In reply to LETTER asking about cause of halo effect around the Moon.]
Lovely picture, Phil. Thanks. Halo effects around the Moon are normally due to some kind of atmospheric effect.
Depending what is on the air you are looking through, in the upper atmosphere (or even lower atmosphere, for that matter), there will be various kinds of light scattering effects.
Water droplets as found in clouds can give various effects, sometimes diffuse, like this, or sometimes arcs of coloured light very comparable with rainbows … the same kind of refraction is involved.
Ice crystals give different effects – coloured rings around the Moon at various distances which can be quite stunning.