There is something transformative about sunset.
As the sun has just set in the west, the sky changes from the intense blue of the evening to a calmer hue, as if settling down for the evening. Soon the east will darken, even as the west passes through a riot of red, orange and yellow. And then, finally, the black of the night will dominate again.
It has a poetry, a liminal feeling of being between two existences, night and day. The Old English prefix “twi-” means “two ways”, and it is certainly this dual state of mutuality that inspired his literary verse. Another term for twilight is “gloaming,” a word as richly textured as I can imagine feel him
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But besides poetry, there is also a science to sunset. Astronomers (among others who look forward to the night) are so familiar with the sunset that we have categorized its different stages. If you haven’t spent a lot of time looking up at the sky as the sun drifts below the horizon, you’ve been missing out on some amazing visual treats.
First, let’s give it up for Earth’s atmosphere. Without air, we’d have some pretty serious problems—not being able to breathe, for example—though given how turbulent and opaque it can be, some astronomers would prefer to do without it. But one interesting thing the atmosphere does is scatter sunlight: photons from the sun hit nitrogen and oxygen molecules in the air and bounce off them. Many of these photons bounce towards the ground, so when we look up, we see light coming from the sky itself.
At night, when most of the solid Earth blocks the sun, there is no sunlight to scatter, so the sky is dark and, depending on the weather, we can see what lies beyond. Between these two times is the sunset cache. But the darkness does not come suddenly; instead, it slips through the sky in eliding stages at dusk.
The first one after sunset is called civil twilight, so called because the sky is still bright enough for people to carry out their normal “daytime” activities. By definition, civil sunset is the period of time when the sun is below the horizon and six degrees lower (roughly three fingers held parallel on your outstretched hand).
The reason the sunset sky is bright is because our planet is round. Our atmosphere curves along with the Earth’s surface. To someone standing on the ground, the sun becomes invisible when it sets, but to an observer at a higher altitude – many kilometers up, for example.he can still see the sun. From their higher point of view, it has not yet come down from the edge of the Earth. This means that sunlight is still scattering through the air up there, providing a brief respite from the emergency night.
The length of civil twilight varies from place to place and season to season. For very high latitudes, near the poles, the sun’s path across the sky takes our star below the horizon only at certain times of the year, and it’s not always low enough for night to actually get dark. Sunset can last many hours in these regions. Closer to the equator, the sun dips more perpendicularly towards the horizon, and civil sunset lasts less than half an hour there.
During civil twilight, the brightest objects in the sky can be seen through cloud cover or the brightness of artificial light sources does not interfere. The moon can be seen in broad daylight, but becomes easier to see as the sky darkens. Venus and Jupiter can be bright enough to see near the end of twilight, or earlier if you know exactly where to look.
This is also when you will see the most colors in the sky towards the west. Those molecules that scatter sunlight don’t do it randomly; the scattering angle depends on the color of the light. Blue light is much more scattered than red, that’s why the sky is blue during the day. Red light, however, tends to pass through these molecules. So at dusk, all the blue light from the sun is scattered, leaving only the reddest colors to shinecreating those gasp-worthy polychromatic sunsets.
However, the darkening continues unabated. The next phase is the marine sunset, when the sun is between six and 12 degrees above the horizon. The sky may be a deep purple, and the stars begin to appear. The term “nautical sunset” was coined because sailors used the stars for their orientation, and at this point in the evening sky navigation becomes possible.
After that comes the astronomical sunset. This is when the sun is 12-18 degrees below the horizon and the sky finally appears black, with the stars really bright. We call it an astronomical sunset because it looks dark to your eyes, but through a telescope, the sky is still clear enough to interfere with some observations. This last phase of dusk ends when the sun drops below 18 degrees; the sky darkens completely, and astronomers can enjoy the start of their night’s work.
Of course, this is all for dark, after sunset. The process happens in reverse order at dawn, when the sun is up it goes up. I remember many all-night telescopic observation sessions; I could always tell when the sky would start to lighten, signaling time to close up shop, back up my data, and go home early to bed. Being from dusk to dawn was my way of life.
This does not mean that sunset and sunrise are perfectly symmetrical, they are different! The sky at dawn is usually brighter and clearer than at sunset. That’s because during the day, sunlight, especially ultraviolet light, interacts with Earth’s air molecules to increase the level of aerosols, particles that float high in the atmosphere. These tend to scatter light differently, giving the sky a whitish, hazy appearance. This is usually more noticeable in the summer, when circulation patterns are more sluggish and fog can persist for long periods of time.
However, I’ve always liked sunsets at sunset. The world calms down, and the sky itself announces the arrival of the stars. For an astronomer, the sunset represents a welcome to the universe, and the anticipation of it is one of the best things I know.
