The green ray is an atmospheric phenomenon , turned by Jules Verne into the protagonist of the homonymous novel, but with a real existence (although rare). It is due to the behavior of the rays of the setting or rising sun when passing through our atmosphere. We will try to understand how it occurs, and under what circumstances it can be seen.
This phenomenon has been documented since ancient times , as inscriptions from ancient Egypt make mention of a setting sun of green color . The first modern scientific reference appeared in the journal Nature in 1883. Since then attempts have been made to “hunt” the phenomenon, photograph it and, above all, look for its causes. As the most favorable conditions for its observation occur on the sea horizon , it was initially thought that it was the color of the sea that was responsible, as the last rays of the Sun pass through the crests of the waves. However, this hypothesis was soon discarded, since the unusual ray also manifests itself on terrestrial horizons if they are flat .
The factors that really determine the appearance of the green ray are the atmospheric phenomena of refraction, diffusion and absorption .
Refraction, as is known, separates – as occurs in the formation of the rainbow – the different colors of the spectrum .
On the other hand, due to the composition of our atmosphere, blue and violet wavelengths are diffused (“scattered” in all directions) by air molecules, and therefore blue light, during the day, seems to come from all parts (Earth’s sky is blue, not that of other planets with different atmospheres).
The absorption caused by dust and other particles suspended in the air can influence the colors that the Sun shows when it sets, depending on the occasions , presenting a more reddish, more orange color …
Likewise, the presence of clouds, atmospheric turbulence, different concentrations of water vapor, etc. produce different conditions every day , and the difference in temperature between different strata or layers of air can give rise to reflection and distortion phenomena that also add to the factors that interest us.
With all this we can already explain the elusive phenomenon: when the Sun approaches the horizon, atmospheric refraction separates the different colors of the solar disk, leaving violet, blue and green on its upper edge, in this order. However, violet and blue are diffused by the atmosphere, so that when only the upper edge of the disk is visible, it is the green color that reaches our eyes .
However, in even more exceptional conditions it is possible to observe a “blue ray” or violet .
To be able to observe the green ray, we have to look for a flat horizon with great visibility (the sea, a plain, a desert) and wait for a day with a clear and clear sky , preferably in the autumn-winter, in which the Sun does not appear. too flushed in its fall towards the horizon. If we are lucky then, we will see a green flash for just a few fractions of a second, the time it takes for the Sun to completely disappear .
The duration logically depends on the speed with which the Sun sets, and this is related to the inclination of the celestial equator with respect to the horizon: thus, the Sun sets vertically and therefore faster at the equator, where the green ray is still shorter; and much more slowly near the Poles, where the Sun can be more than an hour setting and the green ray, therefore, can remain up to several minutes: durations of up to 35 minutes have been recorded .
As for the way it manifests itself, the most common is simply to see the edge or upper limb of the Sun colored green at the last moment . This green stripe is typically about 10 arc seconds wide, which is less than the resolution of the human eye, which is about 25 arc seconds. However, refraction works in our favor by widening the strip up to several arc minutes, and exceptionally, up to almost half a degree!
But it can also happen – and it is its most legendary manifestation – that we observe a flattened and green shape that seems to float in nothingness when the Sun has already set completely . In this case, refraction is again responsible for making it reach us above the horizon.
The phenomenon in question may be preceded by others , such as the distortion of the solar disk that sometimes appears flattened, taking an elliptical shape, or separated into two parts , or appearing to spill over the horizon as if it were liquid, or even duplicating its own image. by reflection.
The green ray can be produced symmetrically at sunrise , the flash appearing just before the solar disk, but obviously this is much more difficult to observe .
Likewise, the Moon and the planets (stars that show an apparent disk, but not the stars) are capable of casting their own “green rays” which, although much more subtle than those of the Sun , can be captured photographically.
