The Red Flash
|Etruscan Vase sunsets and sunrises are mirages of two suns. Let’s talk here only of sunsets – sunrises are the same but with opposite vertical movements. Warm air beneath cooler refracts sunlight to give an upper erect image and a lower inverted image. During a sunset the lower image rises as the upper one sinks.
Interesting things happen where the two images meet. This is at the ‘fold line’.. At the fold the images are stretched vertically. We can see that as the sunset progresses – the two images are liquid-like in that they distort and then join as though attracted and glued.
The high vertical magnification at the fold is the key to the green flash. This is the place of maximum colour dispersion. At the final phase of the sunset, the two images of the sun’s upper limb slide together – the top image sinking and the bottom inverted one rising. Colour dispersion gives the upper limb a slight green rim and as the two images merge the thin green line is all there is. The mirage magnifies the rim vertically – enough to flash out a vivid green.
And earlier in a sunset when the two – red edged - images of the sun’s lower limb meet at the fold? The enhanced vertical colour dispersion then gives a red flash.
The fold line is slightly above the horizon as are the green and red flashes.
Take extreme care when searching for green and particularly red flashes. Never use binoculars and never stare at the sun. Treasure eyes.
|The Etruscan Vase or Omega mirage is formed by refraction across the temperature/density gradients of warm air beneath cooler.|
|Above the fold line is an erect sun with a thin and normally invisible green upper limb and lower red one. Below the fold the second image is inverted with its top edge red and lower one green. Images are stretched vertically at the fold and their colours separate to become visible. As the two red rims meet at the fold we see a red flash.. ..and a green flash when the green rims merge.|