ASK TOM: Giant halo around the full moon

Dear Tom,

 Please explain the amazing giant halo around the full moon that was visible in the Chicago area Monday night.

– Jeff, Schaumburg


Dear Jeff,

Many area residents commented on the magnificent halo that graced area skies Monday night, Oct. 29. The halo resulted from moonlight being refracted through ice crystals in cirrostratus clouds. Those clouds were the western portion of Hurricane Sandy. Some ice crystals take the form of tiny six-sided pillars. Moonlight that enters one of the faces of the pillar and then exits an opposite face is bent (refracted) 22 degrees, generating a circular halo centered on the moon and with an angular radius of 22 degrees. A similar but fainter halo of angular radius 46 degrees occurs when light enters one of the hexagonal faces and exits through the pillar’s base.