Two Crescents
Andrew Kirk at Bishop California captured this unique image of the twin crescents of the Moon (at left) and a circumzenithal arc. March 9, 2011 at 1628 hours when the sun was 16° high.
 ©Andrew Kirk, shown with permission


Atmospheric
Optics

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Both crescents point in the same direction – sunwards.

The Moon, usually a dominant sky object, is dwarfed by the ice-halo.

From the Moon’s known 0.5° diameter we can estimate the width of the circumzenithal arc as about 3°. This broad dispersion of colours arises because it is formed by sunlight refracted through the equivalent of a 90° ice prism.

Light enters the top faces of horizontally aligned hexagonal plate crystals and leaves through a vertical side faces to form the most beautiful of all the halos.

Rays entering along this route when the sun is higher than 32° cannot leave a side face but are instead totally internally reflected to eventually contribute to the parhelic circle.

As the sun sinks below the 32° critical altitude the CZA first forms as a broad smudge close to the zenith. At lower sun altitudes it transforms into the familiar crescent. It moves further from the zenith with decreasing sun altitude.

It is at its brightest when the sun is around 20° high and becomes very faint when the sun is close to the horizon.




Photographing halos? - Still cameras are best for capturing both subtle details and any unnoticed rare arcs that might be present.  Video cameras are not very halo friendly!

                    
HaloSim ray tracing simulations of the circumzenithal arc for changing solar altitudes. The CZA decreases in radius but increases in width as the sun climbs. Eventually at a solar altitude of 32° it can no longer form and its light is instead channeled into the white parhelic circle.