In certain conditions, a bright Moon may appear to be surrounded by glowing “haloes” of light. This happens when moonlight (sunlight reflected by the Moon) passes through ice crystals high in the atmosphere. The light is reflected at certain angles to produce one or two haloes. They are usually incomplete, and they are most often seen when the Moon is at its fullest.
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. Moonlight passes through one facet of the ice crystal, and is then refracted back out at exactly the angle of 22-degrees.
Of course, the atmosphere is filled with an incomprehensible number of crystals, all refracting moonlight off in different directions. But at any moment, a huge number happen to be in just the right position to be refracting light towards your eyes. You just aren’t in a position to see all the other refracted light. In fact, everyone sees their own private halo, because you’re only seeing the crystals that happen to be aligning the light for your specific location. Someone a few meters beside you is seeing their own private version of the halo – just like a rainbow.
The 22-degree size corresponds to the refraction angle of moonlight. We see a rainbow because the different colors are refracted at slightly different angles. This is exactly what happens with a rainbow. The moonlight is broken up into its separate colors because they all refract at different angles, and so you see the colors split up like a rainbow.
Moon dogs (or “mock moons”) are seen as bright spots that can appear on either side of the Moon, when the Moon is closer to the horizon, and at its fullest. These are located on either side of the lunar ring, parallel to the horizon.
In certain conditions, especially in the Arctic, where the ice crystals can be close to the surface, you can get a moon pillar. The light from the Moon reflects off the ice crystals near the surface, creating a glow near the horizon.
Picture Credit : Google
