This past week has seen some remarkably beautiful Sunrises and Sunsets where I live in Queensland Australia. And once again the term ‘Green Flash’ has been mentioned on television. ‘Green Flashes’ and ‘Green Rays’ are optical phenomena that sometimes occur right after Sunset. When the conditions are right, a green spot is visible above the upper rim of the disc of the Sun. The green appearance usually lasts for no more than a second or two. Occasionally, the Green Flash can resemble a ray shooting upward from the point of Sunset. Green Flashes occur because our atmosphere refracts the light from the Sun.

These phenomena may be observed from any altitude and at any latitude; although at the equator the flash rarely lasts longer than a second. They are best seen at an unobstructed horizon, such as over the ocean, and are more likely to be seen in a still, clear sky when more of the light from the setting Sun reaches the observer without being scattered - in other words, a ‘boring’ Sunset. It might be expected that a blue flash would be more likely, since that is refracted more than the other colours, and the blue component of the Sun's light is the very last to disappear below the horizon - but it gets scattered and absorbed by particles in the atmosphere, with the remaining light ending up at the other end of the spectrum – yellow, orange and red. With slight magnification, a green rim on the top of the Solar disc may be seen on most clear-day Sunsets, although the flash or ray effects require a stronger layering of the atmosphere and a mirage, which serves to magnify the green from a fraction of a second to a couple of seconds. A Green Flash also may be observed in association with the Moon and bright planets at the horizon, including Venus and Jupiter.

While observing at the Vatican Observatory in 1960, D.K.J. O'Connell produced the first colour photographs of a Green Flash at Sunset.  With an unrestricted view of the horizon, Green Flashes are regularly seen by airline pilots, particularly when flying westwards as the Sunset is slowed. If the atmosphere is layered the Green Flash may appear in a series - at extreme angles a secondary flash can be seen, and at least one observer claims to have witnessed a triple Green Flash.

Just like rainbows, auroras and solar halos, the beauty of Green Flashes awaits those who go out and look for them.
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