If you look up on a sunny day, you will see a blue sky. This is the color of the sky. Is it the only shade of the sky?

The nature of light, atoms and Molecules, as well as some quirky parts of Earth's atmosphere, are involved in the answers. Big lasers are also used for science.

Blue skies?

When we see a blue sky, what are we seeing? Are we seeing Nitrogen or Oxygen? The simple answer is no.

The spectrum of visible light produced by the Sun is white but includes all the colors of the rainbow. The atmosphere scatters blue light in all directions when the sun shines. It results in a white Sun and blue skies when there is no wind.

This effect can be seen at sunset because sunlight has to travel through more air. We end up with a red Sun with bluer colors surrounding it when the Sun is close to the horizon.

What is the true color of the sky if we don't see any sunlight? At night, we might be able to get an answer.

The color of dark skies

The night sky is dark but not completely black. The night sky glows even though there are stars. The atmosphere is glowing naturally.

You can see the trees and hills silhouetted against the sky on a dark moonless night in the country.

The airglow is created by atoms and Molecules in the atmosphere. Oxygen causes green and red light, OH causes red light, and sodium causes a sickly yellow light. Nitrogen is abundant in the air, but it doesn't make much of a difference to airglow.

The colors of airglow are caused by the release of quanta in the form of light. The green light produced by the split of O2 into pairs of oxygen atoms can be seen at high altitudes.

Yellow light, shooting stars and sharp images

Our atmosphere is made up of a small fraction of sodium atoms, but they make up a large part of airglow and have an unusual origin.

If you're willing to wait, you can see the stars. Grains of dust heating up in the upper atmosphere causes them to travel over seven miles per second.

At 100 kilometers altitude, shooting stars leave a trail of atoms and Molecules. Sometimes you can see shooting stars with different colors. Smoke trails can be left by shooting stars. There is a small amount of sodium among those atoms and Molecules.

The high layer ofsodium atoms is useful to the astronomer. Our atmosphere is constantly moving, it's turbulent, and it blurs images of planets. When you look at a long road in the summer, think of the shimmer.

Calculating how the stars' images are distorted is one way to compensate for the turbulence. The distortion can be removed with a special mirror that can be adjusted. Space telescopes have the advantage of not looking through airglow.

There is a big problem with this technique. There aren't enough bright stars in the sky. Astronomers use lasers to make artificial stars in the night sky.

We can make the sodium atoms glow bright by firing a power laser at them. An artificial star can be used. You can see a shooting star at night.

At times the sky is not blue. The sky is a mix of green, yellow and red. It's colors are caused by scattered sunlight, oxygen, and sodium. Artificial yellow stars can be made with a little bit of physics and lasers.

Matthew Kenworthy is an associate professor in Astronomy at Leiden University.

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