There is something transformational about twilight.
关于暮光，我要讲一些新的事情。
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.
当太阳西沉时，天空的色彩会逐渐由明亮的蓝色转为混合深蓝。东方逐渐暗淡，西方地平线则绽放出红光，最终引入深邃的夜色。
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.
像诗一样，白天和黑夜交际的模糊感觉。twilight 中 “twi”在古英语中表示“in two ways”。另一个表示暮光的词是“gloaming”，它的组成也有丰富的含义。
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.
除了文学的欣赏，暮光也可作为科学研究。天文学家研究暮光的各个阶段。如果没有在太阳落山之际好好观察天空，你会错过很多奇观。
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.
首先，大气层消失。没有大气，会带来很多问题——比如不能呼吸——尽管会很混乱很麻烦，但天文学家依然对大气消失抱有期待。大气层会散射光线：太阳光子遇到空气中的氧气和氮气分子时，会发生碰撞并改变方向。大部分光子转向地面，到达我们眼中，对我们来说，就是整片天空都是亮的。
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.
夜晚，地球自身遮挡太阳的光线，自然没有散射的光线，天空显得黯淡。在日夜交替的时候，我们看到暮光 twilight。但是，黑夜并非一下子来临，相反，暮色在天空中滑过，渐行渐远。
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).
最初出现的阶段被称为“civil twilight”，在此阶段，天空仍然明亮，人们可以像白天一样进行户外活动。此时，太阳位于地平线以下0-6°的位置。
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.
此时天空仍然很亮，因为地球是圆的，太阳光被大气散射后仍能到达地面。如果站的够高，就能看到现在地平线下面的太阳。
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.
civil twilight的持续时间和地点有关，和季节有关。高纬度区域，极夜或者极昼发生，也就没有 civil twilight。而在这些区域一旦发生，暮光能持续数个小时。而在赤道附近，太阳几乎直射，“civil twilight”通常不到半小时。
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.
在良好天气和城市光污染不严重的情况下，“civil twilight”期间，比较明亮的星星便可见。此时，月亮也能被观察到，随着天空变暗，星星愈发显现，金星和木星也可以在此阶段观察到。
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.
西方天空展示了一天中最丰富的色彩。光的散射强度与波长相关，蓝光更容易被散射，因此白天的天空呈现蓝色。而红光则主要以直射的方式穿过空气分子，保持原有方向。黄昏时，蓝光被散射消散，剩下红光可见，形成夕阳余晖时的绚丽色彩。
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.
随着黑暗逐渐降临，接下来的阶段是“nautical twilight”，当太阳低于地平线6-12°时，天空会呈现出深紫色，众多星星显露无遗。“nautical twilight”的名称源于水手们在此阶段利用星星进行导航。因为nautical twilight出现时，就能用星星导航。
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.
随后，进入“astronomical twilight”，此时太阳位于地平线下12-18°，天空变得完全黑暗，星星开始闪烁。这个阶段的命名是因为即便肉眼难以捕捉到什么，使用望远镜仍能够观测到明亮的星空。此阶段的结束标志着太阳已经低于地平线18°，天空彻底黑暗，天文学家则可以开始他们的观测工作。
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.
以上是黄昏，黎明时则会重演，倒着重演一遍这个过程。经过一晚上的天文观测，我能感知天什么时候要亮了，该停止观测，备份数据，回去睡觉了。从傍晚到黎明，这是我的生活。
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.
日出和日落不是可逆的两个过程。它们不同。黎明天空比黄昏更干净。白天，强烈的紫外线与空气相互作用会产生一些气溶胶，光线的散射被改变，视觉上天空看起来更白更不透明。夏天会更明显，当空气流动没那么活跃，模糊雾状的感觉会持续更久。
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.
我喜欢暮光。世界很安静，天空预示着星星的到来。天文学上，暮光像是宇宙发来的邀请，会带来最令人期待的惊喜。
Source: https://www.scientificamerican.com/article/there-are-three-types-of-twilight/