The phenomenon behind a colorful sky.

By Jamie Carter
Updated January 04, 2021
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How many sunset sky photos are there on your phone? Hundreds? It might happen every day, but the sight of the sun sinking below an ocean horizon followed by a sky full of reds, oranges, and pinks makes for perfect vacation photos. Where do those colors come from? 

Here’s everything you need to know about why the sky turns red, orange, and pink at sunset. 

Credit: Eiji Ogura/EyeEm/Getty Images

Why does a setting sun look orange? 

A setting sun is a beautiful orange color that’s unrivaled in nature, but how can a bright yellow ball of fire that’s impossible to look at safely during the day suddenly become a soft, orangey orb as it sinks from view? 

When the sun sets, its light isn’t just grazing the Earth, as seen from where you are, but it’s also coming through a lot of Earth’s atmosphere. That means a lot of molecules and small particles in the air, which change the direction of light rays. Light is made up of lots of different wavelengths, which is why we see color. Bluer light more easily bounces off molecules in the air, while redder light doesn’t. That’s because bluer light has shorter wavelengths while redder light has longer wavelengths. The blue light gets more easily scattered, which is why the sky is blue. 

So when you view a setting sun, the more muted beams of sunlight you’re seeing is composed largely of longer wavelengths, which are towards the redder end of the spectrum. Meanwhile, the blue light is scattered out of your line of sight. The exact same thing happens during a sunrise. 

Why does the sky turn red, orange, and pink at sunset?

The phenomenon of scattering is also why the sky turns red, orange, and pink at sunset. Crucially, you need some cloud to see this. The science is the same, with short-wavelength blue and violet light scattered by molecules in the atmosphere while longer-wavelength red, orange, and pink light passing through and hitting the clouds. If the clouds aren’t there, there’s nothing for the colored light to reflect off. 

Why isn’t the sky violet?

If short-wavelength blue light gets scattered then the sky should be violet, not blue. That’s because violet light has the shortest wavelength of all. It is! The sky is violet. It’s just that human vision doesn’t perceive violet too well, the sky appears blue.

Why does a rising full moon look orange? 

When you watch a full moon rising in the eastern sky during dusk — something that only happens once each month — our natural satellite in space looks orange. Why? It’s for the same reason as why the setting sun looks orange. The moon doesn’t give off any light of its own, but only reflects sunlight. However, when you see its light you’re still looking through a lot of Earth’s atmosphere. So bluer light is scattered and redder light is not. 

Why does a ‘Blood Moon’ look red? 

A "Blood Moon" — the colloquial name for a total lunar eclipse — is thought of as being red. It’s actually more of a copper color, with hints of orange and pink, unless some particularly dense particles are in the atmosphere at the time. For example, after forest fires or an ash-spewing volcano. What happens during a "Blood Moon" is that the moon spends a few minutes or hours in Earth’s shadow in space. Earth has a shadow in space — and it’s red. It must be. Projected around 870,000 miles into space behind the Earth and opposite the sun, the only light within it is sunlight filtered by Earth’s atmosphere. With blue light scattered and red light passing through Earth’s atmosphere, the physics is exactly the same as why a sunset is red — and the result is a beautiful "Blood Moon."

Why does Mars have blue sunsets?

Sunsets on Mars are blue. That’s because the Red Planet has a reddish sky during the day. Again the color is connected to the scattering of light by particles. Since the particles in the dusty Martian atmosphere scatter red light, the sky is reddish. So during a sunset there’s a bluish glow in the sky above the sun. 

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