AT WORLD'S END - CAPTAIN BARBOSSA, MASTER GIBBS AND GREEN SUNSETS

''Ever gaze upon the green flash, Master Gibbs?'' says Captain Barbossa as they journey towards the world's end, searching for a way into Davy Jones' locker. 

No sooner, the veteran pirate and also Jack's first mate goes on saying ''I reckon I seen my fair share. Happens on rare occasion. The last glimpse of sunset, a green flash shoots up into the sky. Some go their whole lives without ever seeing it. Some claim to have seen it, who ain't. And some say...''

''...It signals when a soul comes back to this world from the dead!''. There we have Pintel, who could no longer hold his breath only to get a stern glare from Master Gibbs. 

Keeping aside the myriad of pirate lore, from the fountain of eternal youth to the Black Pearl in a bottle to not-so-dead sharks and a cursed Spanish captain and his bizarre limbless headless crew, the Flying Dutchman and Davy Jones' beating heart locked in a wooden chest, the legend of the green flash is the one and the only thing that checks out to be real. At the same time, let us not forget that they are all pirates. Under the intoxication of pirate's rum and after spending a thousand nights under the endless, landless seas, a pirate's description of even the most ordinary events gets easily plagued with monstrous creatures, cursed objects, and the return of the dead. So if we could excuse ourselves from the pirate's lore, then the true undiluted legend of the green flash is as follows. 

The green flash at sunset happens when the last rays of the setting sun have to travel through a thicker volume of the atmosphere, where it refracts and splits into its constituent colours. When the conditions are just right, and on rare occasions, one can see a faint green smudginess across the upper limb of the sun's disc, looking straight towards the sunset point. 

The green flash at sunset is an extremely rare optical/meteorological phenomenon seen at most for a brief one or two seconds just before the upper edge of the solar disc is about to disappear completely below the horizon. Strictly speaking, the last ray of light emitted from the sun turns into a pale green the moment it (sun) dips below the horizon. Usually, this green light appears as a flickering patch illuminating the sunset point and corresponds to the famed green flash. On phenomenal days, this flickering patch of light can transform into a ray-like appearance rising upwards from the sunset point. Here the reader must be cautioned that this green flash is not bright as an ordinary camera flash but is quite faint. In fact, it is so muted that one might need some practice and encounter several failed attempts before he finally fills his eyes with the last rays of daylight. Hence, it is fair to say, ''he who has seen the green flash will never go wrong in matters of the heart''. 

The following Wikipedia article claims that the green flash can be seen from any altitude and has no preferred geographical location. However, in practice, that is not the case. Since the green flash happens at the exact moment the upper limb of the sun touches the horizon, it would be easier to see if we could look straight towards an obstructed horizon where the sky separates with a sharp, distinct boundary from the land or the sea. It occurs at such a low angle that our eyes need to be perfectly aligned in a straight line looking towards the sunset point when the last upper bit of the sun is about to set below the line of the distant horizon. This is best seen from across the open expanse of the sea or desert landscapes where the horizon appears as a sharp boundary instead of being broken by multifarious buildings and other extensions, both natural or man-made. Therefore, city dwellers have a next-to-none chance of witnessing the phenomenon. An unobstructed horizon is not the only thing that matters if we wish to catch the event. The green flash is an atmospheric effect. It might occur on any random day, depending on the various underlying factors such as temperature or pressure gradients and the moisture content in the atmosphere. Moreover, it happens for such a short span that unless one has gathered enough experience and knows precisely when and where to look, he may not be able to see the green flash on his first attempt. We need to look at the sun moments before it is about to disappear, but not too early. Otherwise, we might get dazzled and miss our chances for that day. 

The green flash can also be seen during sunrise, wherein a speck of green light lines the upper limb of the solar disc as it rises over the flat horizon. But since it is harder to locate the sunrise point in advance, most noted occurrences of the green flash are during sunsets. 

When seen across the open seas, the upper limb of the setting sun turns a shade of green, and on a rare number of days, a faint green flash can be seen lighting up the sunset point for a few fleeting seconds.
Image Credits: flickr.com/photos/genista

Let us now delve further into the underlying physics of the green flash. From our previous articles ⁽¹⁾ ⁽²⁾, we have learned that the earth's atmosphere can significantly alter the appearances of the celestial objects, viz, the sun, moon and the stars, as they lie a few degrees above the horizon. We know that as the incoming rays travel through the layers of air overhead, they interact with the individual atoms and molecules that constitute earth's atmosphere and undergo reflection, refraction, dispersion, scattering and other classes of light-matter interactions, thereby resulting in diverse optical/atmospheric phenomena. Again, from the spherical geometry of our planet, we understand that during sunrise/sunset, the sun's rays reach us following a tangential path and through an increased volume of air. While travelling through the air, owing to the selective nature of the Rayleigh scattering, the blue components are filtered out from the spectrum of white (sun) light, turning the sun yellow to red while it is low on the horizon. Rayleigh scattering, paired with the Tyndall effect and the presence of aerosols in the air, routinely diminishes the net intensity of the incoming sun rays. This completes one part of our story. Over and above, the earth's atmosphere is not uniform everywhere. It is denser near the surface and gets increasingly rarefied with increasing altitude. Now that we know that the path of light suffers a deviation when travelling from one media to another, meaning when travelling across the different layers of the atmosphere corresponding to different temperatures, pressure and humidity, the path of light bends and accordingly curves towards the surface where the air is tightly packed. Light refracts as it travels through the different layers of the earth's atmosphere. During sunrise/sunset, when the sun's rays travel through the most volume of the atmosphere (evident from the graphic below), they suffer the maximum refraction. This atmospheric refraction distorts the shape of the sun, making the round solar disc look like a squashed melon. Also, this same atmospheric refraction causes celestial objects to appear higher above the horizon than they are. Atmospheric refraction coupled with air turbulence resulting from the differential (non-uniform) heating of the air layers leads to the twinkling of the stars when they are closer to the horizon. Here ends our second part of the story.  

