Solar sights

The circumzenithal arc is a rare phenomenon. Much commoner are parhelia or 'sun dogs'. These occur when the sun is low in the sky. They appear 22° to the left and right of the sun, as bright spots of light. You may be able to see part of a 22° halo as well. The light from the parhelion is slightly refracted, giving a slight rainbow effect. Occasionally I have seen bright pillars of light extending a further 5° in the horizontal plane.

Frequent observation of the daytime (and night time!) sky is often repaid by the observation of beautiful phenomena that escapes the casual observer.

Upside down rainbow

Last week we saw a circumzenithal arc, which is a somewhat rare natural phenomenon outside the polar regions. It was right overhead, surprisingly bright, and lasted ten minutes.

Explanation: the Sun is at an angle of about 22 degrees above the horizon. There are zillions of hexagonal ice crystals in the stratosphere. Sunlight hitting the ice crystals at 90 degrees goes through two internal reflections, and sends a rainbow earthwards.

Leave a comment if you have seen one of these.Read more at http://www.cambridge-news.co.uk/cn_news_home/DisplayArticle.asp?ID=349778

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