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Chasing the green flash

Solar phenomena are revealed at Paranal.
Provided by ESO, Garching, Germany
Green flash
The green flash is a rare phenomenon seen at sunrise or sunset, but only when conditions are just right.
Justin Branam
May 6, 2008
Cerro Paranal, Chile, home of the European Southern Observatory's (ESO) Very Large Telescope (VLT), is one of the best sites for observation on Earth.

The Earth's atmosphere is a gigantic prism that disperses sunlight. In the most ideal atmospheric conditions, such as those found regularly above Cerro Paranal, this leads to the appearance of green and blue flashes at sunset. The phenomenon is so popular that it is tradition for the Paranal staff to gather daily on the telescope platform to observe the sunset before starting their long night of observations.

The green and blue flashes are fleeting events that require an unobstructed view of the setting Sun, and a very stable atmosphere. These conditions are very often met at Paranal, a mountain in Chile's Atacama Desert, where the sky is cloudless more than 300 days a year.

ESO staff member Stephane Guisard has been chasing green flashes for many years and has captured them on many occasions. "The most challenging is to capture the green flash while still seeing the rest of the Sun with all its colors," he says.

His colleague Guillaume Blanchard was even luckier. On Christmas Eve, while following the tradition of looking at the sunset, he immortalized a blue flash using his hobby telescope.

ESO astronomer Yuri Beletsky also likes to take photographs from Paranal, but prefers night views. This allows him to make use of the unique conditions above the site to make stunning images. On some of these, he has captured other extremely interesting effects related to the Sun: the so-called zodiacal light and the Gegenschein.

Both the zodiacal light and the Gegenschein (German for "counter shine") are due to reflected sunlight by interplanetary dust. These are so faint that they are only visible in places free from light pollution.

Most of the interplanetary dust in the solar system lies in the ecliptic, the plane close to which the planets are moving around the Sun. The zodiacal light and Gegenschein are seen in the region centered around the ecliptic. While the zodiacal light is seen in the vicinity of the Sun, the Gegenschein is seen in the direction opposite the Sun.

Each of the small dust particles, left over from comets and asteroids, acts as a small Moon reflecting the light coming from our host star. "If you could see the individual dust particles then you would see the ones in the middle of the Gegenschein looking like very tiny full moons, while the ones hidden in the faint part of the dust band would look like tiny crescent moons," explains ESO astronomer Colin Snodgrass. "But even the VLT cannot see such tiny individual dust particles out in space. Instead, we see the combined effect, in photos like these, of millions of tiny dust particles reflecting light back to us from the Sun."
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