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Why is the sky blue?

Why is the sky blue? An aerial view of clouds floating over the ocean surrounded by the glow of a blue sky
Ancient philosophers puzzled over the question, "Why is the sky blue?" Now we know it's all down to Rayleigh scattering (Image credit: Getty Images)

Everyone loves a clear sunny day, but have you ever looked up and wondered exactly why is the sky blue? 

The answer lies in the physics of when sunlight passes through the atmosphere. The light rays are scattered in all directions as they hit the air molecules, and light at the blue end of the spectrum is scattered stronger than other colors. 

Related: Earth's magnetic field: Explained

Author profile pic of Andrew May.
Andrew May

Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer.

In outer space, above the Earth's atmosphere, the sky looks black even in sunlight. That's one of the most striking phenomena experienced by travelers on Blue Origin's suborbital flights.

"To see the blue color go right by you and now you're staring into blackness, that's the thing," as William Shatner put it after his flight, trying to convey the experience he just went through. "The coloring of blue," he added. 

This blackness is easy enough to understand. Unless you're looking directly at the sun, there's no reason for the sky to be illuminated at all. The real puzzle is why it's illuminated down here on the surface of the planet — a longstanding mystery that wasn't fully explained until the end of the 19th century.

The key lies in an effect known as Rayleigh scattering, after its discoverer Lord Rayleigh. This refers to the way light bounces off small particles, up to about a tenth the wavelength of the light itself — which includes the molecules making up the Earth’s atmosphere.

Rayleigh showed that longer wavelengths, corresponding to the red end of the spectrum, aren't scattered as strongly as short wavelengths like blue and violet. Of those two colors, it's the blue that dominates, partly because our eyes are more sensitive to it, and also because the sun emits less violet light to start with.

The sky isn't always blue. When the sun is low in the sky, at sunrise or sunset, it can take on a red hue. This is explained by the same physics — Rayleigh scattering — as the blueness of the sky at other times.

When we look towards the sun at sunset, we're seeing light that has traveled further through the atmosphere than when the sun is high in the sky. So most of the shorter wavelengths have been scattered away, and we just see what's left.

Sunsets like this one appear red because the light from the sun has traveled further through the atmosphere and most of the shorter wavelengths (blues and violets) have been scattered away. (Image credit: Martin Harvey via Getty Images)
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Additional resources

For more information about the physics behind the Earth's atmosphere and light, check out "The Earth's Atmosphere: Its Physics and Dynamics (opens in new tab)" by Kshudiram Saha and "A Ray of Light: A Book of Science and Wonder (opens in new tab)" by Walter Wick.

Bibliography

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Andrew May
Astrophysicist

Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortean Times, How It Works, All About Space, BBC Science Focus, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.

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