A clear blue sky over Earth, shaped by sunlight scattering through the atmosphere (Freepik)
When sunlight strikes our eyes, what we see is a white light. However, hidden within this brightness is a long spectrum of colors red, orange, yellow, green, blue, indigo and violet. This light can then be separated by a prism into all its rainbow components, showing that white is, in truth, a mix of many shades. When this mixed light touches the Earth, it does not continue on naked instead, it bounces against the air to reveal its secrets in the sky above.
An invisible atmosphere surrounds us: a mix of millions of molecules of nitrogen and oxygen with dust and water particles. When sunlight hits the air, the rays are scattered by these tiny goblins. The most interesting thing is that shorter wavelengths scatter more efficiently than longer ones. This means that blue particularly shorter than red or orange ones is scattered in each and every direction.
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That's why when we look up at the sky during the day, it gives us the sense of being filled with a vast blue blanket, which can be explained by Rayleigh scattering the name of this phenomenon after its great learner Lord Rayleigh in the 19th century.
Now then, if blue is less in wavelength than violet, why doesn't that sky appear violet? The answer is partly in the sun and partly in human biology. The sun emits much less violet than blue. In addition, the human eye is much more sensitive to the blue wavelengths and relatively poor at seeing violet.
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Therefore, the perception is further biased, which leads in the end to a blue sky rather than purple.
The story changes completely as the dawn and dusk occur. The sun at dawn and dusk passes through a much thicker atmosphere before entering the eye. During this longer course, most of the blue and violet parts are taken away from sight, leaving the lengthy wavelength reds, oranges and golds to illuminate the horizon with their dazzling colors.
This is why during sunset and sunrise the sky takes on a fiery color, often fading into pink or purple in a deepening evening sky. Both a scientific process and a spectacular natural artwork.
The different colors we can see in the sky are not specific to Earth alone. They would also determine what kind of atmosphere an object has. For instance, during the day, Mars usually has a reddish dusty sky due to the fine particles in its thin atmosphere.
Sunsets there tend to have a bluish hue. On Earth, the horizon sometimes appears white because larger particles scatter light in a phenomenon known as Mie scattering, accounting for the pale appearance of fog and mist. Within each world, the dance of light and atmosphere produces its own version of the color of the sky.
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