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There's a tradition in the tropics that is practiced all too seldom in more temperate geographies: the tradition of watching the Sun set. There's always a feeling that you're witnessing something momentous. And of course, there's the deep, rich, pastel mix of yellow, orange, and red filling the sky, and overwhelming your senses. So what is it that makes a sunset so awe inspiring, and what is it that makes a sunset red?
To tackle the last question first, we must first understand that everything in the Universe vibrates. This is because everything in the Universe (that we can perceive), is made of atoms, and atoms vibrate. Different atoms vibrate at different frequencies, depending on their configuration of protons, neutrons, and electrons. Photons of light, even though they are massless, also vibrate. Like atoms, they vibrate at different frequencies, depending where they lie in the electromagnetic spectrum, as illustrated below. And as you can see, the amount of light we can actually see is a very small part of the amount of light there is.
When a vibrating photon of light bumps into a vibrating atom, and they are both vibrating at exactly the same frequency, the photon and the atom become one, so to speak. The atom absorbs the photon, adding its vibrations to its own, and heat is produced. The photon is never seen again.
When a photon collides with an atom vibrating at a different frequency, it is either transmitted, or reflected. If it is transmitted, it passes through the atom virtually unchanged, and continues its journey. If it is reflected, it bounces off the atom. In the case of a relatively large, solid object, the photon is usually reflected back in the general direction it came from. In the case of smaller objects such as atoms and molecules, the photon can bounce off in any direction, and is said to be scattered. And depending on the vibrational harmonies between the photon and the atom, a photon may be a little absorbed, transmitted and reflected all at the same time. Welcome to the wonders of a photon.
When a photon of light leaves the Sun it takes eight minutes, travelling at 180,000 miles a second, to reach Earth. During that eight minute journey through the near vacuum of space there are so few atoms for the photons to interact with, they reach Earth virtually unchanged, retaining all the frequencies of the electromagnetic spectrum that they started out with.
Then the photons encounter Earth's atmosphere, and something very important happens. As the NASA graphic below illustrates, the atoms in our atmosphere absorb nearly all the harmful and lethal frequencies, such as X-rays, and Gamma rays, allowing only the friendly frequencies like visible and radio light to reach Earth's surface. This is very fortunate for us, and one very good reason not to pollute, and change the composition of our atmosphere, as we are unfortunately currently doing.
The radio light that reaches the surface of Earth is invisible to our eyes, but detectable by technology, such as radiotelescopes, which is why radiotelescopes are used to search for radio light that forms patterns, that could possibly be signs of intelligent life elsewhere in the Universe.
Ninety-nine percent of Earth's atmosphere is composed of Nitrogen (78%) and Oxygen (21%). The atoms composing these gases are vibrating at a frequency that tends to scatter visible light. The higher the frequency of the visible light waves, the more they are scattered in all directions. As you can see in the diagram below, the colour violet has the highest frequency, and is scattered the most. Moving down the scale, indigo and blue are scattered at slightly lesser degrees. Since our eyes are more sensitive to the lighter blue, rather than the darker indigo and violet, the sky appears blue to us.
But the more atmosphere the photons have to pass through, the more they are scattered. As the Sun nears the horizon, the photons are passing through more and more atmosphere, as illustrated below, until the higher wavelengths (blue, indigo and violet) become so scattered they lose much of their energy. Now the lower frequencies (yellow, orange, and red), are becoming scattered as well, and as the Sun sinks lower towards the horizon, we see the sky change from yellow to orange, and finally to that deep overwhelming red that makes a sunset so spectacular. And the more particles in the atmosphere to scatter the photons of light, whether it's ash from a volcano, dust and pollen from harvesting, or pollution from Human industry, the redder the sunset will be.
As for the first question, concerning the power of a sunset to stir the Human soul, I think the answer goes far beyond the depthless beauty of it all, although that is stirring enough all by itself. But there's much more going on than that. For a sunset is a moment of profound transition. We are experiencing nothing less than the duality of life itself, crossing the threshold between light and dark, yang and yin. We can feel it on our skin, and see it in our mind, and smell it in the air. We are watching our source of life disappear, and leave us alone to face the cold darkness of space. But instead of being afraid we are exhilarated, because we know it will be back, and in the meantime we can indulge ourselves in the energy of the night. But perhaps the most awesome part of all is the sheer majesty of a sunset. It is just so big, and glorious, and powerful, that we are humbled and inspired at the same time. We have a front row seat in the Church of Life, electricity is crackling in the air, and we feel a little closer to the truth. A little closer to each other. And maybe even a little closer to God.
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