The visual effect of a “fuzzy glow” or a ring around the moon is a common atmospheric optical phenomenon, often seen on clear nights with high clouds. These luminous rings are formed when moonlight interacts with tiny particles suspended in the Earth’s atmosphere. What an observer sees is one of two distinct phenomena: a large, faint halo or a smaller, colorful corona. The appearance is largely determined by the type of particle and the physical mechanism at work.
The Primary Cause of the Lunar Halo
The most frequently observed ring is the large lunar halo, a translucent circle with a radius of 22 degrees that appears to surround the moon. This effect is caused by refraction, the bending of light as it passes through one medium into another. Moonlight is bent as it travels through millions of hexagonal ice crystals suspended in the upper atmosphere. These ice crystals accumulate in high-altitude cirrus or cirrostratus clouds, which often appear thin and wispy.
The hexagonal shape of the ice crystals determines the specific 22-degree radius of the halo. Moonlight enters one face of the crystal, is refracted, and then exits through a different face, undergoing a second refraction. This process consistently deflects the light by a minimum angle of 22 degrees, regardless of the crystal’s orientation. Since no light is refracted at an angle smaller than 22 degrees, the sky inside the halo often appears noticeably darker than the surrounding area.
When Diffraction Creates a Glow
While the large halo is caused by refraction, the “fuzzy glow” can also be the result of a separate phenomenon called a lunar corona, which is caused by diffraction. Diffraction is the slight bending of light waves as they pass around the edges of a small obstacle. A lunar corona appears as a smaller, typically more colorful ring that hugs much closer to the moon’s disc, often with an angular size of around 10 degrees.
This effect is created when moonlight encounters small, uniform liquid water droplets found in lower-altitude clouds, fog, or haze. These tiny droplets act as the obstacles, causing the light waves to spread out and interfere with each other. The result is a series of concentric, pastel-colored rings, with the central area immediately around the moon often appearing as a bright, bluish-white disk called the aureole. The size of the corona is inversely related to the size of the droplets. The colors, which can repeat in alternating rings of blue-green and red, are a result of the different wavelengths of light diffracting at slightly different angles.
Atmospheric Requirements for Optical Effects
The visibility of both halos and coronas depends on specific atmospheric conditions governing the particles. For the lunar halo, the hexagonal ice crystals must be suspended in high-level cirrus or cirrostratus clouds, typically existing at altitudes of 20,000 feet (6,000 meters) or higher. These ice clouds must be thin enough to allow moonlight to pass through but dense enough to contain sufficient crystals for refraction.
Conversely, a lunar corona requires the presence of small, uniform water droplets found in lower-level clouds, mist, or fog. The uniformity of the droplet size is particularly important, as non-uniform droplets would blur the distinct colored rings into a less defined glow. For both optical effects to be visible, the particles must remain suspended, allowing the light to interact consistently and create the recognizable rings.