Why Are Sunsets And Sunrises Red, When The Sun Is Blue-Green?

Here’s a fun fact to annoy people with; depending on how pedantic you are as a person, you could call the Sun blue-green. In fact, you have NASA’s permission to do so.

“So, the sun actually emits energy at all wavelengths from radio to gamma ray. But,” NASA explained ahead of the 2017 eclipse, “it emits most of its energy around 500 nm, which is close to blue-green light. So one might say that the sun is blue-green!”

Of course, we don’t see it like that, thanks to how our eyes work.

“Though the sun emits strongest in the green part of the spectrum, it also emits strongly in all the visible colors – red through blue,” NASA continued. “Our eyes which have three color cone cell receptors, report to the brain that each color receptor is completely saturated with significant colors being received at all visible wavelengths. Our brains then integrate these signals into a perceived white color.”

Now that we’ve got the fact that the Sun is blue-green but perceived as white out of the way, why are sunsets and sunrises red?

When the Sun hits our atmosphere, light in the blue spectrum is scattered more efficiently than red light by particles within it, known as Rayleigh scattering. With less blue light hitting your eyes, you will perceive the Sun as tinted slightly yellow. The more atmosphere the light has to travel through – say at sunrise and sunset – the more blue light gets scattered, making the Sun appear yellower or red.

Conversely, when the Sun is directly above you it will appear whiter, as the blue light has less atmosphere to scatter through in order to reach your eyes. 

On Mars, with a far thinner atmosphere filled with carbon dioxide and dust, red light scatters more efficiently, meaning all the robots we have sent there look out on an eerie blue sunset, contrasting nicely with all the red.

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