sun's angle and the dust and particles in the air.
Facts checked October 1, 2026.
Sources
- NASA Space Place, "Why Is the Sky Blue?" (last updated August 29, 2022): https://spaceplace.nasa.gov/blue-sky/en/