A Moon Corona

Yesterday evening, we sat a round the campfire with our guests. It was almost full Moon but it was quite clouded. A little later in the evening the sky became clearer, except for some fast moving, high clouds. Then we saw something beautiful; as the thin clouds passed the Moon, a darker ring formed around it at a little distance. At some point the ring had even some colours in it and almost looked like a pale, circular rainbow. When the clouds moved on, the corona disappeared again.
Corona’s are produced by refraction of the moonlight (which is of course reflected sunlight) from ice crystals in the upper atmosphere. The hexagonal (6 sided) shape of the ice crystals focuses the light into a ring. If there are few ice crystals, you will see no clouds but a thin ring
will form at a little distance from the Moon. This is called a halo. In this case the thin clouds provided more ice crystals, which resulted in a thicker band around he Moon, called a corona. The colours sometimes seen are the result of the prism-effect of the hexagonal ice crystals. This separates the light into its various colours because each colour bends slightly different.

Anatomy of the Moon halo

The ring that sometimes appears around the Moon is caused by the moonlight being refracted, or bend, by ice crystals high in the atmosphere. These hexagonal (6 sided) crystals, bend the light at a 22 degree angle, creating a halo 22 degrees in radius, or 44 degrees in diameter.

Weather folklore

Folklore has it that a ring around the Moon is a sign that bad weather is coming, especially in winter. In many cases this may be true. So how can a ring around the Moon predict the weather?
The ice crystals that produce the halo are a sign that there are high altitude, thin cirrus clouds present that normally precede a warm front by one or two days. Typically, a warm front will be associated with a low pressure system, which is commonly referred to as a storm. The days before this storm arrives are usually nice and warm.
In ancient times they believed that the number of stars inside the ring will tell you how many days you have before the storm arrives but if there is any truth in that….?

Lunar halo’s and even corona’s are in fact fairly common, especially around full Moon. So don’t forget to look up to the night sky every now and again. Even when it is clouded there can be something interesting to see!
Regards from the bush
Miriam