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Last Updated:  Tuesday, 4 March, 2003, 16:19 GMT
Strange rays seen over Horseshoe Canyon
By Dr David Whitehouse
BBC News Online science editor

(Image copyright Peggy Peterson)

While enjoying a spectacular sunset over the Horseshoe Canyon in the US state of Utah, Peggy Peterson noticed something unusual going on behind her.

Light beams scattered by clouds in front of the setting Sun appeared to re-converge on the opposite side of the horizon. Meteorologists have a technical term for this beautiful but rarely seen phenomenon: anticrepuscular rays.

Although the Sun's rays strike the Earth almost parallel to one another, we see them appear to come from a point because of perspective - because at a distance, parallel lines seem to converge, like railway lines disappearing over the horizon.

So-called crepuscular rays are the rays that appear when parallel solar beams are redirected through clouds and appear to diverge.

Sometimes when crepuscular beams are visible, they can cover the entire sky.

Anticrepuscular beams appear at the other end of the sky from crepuscular beams. They converge on the horizon at exactly 180 degrees from the place where the beams appear to come from.

This trick of perspective is rarely seen as well as that depicted in Peggy Peterson's picture.


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