Green flash in the sky: If you’re also a fan of Disney’s famous pirates saga featuring Johnny Depp in the role of Jack Sparrow, you might remember that one of the third episode’s riddle implicates a map. The Earth rotates upside down and a green flash appears at sunset/rise, allowing the passage of living characters into the realm of the dead. What you might not know is that these green flashes actually exist. Well, less dramatic and loud, but still there on the horizon. What sorcery is this Green flash in the sky though?
Some might think there is some kind of magic at work here but this phenomenon has all to do with light and optics. First and foremost you are probably dying to know why the sun or the moon appear completely crooked when the near the horizon like in the picture above. The phenomenon is a pure optical illusion and even though the moon or the sun appear to be above the horizon line, they actually are beneath it! Since the Earth is round (apologies to all the remaining skeptical opinion for breaking it to you), its atmosphere creates a loupe effect and bends the rays of light. You have probably made a similar experiment in your physics class: putting a penny at the bottom in a water-filled container and getting far away from the rim. You still seem to see the penny lying at the bottom. These refraction properties enable us to see the sun or the moon while they still are under the horizon! Pretty cool, isn’t it?
Hold on! It doesn’t stop there. As the rays of white light travel through the atmosphere to reach your eye, the different wavelengths that compose its light get separated. It is exactly what happens with sunlight passing through any transparent surfaces: you start seeing all the colors of its spectrum. The dominant color is orange/yellow as the object gets closer to the horizon, but sometimes the conditions are right and even more diffraction happens, and some other colors appear: green and red.
That is what we call green and red flash. They are the ephemeral diffraction of white light from the moon or the sun and are visible when the diffraction properties of the atmosphere are extreme. Here’s a short gallery of images taken in December 2016 when the moon was full:
The Green flash in the sky lasted for thirty seconds and the red flash is barely visible at the bottom edge of the moon. But all in all it is still visible with the naked eye, especially the Sun flash. So next time you watch those objects rise and set make sure you catch the flash!
