Lunar Elliptical Halo
Sighted by Stewart King at Bennington, Vermont, USA. It was 23:45 local time on 19th January, ’16 with the moon 42° high. Aldebaran is at 5 o’ clock. The ground level air temperature was -9 Celsius.
The halo was visible for several minutes with small but significant changes. The two images were only 36 seconds apart yet the ring structure has already altered.
Elliptical halos are rare. They are small compared to the common 22°. This one was 1/7th that size – about 6° wide. Shield the sun or moon well to find one.
Images ©Stewart King, shown with permission
Snowflake-like sectored plates are another crystal possibility. Flakes with curved surfaces are another.
The crystals responsible for ellipticals are problematic.
Ray paths like those below through plate-like pyramidal crystals at first sight generate plausible computer simulations but the match with reality is not good enough.
Furthermore, the crystals are crystallographically improbable. That is not to say they cannot exist and growth nuclei in power plant exhausts, for example, might favour them.