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October 14, 2017

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Straight out of Pirates of the Caribbean?

December 21, 2016

On December 14th, 2016, you, too, might have been able to witness one of nature's rare and ephemeral light phenomena: a green and a red flash on the super moon...

 

 

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 rotating upside down, and a green flash appearing 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 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 up 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 composes its light get separated. It is exactly what happens with sunlight passing through any transparent surfaces: you start seeing all the colors of the 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. 

 

On the night of December 14th, 2016, I went out to take pictures of the near supermoon rise against a very clear sky. I got out of work at around 4:30 pm and hurried home, thinking about going out but having no idea as to when the moon was rising. When I found out it was 20 minutes later, I stormed out of the house with my cameras and long lenses, hoping not to forget anything. I jumped in my car, headed towards Esterhøj where I could have a great field of view. I barely had the time to set up and adjust my cams that some orange was already painting the north-eastern horizon, indicating the imminent moonrise. As the moon was slowly making its way up above the horizon, I was not realizing that I had captured a the green and red flash for the first time. Here comes a little gallery: 

 

The green 'flash' lasted for thirty seconds (it took three frames on my time-lapse) and the red flash is barely visible at the bottom edge of the moon.

 

I put a small time-lapse together for the sake of it, along with some shots taken later that night when the night frost was falling on the landscapes of Odsherred. 

 

 

As I was finished, I took advantage of the view point to take a picture of the northern horizon. No aurora but another strange bright angel in the sky. It took me several minutes to put the pieces together and connect the dots. The Høve community had installed a projector to illuminate the ancient Viking tombstone on top of the hill. I was actually looking at the rays of light emerging from the stone that was behind me and casting its shadow in the bright night sky. Take a look: