To see this display in sunlight would be memorable. To see it at night generated by Selene�s silvery, gentle and subdued rays is exceptional.
Ice crystals with three classes of orientation helped make it. The familiar 22� radius halo came from light refracted through randomly tilted crystals and prism side faces inclined 60� to one another and. Horizontal column crystals generated the upper tangent and supralateral arcs, the former via refraction through side faces inclined 60� and the latter by rays passing between a side face and near vertical end face. Plate oriented crystals generated the moon dogs or paraselenae and most of the circumzenithal arc. There is a weak and rare Parry arc from horizontal columns constrained so that two prism faces remain in a horizontal plane.
At left HaloSim traces the rays with the contributing crystals identified.
Why the particular tilts from perfect alignment? Why the crystal thicknesses and lengths (c/a ratio = length/width)? There was no 46� halo and the randomly oriented columns were deliberately made long to avoid making it. In Nature, imperfect end faces commonly achieve the same. The upper tangent arc was not particularly sharp not extensive. The columns of the ray tracing were given large wobbles of standard deviation of 1� to reproduce it. Similarly, the moon dogs were fuzzy and two populations of plate crystals were set wobbling at 2 and 5�. Parry crystals are very efficient at generating Parry�s arcs and a mere pinch of them (with wobbles) did the trick. They were made long to avoid very rare Parry supralaterals forming � again, imperfect ends also do that. Do not believe too strongly the resulting selection in �Crystal Selection� for Nature is richer with a variety of crystal habits, diverse regularity or otherwise and a whole range of perfection to imperfection.
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