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There is something transformational about twilight.
When the sun has just set in the west, the sky changes from the vibrant blue of afternoon to a more relaxed hue, as if it's settling down for the evening. Soon the east will darken even as the west goes through a riot of reds, oranges and yellows. And then, at last, the black of night will reign once again.
There's a poetry to it, a liminal feeling of being between two existences, that of night and day. The prefix "twi-" in Old English means "in two ways," and certainly this twofold state of in-betwixt-ness has inspired its share of literary verse. Another term for twilight is "gloaming," a word so rich in texture I can feel it.
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But besides poetry, there is a science to twilight, too. Astronomers (among others who eagerly await the night) are so familiar with twilight that we've classified its various stages. If you haven't spent much time gazing at the sky as the sun drifts below the horizon, then you've missed out on some wonderful visual treats.
First, let's give it up for Earth's atmosphere. Without air, we'd have some fairly serious problems -- such as not being able to breathe -- though given how turbulent and opaque it can be, some astronomers would rather do without it. But one interesting thing the atmosphere does is scatter sunlight: photons from the sun hit molecules of nitrogen and oxygen in the air and bounce off. Many of those photons wind up caroming toward the ground, so when we look up, we see the light coming from the sky itself.
At night, when the bulk of the solid Earth blocks the sun, there's no sunlight to scatter, so the sky is dark, and, depending on weather, we can see what lies beyond. Between these two times lies the lair of twilight. But the gloaming doesn't come all at once; instead twilight slides over the sky in eliding stages.
The first that occurs after sunset is called civil twilight, so named because it's when the sky is still bright enough for people to conduct typical "daytime" outdoor activities. By definition, civil twilight is the range in time when the sun is just below the horizon to when it's six degrees lower (roughly the same span as three fingers held parallel in your outstretched hand).
The reason the early twilight sky is bright is because our planet is round. Our atmosphere curves along with Earth's surface. To someone standing on the ground, the sun becomes invisible once it sets, but an observer at higher elevation -- many kilometers up, for example -- can still see the sun. From their loftier perspective, it hasn't dropped below Earth's edge yet. This means sunlight is still scattering off the air up there, offering a brief reprieve from the onrushing night.
Civil twilight's duration varies from location to location 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 the night to get truly dark. Twilight can last for many hours in these regions. Closer to the equator, the sun dives in a more perpendicular fashion toward the horizon, and civil twilight there lasts less than half an hour.
During civil twilight the brightest celestial objects may become visible if cloud cover or glare from artificial light sources doesn't intervene. The moon can be seen in broad daylight but becomes easier to spot as the sky darkens. Venus and Jupiter can easily be bright enough to see toward the end of civil twilight -- or earlier if you know exactly where to look.
This is also when you're likely to see the most colors in the sky toward the west. Those same molecules that scatter sunlight don't do so randomly; the angle of scattering depends on the color of the light. Blue light is scattered much more than red, which is why the sky is blue during the day. Red light, however, tends to stream right past those molecules. So at dusk, all the blue light from the sun is scattered away, leaving only the redder colors to shine through, creating those gasp-worthy polychromatic sunsets.
Still, the darkening inexorably continues. The next phase is nautical twilight, when the sun is between six and 12 degrees below the horizon. The sky may be a deep purple, and stars begin to appear. The term "nautical twilight" came about because sailors used the stars to get their bearings, and it's at this time of the early evening that celestial navigation becomes possible.
After that comes astronomical twilight. This is when the sun is 12 to 18 degrees below the horizon and the sky finally appears black, with the stars truly shining. We call it astronomical twilight because it may look dark to your eye, but through a telescope, the sky is still bright enough to interfere with some observations. This last phase of twilight concludes once the sun drops below 18 degrees; the sky becomes fully dark, and astronomers can enjoy the beginning of their work night.
Of course, this is all for dusk, after sunset. The process happens in reverse order at dawn, when the sun rises. I remember many a telescopic observing session running all night long; I could always tell when the sky would just start to brighten, signaling the time to close up shop, back up my data and head home for a good morning's sleep. Being up from dusk till dawn was my way of life.
That isn't to say that sunset and sunrise are perfectly symmetric -- they're different! The dawn sky tends to be clearer and cleaner than dusk. That's because during the day, light from the sun, especially ultraviolet light, interacts with molecules in Earth's air to boost levels 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 obvious in summer, when circulation patterns are lazier and the haze can stick around longer.
Still, I have always loved twilight at dusk. The world quietens, and the sky itself heralds the coming of the stars. For an astronomer, twilight signals a welcoming to the universe, and the anticipation of that is one of the best things I know.