120 degree White Parhelion
David Thomas saw this halo display over Egilsay, one of the Scottish Orkney Islands. It lasted 20 minutes. Part of a 22° halo circling the sun is at far right. At left, the colourless bright patch is a rare 120° parhelion. It is linked to the more familiar 22° parhelion or sundog by the parhelic circle.?
The 120° parhelia’s rarity comes from its low visibility against cloud coupled with how it is made from unusual crystals and the necessary tortuous sunlight path through them.?Sunlight enters the tops of plate crystals floating horizontally. It reflects twice from vertical side faces then leaves through the lower face. The two refractions with two internal reflections always give a ray deviated in azimuth by 120° and there is no net separation into colours. ?Unusually thick plate crystals and/or ones with roughly alternate long and short side faces are best feed enough light into the contorted ray path.??
120° parhelia feature as two of the “6 suns” of the famous Vädersolstavlan painting in Stockholm Cathedral. Its portrayal of halos on April 20, 1535 is both beautiful and accurate.?Image by Måns Hagberg.??
David Thomas assembled this composite.?From top downwards there was a circumzenithal arc, 46° halo, upper Parry arc, upper tangent arc, 22° halo, 22° parhelia, parhelic circle and an infralateral arc.?