Why is there no green sky

Magical phenomenon in the sky: the green glow

16.01.2008

A novel by Jules Verne is even related to this very rarely observed event

Few people are ever allowed to see it. Anyone who has seen it will never forget it - and, at least as the legend promises, from now on they will no longer be wrong about matters of the heart. We are talking about a color phenomenon in the sky: the green glow, also called the green ray. For a brief moment, usually only for a fraction of a second, a green light flashes over the setting sun.

The green glow is sometimes accompanied by a strong deformation of the image of the sun. Photo: Mbz1, wikipedia.de

Only rarely seen for more than a second: the green ray, a particularly impressive phenomenon in the sky. Photo: Cfoellmi, wikipedia.de

As short as the green glow is, it has always fascinated people. The ancient Egyptians reported about it on stone tablets, and the French author Jules Verne even made it the main theme and title of an entire novel. "Le Rayon Vert" is the name of the love story, published in 1882, of a young woman who does not want to marry until she has seen the green glow. So she moves around restlessly in search of the legendary green, which, as Jules Verne describes, "no painter can get on his palette. A green that nature has nowhere else produced, neither in the variety of colors of the plants nor in the color of the clearest seas! If there is a green in paradise, then it cannot be any other than this green, the true green of hope. "

This enthusiasm for the beauty of the phenomenon does not detract from the fact that the physical background is comparatively simple: Since the density of the air continuously decreases towards the top, the sunlight falling from the sun on the earth is refracted like a lens and reaches the Earth's surface on a curved path. This refraction of light is not the same for all wavelengths of sunlight. Blue light is refracted more strongly than green, green light more strongly than red. This effect can also be observed in the rainbow, only that the light here does not refract on the layers of air of different densities, but in the water droplets. In the case of sunlight in the atmosphere, however, this splitting is so small that it cannot usually be seen with the naked eye.

If the sun now sets behind the horizon, the red part of the light disappears first, as this is the weakest part. With the green and blue parts, the tracks are more curved. The light can therefore follow the curvature of the earth longer and can therefore be seen longer. However, the air filters out a large part of the blue light, as the path through the atmosphere is particularly long when the sun is low. A few seconds after sunset, it can happen that only the green part of the light remains. Then there is a flash of green, the green glow.

Because of its short duration, often only a fraction of a second, the phenomenon is difficult to observe. In addition, the view to the horizon must be free and the air must be very clear. The green glow is therefore most likely to be seen at sea or on the coast - especially in the far north, where the sun sets at a very acute angle to the horizon and the time in which a green ray can flash is a little longer.

An island in the North Atlantic is also the place where Jules Verne's heroine Helena Campbell finally gets the opportunity to see the green glow: on the uninhabited Scottish island of Staffa, she experiences a sunset that cannot be surpassed in terms of clarity. But at the crucial moment she does not direct her gaze to the setting sun. Instead, she looks - effectively staged by Jules Verne - into the eyes of the young painter Olivier Sinclair who accompanies her and recognizes in him her future. (ud)