Atmospheric
Optics
Lunar Halos
Walter Piorkowski imaged these at South Beloit, Illinois, USA.
Images ©Walter Piorkowski, shown with permission
A paraselenic circle sweeps through the moon and across the image. Lunar halos take their name from Selene, the Greek goddess of the moon and sister of Helios the sun god.
A 22 degree halo circles the moon. Touching it at top and bottom and extending outwards at its sides is a circumscribed halo. Refraction though horizontal column crystals produces it. Further out still is a moondog or paraselene, exactly analogous to the more often seen sundog and produced by plate crystals.
Some unreliable sources teach that moon/sun dogs are 22 degrees from their parent. Only for the instant when the sun or moon is on the horizon is that the case. At higher altitudes the moon/sun dog is further and further away.
The HaloSim ray tracing at left shows 120 degree paraselenae. One is possibly shown in Walter Piorkowski�s second image below. At this lunar altitude 120 degree paraselenae from regular hexagonal plates are very faint. To make them visible the ray tracing used less regular hexagons with alternate side long and short.
