by Caleb, KE0FOE
A Green Flash (also known as a Green Ray) is an optical phenomenon that can sometimes be seen right before sunset.
When conditions are right, a brief area of green light appears to envelop the upper area of the sun’s disc just before it sinks below the horizon.
When the sun is low in the horizon, moisture in the atmosphere absorbs the yellow and orange colors, and air molecules scatter the violet light. What’s left is the red and the green light. Near the horizon, the sun’s light is so strongly refracted to the point that it’s almost as if there are two suns – a red one and a green one overlaid on top of one another. The red light overpowers the green which is why the sun looks red while it’s setting. However, since the refracted red light tends to be shifted a little lower toward the horizon compared to the green, as soon as all of the red refracted disc of the sun sinks below the horizon, we’re left with a brief glimpse of the green refracted disc before it quickly slips below the horizon as well.
Green Flashes and Green Rays were the inspiration for the novel The Green Ray by famed author Jules Verne.
More information about Green Flashes can be found on Wikipedia: https://en.wikipedia.org/wiki/Green_flash, on EarthSky: http://earthsky.org/earth/can-i-see-a-green-flash, and you can watch one for yourself on YouTube: https://www.youtube.com/watch?v=lwus2nqU0SY