As the sun sinks slowly in the West, there is cause for pause by people as they wish their luck in seeing the elusive “green flash.”
I remember the daily ritual on our deck in Kahaluʻu mauka in Kona (as we were growing up, it was the only home we ever lived in with a western orientation and view of the Pacific Ocean.)
Scientists say green flashes come in two common forms; these were described by James Prescott Joule in a letter to the Manchester Literary and Philosophical Society in 1869.
First, he noted that “at the moment of the departure of the sun below the horizon, the last glimpse is coloured bluish green.” This “last glimpse” flash is associated with the inferior mirage, familiar on asphalt roads on sunny days.
It is best seen from a few meters above sea level, and becomes compressed to a thin line at the horizon when seen from considerable heights.
Joule also observed that “Just at the upper edge, where bands of the sun’s disk are separated one after the other by refraction, each band becomes coloured blue just before it vanishes.”
This second form of flash is associated with a mock mirage, which is caused by a thermal inversion below eye level; so it is mainly seen from elevated positions.
As light passes from the vacuum of space into the atmosphere, which acts like a prism, it slows down and causes the light to bend or refract towards the surface of the earth.
The white from the sun is made up of many different colors of light, all of which have a different wavelength. The wavelength (or color) of light affects how much it is refracted on entering the atmosphere, with red light refracted the most and blue least (as in rainbows).
Imagine the image of the sun as being made up of red, green and blue images. Light from the ‘red image’ will be refracted more than that from the green and blue.
So, the ‘red image’ will appear lower than the green, which will similarly appear lower than the blue. At sunset, or sunrise, this effect is intensified as light travels through a slightly thicker atmosphere.
As the sun disappears below the horizon, the ‘red image’ will disappear first and the blue last.
The atmosphere causes blue light to be scattered more than red or green – the reason why the sky appears blue – so light from the ‘green image’ … the ‘green flash’ … will normally be the last thing you see as the sun disappears below the horizon.
On very rare occasions, the atmosphere may be clear enough to allow some of the blue light to reach us and cause a ‘blue flash’ as the sun sets.
The phenomenon lasts only a fraction of a second, so unless you know where to look and when, the chances of seeing one are very slim. Viewing conditions need to be just right, too.
Watching the sun set over an ocean horizon on a clear evening creates optimal viewing conditions.
Your line of sight should be almost parallel to the horizon and you need to really concentrate at the top edge of the sun as it is about 98% set.
If you are lucky, you will see the top edge of the sun turn green for a brief moment, before disappearing below the horizon.
Jules Verne’s 1882 novel “Le Rayon Vert” (The Green Ray) popularized the green flash, described as “a green which no artist could ever obtain on his palette, a green of which neither the varied tints of vegetation nor the shades of the most limpid sea could ever produce the like! If there is a green in Paradise, it cannot be but of this shade, which most surely is the true green of Hope”.
Be careful. Even with the sun low in the sky, concentrated observation with the naked eye can cause damage to your eyesight.
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SONY DSC 
At sunset, the sky is often painted with an array of oranges, reds and yellows, and even some shades of pink. There are, however, occasions when a green flash appears above the solar disc for a second or so. One such occurrence was captured beautifully in this picture taken from Cerro Paranal, a 2600-metre-high mountain in the Chilean Atacama Desert, by ESO Photo Ambassador Gianluca Lombardi. Cerro Paranal is home to ESO’s Very Large Telescope. The green flash is a rather rare phenomenon; seeing such a transient event requires an unobstructed view of the setting (or rising) Sun and a very stable atmosphere. At Paranal the atmospheric conditions are just right for this, making the green flash a relatively common sight (see for example eso0812). But a double green flash such as this one is noteworthy even for Paranal. The green flash occurs because the Earth’s atmosphere works like a giant prism that bends and disperses the sunlight. This effect is particularly significant at sunrise and sunset when the solar rays go through more of the lower, denser layers of the atmosphere. Shorter wavelength blue and green light from the Sun is bent more than longer wavelength orange and red, so it appears slightly higher in the sky than orange or red rays from the point of view of an observer. When the Sun is close to the horizon and conditions are just right, a mirage effect related to the temperature gradient in the atmosphere can magnify the dispersion — the separation of colours — and produce the elusive green flash. A blue flash is almost never seen as the blue light is scattered by molecules and particles in the dense blanket of air towards the horizon. The mirage can also distort the shape of the Sun and that of the flash. We see two bands of green light in this image because the weather conditions created two alternating cold and warm layers of air in the atmosphere. This stunning photo was taken by ESO Photo Ambassador Gianluca Lombardi on 28 March 2011. The phenomenon was captured on camera as the Sun was setting on a sea of clouds below Cerro Paranal. 
