Want to stay on top of all the space news? Follow @universetoday on TwitterA moon halo is an optical phenomenon produced by ice crystals(ice halo). The ice crystals create colored or white arcs and spots in the sky. A halo does not always occur around the Moon. They can be near the sun, around artificial lights, even human bodies. They form in very cold weather when ice crystals called diamond dust are floating in the air.
Ice halos are produced by the ice crystals in cirrus clouds in the upper troposphere. Usually around 5-10 km up. The shape and orientation of the crystals are responsible for the type of halo observed. The light is split into colors by dispersion. The ice crystals act like a series of prisms and mirrors and refract and reflect the light to send shafts of colored light in multiple directions.
A moon halo is a particular type of ice halo called a 22° halo. These halos only form around the Sun and Moon. It forms as light is refracted in hexagonal ice crystals and the light beam passes through two sides of the prism forming a 60° angle and the angle of minimum deviation is almost 22° and determine the color wavelength. This causes the inner edge of the circle to be reddish and the outer edge bluish. A 22° halo may be visible on as many as 100 days per year.
Light passing through the hexagonal ice prisms is deflected twice, which produces deviation angles ranging from 22° to 50°. Lower deviations result in brighter halos along the inner edge of the circle, while greater deviation contribute to the weaker outer part of the halo. Since light is not refracted at less than 22°, the sky is darker inside the halo.
Moon halos are an interesting phenomenon. It is always interesting discover what ice and water can do to light.
We’ve also recorded an entire episode of Astronomy Cast all about the Moon. Listen here, Episode 113: The Moon, Part 1.