Credit: ASTRONOMY: ROEN KELLY, after Aurora Borealis Observatory In twilight As each type of twilight progresses, these changes signal to astronomers that fainter objects will start appearing in the night sky. Twilight is defined mathematically as the Sun’s center travels by varying amounts below the horizon, the colors in the sky morph from a vibrant light show to a dark sky. The momentary green flash can result because the human eye is most sensitive to green light - but to avoid serious harm, be sure never to stare directly at the Sun. The short wavelengths of violet and blue appear slightly above the Sun, while red is consigned below the horizon. This occurs because Earth’s atmosphere acts as a prism, refracting the light and splaying out its colors. If conditions are just right, a brief flash of emerald may appear at the top of the flattened Sun as it sinks below the horizon. (The same effect can flatten the look of the Full Moon as well.) When the Sun is just on the horizon, the light from its lower limb refracts more strongly than the light from the top of the Sun, creating the illusion of a flattened orb. These distort and refract the Sun’s light unevenly, and can even cause the Sun to look flattened as it sets. Our atmosphere is made up of layers consisting of different densities and temperatures. There are other effects at play here, too. Both the Sun and the horizon start to take on a reddish-orange hue. Now most of the Sun’s light is traveling through even more of the atmosphere and blue light is further scattered away, leaving the red and orange colors. This is because you’re looking through more of the atmosphere at low altitudes, so sunlight travels through more air molecules and scatters even more.Īs the Sun sets, this scattering effect is intensified. However, the bright blue of the daytime sky seems to fade if you look toward the horizon. This effect is known as Rayleigh scattering. As a result, the sky looks blue in every direction. The visible portion of the Sun’s light represents only a fraction of the solar spectrum: With its energetic short wavelength, blue light is scattered more easily by dust and air molecules, such as oxygen and nitrogen, than redder light. This results from a complex interaction of sunlight and air molecules. On a clear day, the sky can appear an intense and beautiful blue. Whether you’re an early riser or have stayed up all night, learning the three kinds of twilight will ensure you can catch the radiant atmospheric light show. What’s more, understanding twilight in its different stages helps to define how we see the sky at its best. But with the anticipation of setting up equipment and making plans for observing, we often forget to watch one of nature’s most beautiful spectacles, the time of transition from our busy day into the vibrant night. Twilight is a time when sunlight and Earth’s atmosphere intermingle to produce wonderful effects. A spectacle of sparkling stars and shining planets dance in hues of yellow, orange, and blue, as if creating a painting made by nature during the transition from civil twilight to nautical twilight.
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