Sunday, 13 February 2011

Fake sun

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Julie Bronson's photos above show a largely forgotten effect of a sun pillar, the fake sun. It was photographed on 19 January 2009 in South Central Minnesota, USA.

Sometimes, when low sun is just behind an edge of an opaque cloud that is precipitating ice crystals, an image of the sun can been seen below or above the cloud in the icy virga. The effect is so perfect that one is easily fooled to think it is real sun. It is blindingly bright, circular and about the size of the sun. Only when the true sun comes up from behind the cloud, the cheat is revealed.

Today this optical effect of sun pillar is almost unknown, but in the past sky watchers have made detailed notes on it. Auguste Bravais listed in 1845 five persons who have observed the fake sun, including Johannes Hevel, Pierre Bouguer and Giovanni Cassini. Above is shown the observation by Cassini on 18 January 1693. Unfortunately it is a bit cropped from the left.

Julie Bronson's images of the fake sun may be the only ones existing. Her documentation of the phenomenon is commendably extensive and comparison with Cassini's 300 years old observation is good. I have seen fake sun a couple of times during 25 years of sky watching. Appearances of this effect are short and camera must be handy when the time comes.

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