High Moon Parry Arcs

Lunar halos imaged by veteran observer, Michael Ellestad. The circumscribed halo from column crystals is spectacular but is not the star of this show. Look below it for a faint loop of light, a rare lower Parry arc. ©Michael Ellestad, shown with permission.
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An upper Parry arc in daylight is a rare sighting.    A lower arc by moonlight even more so.    A high moon and a dark sky are needed – here the moon was 57° high.

Depending on the lunar or sun altitude, there are four routes by which rays can generate 22° Parry arcs.  

Hexagonal ice crystal side faces are, by convention, labelled from 3 to 8.  The end faces are 1 and 2.   

In Parry oriented crystals the near horizontal uppermost face is 3.   The lower Parry arc ray path is into a sloping face to leave through the lower horizontal facet.   This is (8,6) in the diagram which is equivalent to (4,6).   The upper arc‘s ray path is (3,5).

At right, the HaloSim ray tracing shows the two Parry arcs