American author and critic Pamela Hansford Johnson could have been inspired by a Vineyard sunset when she enthused, “The sky broke like an egg into full sunset and the water caught fire.”
Mahatma Gandhi was also clearly convinced of the divine nature of sunsets, observing that “When I admire the wonder of a sunset or the beauty of the moon, my soul expands in the worship of the Creator.”
It is hard not to marvel at the sky as day turns to night. Last week’s sunsets were nothing short of miraculous. The magic, though, is simple science.
Light from the sun is the origin of the colors. Though we say our sun is yellow, it is white light that it produces. The white light encompasses all the colors of the rainbow, each occurring at different wavelengths. The notion that the different-colored wavelengths could be separated out again from the white light of the sun was first demonstrated by Isaac Newton, who used a prism to scatter each color.
Blue, violet and green have short wavelengths, while yellow, orange and red have long wavelengths. These different lengths make a difference, since the shorter wavelengths are more vulnerable to interference.
As light moves through the atmosphere, it encounters molecules of gas and other particulates. These molecules might be considered “bumps” in the atmosphere that get in the way of light. When light encounters these bumps, the different-colored parts scatter, starting with the shorter wavelengths. This selective scattering of light is referred to as Rayleigh scattering.
During the day we see a lot of blue (but not violet; our eyes see the blue more easily than the violet) because the direct path that the sunlight is taking through the atmosphere is at its shortest and there typically isn’t enough interference between the sun and our eyes to scatter out the blue.
At sunset and sunrise, however, light must travel through 10 times more atmosphere than it does at noon (due to the oblique angle of the sun from the vertical). Thus, the shorter wavelengths of light encounter 10 times the number of particles and molecules of gas, and they get scattered first. What are left are the longer wavelengths, giving rise to our spectacular evenings of reds and oranges. This phenomenon, too, has a name — the process whereby short wavelengths are scattered more than large ones is called the Tyndall effect.
There are other factors that can affect the sunset we see. One is pollution. Though many think that pollution enhances sunset because it adds particles that would scatter light, this is not actually the case. Pollution dims sunsets due to the size of the particles, which are too large to be effective Rayleigh scatterers and, instead, cause gray haze and subdued skies. Salt from the sea, however, encourages blazing red and orange skies. That may be one of the reasons that seaside sunsets are so incredible.
Seasonality matters, too. Fall and winter have some of the most intriguing and colorful sunsets because air circulation is less lively than in the summer and the chemical reactions that cause smog and pollution are slowed due to lower temperatures. High-level clouds, like cirrus and altocumulus, also add to a sunset’s palette, because they are above the atmospheric layer that has the most color-suppressing dust and haze.
Rabindranath Tagore, Indian playwright and poet, referred to this effect and remained ever the optimist when he waxed both poetic and philosophical about the intrusion of clouds in his sky: “Clouds come floating into my life, no longer to carry rain or usher storm, but to add color to my sunset sky.”
Suzan Bellincampi is director of the Felix Neck Wildlife Sanctuary in Edgartown.