Alps - Moon - Ice Halo - Stars ~ Toothed Alpine peaks, slopes of night chilled snow crystals frame an ice sky, its crystals glint to encircle the moon � a 22� halo. Imaged by Roger Benoit from M�rren, Switzerland. Left to right are the Jungfrau, Ebnefluh, Mittaghorn and Grosshorn. ©Roger Benoit, shown with permission

About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed

Moon rays cross hexagonal ice prisms.    Their path of minimum deflection is one of symmetry. Entrance and exit angles the same, the ray inside the crystal parallel to a face.   But these crystals, unlike those of other halos, have no special sky orientation and most rays have a less symmetric journey.   They cross askew. Even those few whose plane is right angled to the prism axis deflect more to create a glinty glow stretching out from the 22° radius edge to more than twice as far from the moon.

22° halo crafting crystals are not known. They must be optically good. Parts must have hexagonal cross sections poorly oriented in the air. They are not the clean columns and plates of other halos for they would be better oriented.

They might be clusters, they might sometimes be very large crystals. But that is speculation. Samples or measurements are needed.