Haloes

Haloes are circles of light around the sun or moon, due to the presence of ice-crystals in the air. They are not usually distinguished from coronte, which, strictly speaking, are smaller circles appearing round the sun or moon when seen through a faint cloud or mist. The formation of haloes is very similar to that of rainbows; it is due to the refraction of light into the ice-crystal, and its reflection from the inner surface back through the crystal at a definite angle to its original path. A spectator placed at a certain position will receive light that comes from a certain set of crystals placed symmetrically with regard to him and the sun or moon. Such crystals will lie in a circle whose centre is in the line between his eye and the source of light, and he therefore sees a circle of light surrounding that source. The angular distance of the circle from the sun or moon will depend upon the arrangement of the ice-crystals. It is about 22° usually, but secondary haloes of 46° and even tertiary of about 90" have been observed. The different refrangibilities of light of different colours causes primary and secondary haloes to be tinged red on the inside, and blue on the outside edges. Tertiary haloes appear to be uncoloured, but no satisfactory reeison has yet been assigned for this deficiency. [Rainbow, Mirage.]