Sunday, March 6, 2011

A green flash from the sun

Credit & Copyright: Andrew Penketh
(Eva Cassidy singing Ain't No Sunshine)

From NASA (Astronomy Picture Of the Day) & Cornell University

Green flash : many think it is just a myth.
Others think it is true but its cause isn't known.
Adventurers pride themselves on having seen it.

It's a
green flash from the Sun.
The truth is the
green flash does exist and its cause is well understood.
Just as the setting Sun disappears completely from view, a last glimmer appears startlingly green.
The effect is typically visible only from locations with a low, distant horizon, and lasts just a few seconds.
green flash is also visible for a rising Sun, but takes better timing to spot.

Credit & Copyright: Juan José Manzano (Grupo de Observadores Astronómicos de Tenerife)

A dramatic green flash, as well as an even more rare blue flash, was caught in the above photograph recently observed during a sunset visible from Teide Observatory at Tenerife, Canary Islands, Spain.

Credit & Copyright: Pekka Parviainen (Polar Image, 1992)

The Sun itself does not turn partly green or blue -- the effect is caused by layers of the Earth's atmosphere acting like a prism.
The green flash is real but it is rarely seen, since it requires special conditions to be observed. The green flash is usually a band or vertical ray of green light just above the setting or rising sun, and can be green, violet, or blue.
To see it, you need a clear, flat horizon and a haze free sky.
An ocean works well - so do deserts.

"Rayon vert" Côte des Basques, Biarritz ©Sylvain Cazenave

The green flash is caused by rays of sunlight refracting (bending) in the atmosphere.
Because refraction depends on the wavelength (color) of the light, blue, violet and green light are refracted more than yellow, orange, and red light.
So at sunset, when the light has the most atmosphere to be bent by, the sun is surrounded by "shadows" of different colors, with the blue/violet/green shadows farther out.
The red, orange, and yellow shadows are abosrbed by the atmosphere, and the blue and violet shadows are scattered by the atmosphere, so the strongest shadow left is usually the green one.
This effect is only strong enough to see for a few seconds during sunrise and sunset, hence the "green flash."

To see the green flash again, you'll probably need to watch a lot of sunsets on clear days over the ocean.
It really is mostly just a matter of being in the right place at the right time.

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