Question

Why do we sometimes observes haloes rings round the sun or moon

Answer 1

This ring surrounding the Moon is caused by the refraction of Moonlight (which is really reflected sunlight, of course) through ice crystals suspended in the upper atmosphere between 5-10 km in altitude. It doesn’t have to be winter, since the cold temperatures at high altitudes are below freezing any time of the year. Generally they’re seen with cirrus clouds; the thin, wispy clouds at high altitude.

The ice crystals themselves have a very consistent hexagonal shape, which means that any light passing through them will always refract light – or bend – at the same angle.