Atmospheric refraction and the bending of light 
Image Credits: via Wikimedia Commons

Now the third part. The green flash is nothing but a pronounced effect of this atmospheric refraction. Earth's entire atmosphere behaves like one giant prism splitting the white light (of the sun) into its constituent colours. As a result, the light of any celestial body closer to the horizon split apart, wherefore the object is seen with a blueish lining on its upper limb and a reddish lower limb. But the blue and the violet wavelengths suffer the maximum refraction and, via Rayleigh scattering, are scattered away from our direct line of sight. Therefore, we see a faint green tinge lining the upper circumference of the celestial object, be it the sun, moon, Venus or any of the stars. The lower circumference of the said objects similarly turns a shade of red. If the air is particularly free from pollutants, aerosols and other impurities, then, in that case, the upper rim might look blue to violet instead of green. In that case, it is possible to witness a blue flash or maybe a violet instead of green. The following external article - Observed colors of ''green'' flashes gives a detailed account of the colours of the last light that leaves the sun. 

But since the blue components are filtered out, the upper rim of the celestial object looks predominantly green. If we could just remove the horizon or make it transparent, we would notice that the lower limb of the sun already below the horizon appears redder than the rest of its parts. These colourations are always present on a celestial object as it hangs low over the horizon. However, these colours are too faint to be seen without the aid of a pair of binoculars or a telescope. Plus, atmospheric reddening subdues the green rim, which is why it is so hard to spot. Atmospheric reddening is the removal of high-frequency short wavelength components due to scattering and absorption by the various impurities in the earth's atmosphere. If the sun looks bright red, it is a sure indicator that the green flash will not happen on that day, for the higher frequencies have been scattered away from our line of sight, where only the red wavelengths remain. 

The upper green rim and the lower red rim seen on the sun as it sets
Image Credits: Wikimedia Commons 

The green flash is a rarity because of the various factors responsible for it, most importantly, the miraging effect. Mirages happen due to the temperature gradients in the atmospheric layers closer to the earth's surface. It is during a mirage when the entire atmosphere between our eyes and the sun starts to behave as a magnifying glass, thus bringing the effects of refraction closer to our eyes and transforming the faint green rim into an ephemeral burst of green light. Depending on the type of the mirage, green flashes come in different flavours. There can be more than a single flash, occurring back to back, one after the other. Mirages are interesting atmospheric/optical phenomena and are predominantly seen in places where there is a large non-uniform variation in air temperature, pressure and humidity across desert landscapes and open bodies of water. The green flash, as in the below photograph occurs due to a strong mock-mirage where the solar disc has separated into some discrete layers. Mirages can drastically alter the appearance of the sun. 

A mock-mirage green flash observed from San Francisco
Image Credits: Wikimedia Commons 

Apart from green and blue flashes, there is one more type. Robert Greenler, in his book, ''Rainbows, Haloes and Glories'', speaks of the red flash. He writes, ''Given the situation that produces a green flash, an obscuration of all the sun's disc except for the extreme lower edge should produce a red flash. This flash can be seen as the sinking sun reappears from beneath an obscuring cloud; you can see an indication of the effect in the first photograph of plate 7-II. It is not as dramatic as its green counterpart, because we are accustomed to seeing parts of the sun appear quite red from scattering effects''. 

Although the green flash sounds like an elusive sight to behold, it is quite hard to catch a glimpse of the last light that ends the day. Unless one has a clear, unobstructed view of the horizon, such as from the deck of a ship or across desert landscapes, there is virtually no way one can see the green flash. This is why most photos of the green flash are across the open expanse of the seas. It occurs at such a low angle that our line of sight has to be almost parallel to the ocean surface and straight towards the sunset point. Again, since we are looking into the bright sun, there is always a possibility of missing out on the last-moment green light. On too rare an occasion, the effect can last for 20 or more seconds, and in one particular instance, the green flash was seen with interruptions for 35 minutes across the Antarctic coast. The occurrence of the green flash is not like rain or snow, which can be predicted in advance. The air has to be free from pollution, the temperature variation has to be just right, the humidity has to be at an optimum percentage, and the amount of refraction, miraging and other factors have to be just right, like Goldilocks's porridge. On the contrary, the green rim is present at every sunset, and the odds of seeing it are good even if we do not have a sharp horizon. 

Considering all things, we should never stop looking, for Mother Nature seldom disappoints a keen observer. 

References: 

1. https://www.nationalgeographic.com/science/article/green-sunsets-are-100-real?loggedin=true
2. https://adsabs.harvard.edu/full/1958RA......4.....O
3. Greenler, Robert. Rainbows, Haloes and glories. Cambridge University Press
4. Minnaert, M. The Nature of Light and Color in the open air. Dover Publications Inc. 
5. https://skyandtelescope.org/observing/celestial-objects-to-watch/catch-the-green-flash/
6. https://aty.sdsu.edu/reading.html