The night of 29th January was a chilly one on Rathlin, but the moon was shining brightly and almost full. We happened to look up to admire the night sky, and noticed a big ring of white light around the moon.
It was a 22 degree halo, which occurs due to the refraction of light in small hexagonal ice crystals high in the atmosphere. The 22 degrees refers to the radius of the ring.
Actually there were two rings around the moon at the time – the 22 degree halo and also the corona, which circles the moon more closely, and which we observed as a ring of orangey-red colour showing against the thin cloud surrounding the moon. The corona differs from the 22 degree halo in being the result of refraction of light in water droplets or ice crystals of a larger size.
We’d seen the solar version of this phenomenon before, both on Rathlin and in Kamchatka, as shown in the photo below.