Celestial Jewellery: Moon Halos Illuminate the Night

By Mary Hughes

There has been some chatter around recently about current sun activity and subsequent aurora phenomena. I’ve al-ways been a bit fascinated by aurora, but I tend to gaze at the sky anytime I’m outside and the other intriguing things I like are Moon Halos. We all tend to take the Moon for granted and most of us hardly give it a second glance. But when atmospheric conditions are right the moon can produce some interesting and rather stunning effects. Scientists call a ring or circle of light around the sun or moon a halo.

A halo (from Greek) also known as a nimbus, ice bow or gloriole is an optical phenomenon produced by light interact-ing with ice crystals suspended in Earth’s atmosphere, resulting in a wide variety of coloured or white rings, arcs, and spots in the sky. Many halos are near the Sun or Moon, but others occur elsewhere or even in the opposite part of the sky. Among the most well-known halo types are the circular halo (properly called the 22° halo), light pillars and sun dogs, but there are many more; some of them common, others (extremely) rare.

Perhaps the most common of these effects are the Moon halos, large rings which circle the moon on a hazy cold night and have a radius of either 22° or 46°. They are more intense when the Moon is at its brightest, around full Moon but can be witnessed to a lesser degree at other bright phases of the Moon. For the rings to appear there needs to be a layer of thin cloud containing millions of tiny ice crystals covering much of the sky high in the upper atmosphere. These ice crystals are typically suspended in cirrus or cirrostratus clouds high (5–10 km) in the upper troposphere, but in cold weather they can also float closer to the ground, in which case they are referred to as diamond dust. Each of these ice crystals behaves like a small lens and because most of these crystals have the same elongated hexagonal shape the moonlight entering one face of the crystal and exiting its opposing face is bent at an angle of 22°. The larger 46° ring which is far less typical may be caused by the light exiting at different angles from the crystals.

A lunar corona is a ring of light, usually in the form of a rainbow, seeming to appear around the moon in the night sky. They are also caused by high thin cloud, but the moonlight is being refracted by water droplets in the clouds and not by ice crystals. On rare occasions Moon Halos and Lunar Coronae can be seen together. The Lunar Coronae have been known to swell or shrink slightly depending on the size of the water droplets that are present in the passing clouds. There can also be colours present on the coronae, but these colours are always very subtle and more likely to be obvious when the Moon is full.

So, the next time you are sitting out under the night sky check it out.