On a clear sunny day, the sky above us looks bright blue. In the evening, the sunset puts on a brilliant show of reds, pinks and oranges. Why is the sky blue? What makes the sunset red?
To answer these questions, we must learn about light, and the Earth’s atmosphere.
Like most curious people, you have probably asked at some time, “Why is the sky blue?” Or if you saw a beautiful sunset or sunrise, you might have asked, “Why is the sky red?”
It’s so obvious that the sky is blue, you might think the reasons would be just as obvious. They aren’t! Of all the colors of the rainbow, why blue?
Couldn’t the sky just as easily be green? Or yellow? When we see a rainbow, we do see green and yellow in the sky, as well as blue, violet, orange, yellow, red, and everything in between.
Why is the Sun Yellow?
If the Sun is supposed to be producing white light, why does the sun appear yellow to the eye instead of white?
The direct sunlight? Nearly perfectly white! But the light that passes through the Earth’s atmosphere, reflects off the ocean, and heads back through the atmosphere to you? Yellow!
So that tells us that the atmosphere definitely makes the Sun look yellow to us.
We need to start with the fact that white light is a combination of all colors produced equally by a glowing object. A glowing object that appears blue is blue because it’s producing more blue light than it is producing red, orange, yellow, green light. The color of a glowing object depends on the temperature of the object. Now we can proceed to your question. Two Reasons why the Sun appears yellow:
The Sun’s surface temperature (5,500 degrees C) produces a range of visible light (red to blue) in which yellow is the most plentiful, but not much more than other colors it produces. If the Sun were cooler, say 2,500 degrees C, it would look red, like the stars Antares and Betelgeuse. Or if the Sun were hotter, say 15,000 degrees C, it would look blue, like the star Rigel.
Why is the Sky Blue?
The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.
However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.
As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.
Why is Space Black?
We’ve all see pictures of the blackness of space. And who hasn’t enjoyed a colorful sunset or sunrise? Once you understand the science behind why the sky is blue, these other questions are easy to answer.
On Earth, the sun appears yellow. If you were out in space, or on the moon, the sun would look white. Why? In space, there is no atmosphere to scatter the sun’s light.
(Of course, you should NEVER look directly at the sun without adequate protection for your eyes. Regular sunglasses do NOT provide enough protection.)
On Earth, some of the shorter wavelengths of light (the blues and violets) are removed from the direct rays of the sun by scattering in the atmosphere. The remaining colors together appear yellow.
What Makes a Sunset red?
Photo of a red sunset.As the Sun gets lower in the sky, its light passes through more of the atmosphere to reach you. Even more of the blue and violet light is scattered, allowing the reds and yellows to pass straight through to your eyes without all that competition from the blues.
Also, larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows, sometimes making the whole western sky glow red.