A friend of mine wrote to tell me of a phenomenon that she and her husband witnessed – an enormous white foggy ring around the moon. If you do happen to see it, this ring around the moon is an awesome sight.
The ring around the Moon, or lunar halo, is caused by the refraction, or bending, of moonlight as it passes through ice crystals in high-altitude cirrus or cirrostatus clouds in the upper atmosphere. Light from the moon, or more specifically, sunlight reflected off the surface of the moon, passes through ice crystals causing the light rays to bend. The ice crystals are shaped like hexagons, much like snowflakes, which means the light will bend in exactly the same way around the moon, and since the moon is round, this results in a perfectly circular ring.
This effect can actually be explained through math. Because of their hexagonal shape, the ice crystals act like 60 degree prisms, which cause light to be bent at about 22 degrees. Since the light is coming from a single object (the moon), it bends in all directions to form a circle with is 22 degrees in radius, or 44 degrees in diameter. The moon itself is about a 1/2 degree wide as we see it in the sky, meaning lunar halos are about 80-90 times large than the diameter of the moon!
The “prism effect” of the ice crystals is interesting in another way as well. If you look very closely at a lunar halo, you may notice that the light has separated into its various component colours, meaning that the halo itself is, in fact, a rainbow! Red light bends more than blue light, so you may see a redish tinge on the inside edge of the ring, transitioning to blue on the outside edge.
The appearance of lunar rings is not particulary common, and is often overlooked becuase of the sheer size of the ring. According to folklore, a ring around the moon was a harbinger of bad weather to come. While not typically very “sciencey”, there may be some truth to this. Since the halo forms when high altitude cirrus clouds are present, and these clouds typically precede a warm front which can be associated with a low pressure system bringing stormy weather. It’s always fascinating to realize that our ancient ancestors may have been able to piece together a pattern from observatsion of the moon that turns out to have a basis in fact!
Thank you Risa! This is one very cool BLOG! We are so lucky to be able to reach out with weird questions and have someone do the research and provide a direct and meaningful explanation. We feel even more blessed to know that what we saw in the sky that night was rare. It was really overwhelming to have this lunar halo sitting right over us. And yes … the next day, we had a very warm day followed by a day of relentless strong winds that rocked the house. Hummm ….