Why do Humans Perceive the Earth’s Skies as Blue?

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A common question everyone asks is – Why is Earth’s sky blue? From sunrise to sunset, the color of the sky changes to various colors, ranging from yellow, orange, and red. What is the reason for this phenomenon?

To understand why Earth’s sky is blue, you have to understand light and how it works and how we perceive blue objects. It is also essential to know how light interacts with various gas molecules present in this planet’s atmosphere.

Light – What you need to know?

Light, a form of energy, from the sun looks white as it is a combination of all the colors you see in a rainbow. You can observe this phenomenon by letting sunlight to pass through a prism. The object separates the light into all its colors, allowing you to see them individually.

As it is an electromagnetic wave, it causes the protons and electrons in the air to oscillate. Due to this movement in the particles, they start producing electromagnetic radiation. This will be in the same frequency as the sunlight entering the Earth’s atmosphere. As a result, it starts to spread the light in different directions, resulting in a phenomenon known as scattering.

Red light is the one with the longest wavelength, while violet and blue have shorter wavelengths. The tiny molecules in the sky (oxygen and nitrogen) scatter the light depending on the wavelength. It happens due to an effect known as Rayleigh Scattering, a phenomenon that draws its name from its discoverer, Lord Rayleigh.

Blue light, unlike red light, has higher frequencies, despite its shorter wavelength. When it comes in contact with the particles in the sky, blue makes the molecules oscillate at a faster rate. Higher the level of oscillation, greater is the scattering effect.

What makes the sky look like blue objects?

As a result, the intensity of blue light scattering in the atmosphere is greater than red light. From your point of observation on Earth, the color you see depends on which light the atmosphere redirects towards your eyes.

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In this case, blue is the one that scatters more than red, resulting in this hue. You should keep in mind that violet also gets scattered stronger than blue. However, as the majority of the sunlight entering the Earth’s atmosphere is blue, our eyes see this color.

Also, it is essential to keep in mind that the sensitivity of our eyes towards blue is greater than it is for violet. Combining all these factors explain why the sky looks like a blue object.

Why isn’t this the case during sunrise and sunset?

Naturally, as you understand how sunlight interacts with the atmosphere, it explains why the sky looks like blue objects. However, when it comes to sunrise and sunset, shouldn’t it also be in the same color?

In this situation, you have to consider the location of the sun. When the star is at Earth’s horizon, there is a change in the color of the sky. Why? It’s because the light from the sun has to travel a longer distance to reach the atmosphere. At the same time, as the position of the star is at the horizon, the scattering effect has an opposite effect on the color of the sky.

Remember that red light has a longer wavelength than other colors in the rainbow? This comes into play as the sun starts to approach the horizon. Blue light gets scattered, due to which, it doesn’t reach your eyes.

Instead, red light becomes dominant, due to which there is a change in the color of the sky. You should also note that the greater the distance light has to travel, there will be more opportunities for blue light to undergo significant scattering.

There is also another factor that comes into play, during sunrise and sunset. Due to pollution, there are plenty of particles such as mineral dust, carbon, and sulfate in the sky. The molecules of these substances increase the level of scattering that takes place in the atmosphere. As a result, you get different colors during sunset.

Shouldn’t clouds also be blue objects?

If the sky looks like blue objects due to scattering, shouldn’t the same effects apply to clouds? Keep in mind these bodies contain water droplets, which are larger in comparison to the wavelengths of light.

Due to this reason, these particles don’t scatter red and blue light as much as the Earth’s atmosphere. As a result, the clouds have a whitish appearance, while the sky looks like blue objects.

What about other planets?

Now that you understand how light behaves on our planet, you can apply the same logic to other celestial bodies. Let’s take the closest object in space to Earth – Moon. If you were to look at any photographs from astronauts, satellites, and telescopes, you observe a common phenomenon – the sky is black.

In this case, the Moon’s atmosphere is thin in comparison to Earth, the scattering effect is negligible, resulting in a dark sky. Whether it is day or night on the Moon, it will always appear to be black to the human eye.

Another planet where you can see how this phenomenon affects the color of the sky is Mars, a celestial body everyone wants to go to, and transform it into a livable environment.

From any photograph of this planet, you can see that the sky is yellow. The dust particles in Mars atmosphere are large enough to absorb incoming blue light from the sun. However, it scatters the remaining colors, resulting in a yellowish sky.

Again, during sunrise and sunset, there is a change in the color of the sky. On Mars, both the distance and density of the atmosphere play a huge role in scattering the light. Blue light undergoes the most amount of scattering on this planet. However, the longer wavelengths don’t undergo significant scattering. As a result, if you were on Mars during dusk or dawn, you will notice the Sun has a blue glow around it.

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