Here’s something you might remember from your high school earth science class. Sunlight, which appears pure white, is refracted or bent as it passes through a glass prism. Different wavelengths of light are bent by different amounts. In this way, the prism can spread the white light into a rainbow spectrum.
The diffuse color of refraction can be observed in poor quality binoculars and telescopes. The optics tend to be misaligned on these cheap instruments, so you’ll see the full moon appear yellow-orange on one side and a bluish tint on the other.
But again, because the sun and moon reflect direct sunlight back to Earth, they actually consist of “stacks” of images of many different colors. You can probably remember the fictitious name “Roy G. Biv”. Each letter here represents a color in the spectral lineup: red, orange, yellow, green, blue, indigo and violet.
Of course it looks somewhat white because we usually see the color stack all at the same time.
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Mood can act like a lens.
Now, when the sun is near the eastern or western horizon, Earth’s atmosphere behaves in a similar way to poor-quality optics by separating stacks of images of different colors. Green, especially blue images, consist of shorter wavelengths and are scattered by air. The red and orange images are composed of longer wavelengths with higher transmittance, resulting in the red and orange colors seen at sunrise and sunset.
However, in very special cases where the sky is very clear and exceptionally clear, a very small portion of the green image can make the highest point of the sun appear to rise or set. This rather rare phenomenon is known as the “green flash”. Green is created by atmospheric refraction of light, which causes objects near the horizon to appear slightly higher in the sky than they actually are. Because refraction is stronger for blue and green light than for yellow and red light, the sun’s blue light is lifted somewhat more than red light. This weak prismatic effect in the atmosphere creates a bluish or greenish streak across the sun’s upper edge. This is very similar to the effect of viewing the moon through misaligned optics.
Surprisingly, the earliest references to green flashes are relatively recent. It is only about 160 years old. This seems a bit strange, as we would have thought that diligent sky observers in ancient China, Japan, and Babylonia might have noticed this phenomenon.
The first definitive scientific record of a green flash was made in 1865 when “W. Swan” saw a “dazzling emerald green” flash at sunrise over a distant mountain. Interestingly, however, Swann did not publish his observations until 1883. In the same year, people around the world reported views of a green sun and other strange phenomena that occurred after the massive eruption of Krakatoa Volcano.
Some say that Jules Verne’s 1882 novel green ray (“green rays”) drew general attention to this phenomenon, and the great British mathematician, physicist and engineer William Thomson (known in later years as Lord Kelvin) wrote that he had seen even numbers in 1899, referring to Verne’s novel . rare “blue The flash he saw as the sun rose over Mont Blanc, Switzerland’.
To emphasize how elusive the green flash can be, I’d like to point out that I’ve been an avid skywatcher for over half a century and have looked for the green flash on several occasions, but have definitely seen it. only twice.
The first incident occurred on the morning of May 14, 1977 and was reported in the August 1977 issue. sky and telescope magazine pages 150-151). Me and three other stargazing buddies drove to Caumsett State Park, a Long Island bird sanctuary about 30 miles (50 km) east of Manhattan, to observe not one but two planets (Venus and Mars) orbiting within 2 degrees of the crescent moon. ) was seen. Just before sunrise, it was clear that the sky was exceptionally clear and we all had a good feeling we had a good chance of catching a glimpse of the flash. I was looking through my Celestron 8 telescope at the magical moment when the upper edge of the sun suddenly suddenly suddenly appeared over the nearby Connecticut coastline in a beautiful emerald green that lasted nearly two seconds.
Another opportunity came many years later, in June 2019, on a cruise ship sailing to Bermuda. As my wife, Renate, and I watched the sun set on the horizon, we noticed that although the sky wasn’t clear enough to show off the green flash, the clearness of the sky looked pretty good. Slowly, the sun’s disk sank below the horizon, and just as the uppermost part of the sun was about to disappear, to our surprise, we saw a very brief, brilliant dot of green light on the horizon.
And that’s the fickleness of witnessing a green flash. It seems to be the exception rather than the rule. Sometimes it can appear in adverse conditions, other times conditions may look good but not appear.
Here are a few things to look for to see a green flash.
- The general conditions needed are a distant, sharply defined, low (preferably sea) horizon.
- For possible sunset viewing, don’t look at the sun until the last minute. Eye damage is possible. Furthermore, a bright red image of the sun can leave a green afterimage in the eye that can be mistaken for a green flash after sunset.
- Cool weather and the absence of haze and red tints seem to favor visibility.
- For possible sunrise sightings, rising points along the horizon a few minutes before sunrise may be surrounded by a bright white or yellow glow. Some call this the “bonfire effect” and it seems like a good sign to see a green flash from time to time. Before capturing the 1977 flash, I saw a glowing “bonfire” along the distant horizon. That was the prelude to the moment I first saw the green flash.
good luck. We hope you will experience the same joy!
Joe Rao is a lecturer and guest lecturer at New York’s. Hayden Planetarium. He writes about astronomy. natural history magazine, farmer’s almanac and other publications.
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