Green Flash

The Green Flash is an atmospheric refraction phenomenon, appearing at the top edge of the sun as a brief green flash. It is rarely seen with the naked eye, primarily because specific conditions are needed for its occurrence, but also because many do not know when and where to look for it. The green flash can take three different forms:

1. The Green Rim, which is almost always visible at the upper edge of the sun with binoculars and becomes wider as the sun sinks lower; at the same time, the lower edge turns red. Often, it remains visible as a green strip even after sunset.
2. The Green Segment, which occurs when the upper segment of the setting sun detaches due to mirage effects and turns green. This form is usually preceded by a strong distortion of the solar disk.
3. The actual Green Flash or Green Ray is extremely rare to see with the naked eye. It resembles a green flame or tuft and is observed at the moment the sun disappears behind the horizon. This phenomenon is often visible for only a fraction of a second, or under favorable conditions for a few seconds.

The fundamental cause of the Green Flash is the refraction of sunlight. Since refraction is strongest near the horizon, the last light segment of the setting sun is split into its spectral colors there. There is a red, green, and blue solar rim. This splitting increases toward the horizon, but even there it's only a few arcseconds small (one arcsecond is 1/3600 degree). Since the red rim sets first, only the blue and green rims are above the horizon for a very short time. Due to atmospheric pollution, the blue light is greatly weakened, and for a few seconds, the green rim is actually visible. Under extraordinary conditions, the blue light might be seen in very rare cases. But what conditions are necessary to perceive the green flash? The green flash can be well observed when there is a clear view of the horizon and the air is very clear and clean. This is also why a green flash is often observed at the infinite horizon over seas and oceans, where the geometric view to the horizon is almost parallel. Observations are also feasible from mountain peaks, as you are often above the inversion layer. To observe the Green Flash well, it is important to take careful precautions to prevent serious eye damage! Although the sun is near the horizon and its brightness is reduced by the atmosphere, the sun is still very bright, and prolonged observation with the naked eye or optical instruments is permanently damaging to the eyes. The Green Flash can appear not only on the sun but also on the moon, and it has even been observed on Venus, Jupiter, and Saturn. Besides the Green and Blue Flash, there is logically also the Red Flash. This occurs when the lowest rim of the sun or moon is just rising above the horizon or when the sun has set behind a sharply defined cloud bank near the horizon and its lowest section emerges from under the cloud bank. This phenomenon is even more fleeting than the Green Flash. Moreover, it is rare that a "true" beam shoots up here. Hence, it is very difficult to observe the Red Flash. For the formation of the Green Segment, a calm and layered atmosphere with layers of differently dense air masses is required. When the sun is a few degrees above the horizon, it appears distorted and vertically flattened. The more it approaches the horizon, the more flattened it appears. Soon the solar rim appears notched on both sides. These notches, caused by refraction and mirages, give the impression of "riding" on the top or bottom of the sun and then suddenly turn green. The intensity of the color depends on many factors. If the sun is too bright, the colors fade. Conversely, if it is too weak, hardly any colors are visible. Consequently, clear photographs of the green segment are very difficult to achieve. Atmospheric mirage effects also cause the Green Rim or the so-called Novaya Zemlya Effect (named after an archipelago extending from the Ural Mountains in Russia to the Arctic Ocean). A narrow green strip rims the upper edge of the sun and is sometimes still visible even when the sun itself has long set. As a narrow green slit of light, it moves along the horizon as the sun below it. This effect mainly occurs in higher latitudes, where the angle of the sun's path to the local horizon can be very flat. However, even when this effect can be observed in mid-latitudes, it is occasionally visible until a few minutes after sunset.