Circumzenithal Arc and Sun Altitude

The circumzenithal arc is broader and more tightly curved when the sun is higher. In this image the sun altitude was 27 degrees, only five degrees less than the upper limit for the arc to be visible.

This image is at the same scale and was taken about 90 minutes later with a solar altitude of only 12 degrees. The arc is now obviously longer and thinner than it was earlier on.

Canon EOS 20D   16th June 2006
Berkshire, England

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