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Lunar Halos by Martin McKenna (site) in Northern Ireland during the early hours of December 14, '08.. ©Martin McKenna, shown with permission.

"I was out observing the Geminid meteor shower before dawn. The Moon (1 day after full approx) was in the W. Some high level cirrus clouds drifted over periodically.."

The camera is pointing westwards, away from the 36° high moon. The bright arc is the 'paraselenic circle' the lunar counterpart of the parhelic circle. When complete it goes all around the sky parallel to the horizon and at the same elevation as the moon.

The slight colour fringes are due to the presence of water droplets or much smaller ice crystals than those producing the halo. The bright halo is acting as a light source and the small particles diffract light to produce coronal or iridescent coloured edges.

While the paraselenic circle is moderately rare, the cross-like halo is more so. This is a Wegener arc which crosses the parhelic circle directly opposite the moon. The star field (at 05:30UT) confirms this, for reference the bright star at lower right is Arcturus.

Horizontal column crystals form Wegener arcs. Rays enter and leave side faces inclined at 60° but are internally reflected from an end face. Without the internal reflection, a circumscribed halo would be formed. Martin Mckenna saw fragments of a circumscribed halo. It is likley that column crystals rather than plates were responsible for the paraselenic circle.



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