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A Double Green Flash

At sunset, the sky is often painted with an array of oranges, reds and yellows, and even some shades of pink. There are, however, occasions when a green flash appears above the solar disc for a second or so. One such occurrence was captured beautifully in this picture taken from Cerro Paranal.

The green flash is a rather rare phenomenon; seeing such a transient event requires an unobstructed view of the setting Sun and a very stable atmosphere. At Paranal the atmospheric conditions are just right for this. But a double green flash such as this one is noteworthy even for Paranal.

The green flash occurs because the Earth’s atmosphere works like a giant prism that bends and disperses the sunlight. This effect is particularly significant at sunrise and sunset when the solar rays go through more of the lower, denser layers of the atmosphere. Shorter wavelength blue and green light from the Sun is bent more than longer wavelength orange and red, so it appears slightly higher in the sky than orange or red rays from the point of view of an observer.

When the Sun is close to the horizon, a mirage effect related to the temperature gradient in the atmosphere can magnify the dispersion and produce the elusive green flash. The mirage can also distort the shape of the Sun and that of the flash. There are two bands of green light in this image because the weather conditions created two alternating cold and warm layers of air in the atmosphere.

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