|Lunar Pyramidal Crystal Halos
Gaida Jakson captured this display over Estonia on 5th April '14.
Pyramidal ice crystals created most of the features. Unusually, some crystals were aligned to give pyramidal plate arcs.
The crescent moon had to be overexposed to show the halos and stars.
©Gaida Jakson, shown with permission
|A 9° radius halo hugs the Moon. Ray paths [3,26] generate it. They pass between a central prism face and a pyramidal facet inclined at 28°. The top and bottom of the halo is slightly brightened - perhaps the presence of 9° parhelia from partially aligned crystals.
Hints also of 18° parhelia from similar crystals - This time the ray paths are [13,25].
The HaloSim ray tracing used pyramidal crystals with only a narrow central section (c/a=0.1) and truncated pyramids of height 0.3. A trace of 'ordinary' plate crystals added the 22° parhelia.
Tumbling randomly oriented crystals create circular odd-radius halos
|Some pseudo-plate aspect pyramidal crystal are weakly aligned to generate parhelia or 'plate arcs'. More rarely, aligned column aspect crystals give pyramidal column arcs.|