The large, glowing circle around the moon is officially known as a 22-degree halo, also commonly called a Moon Ring or lunar halo. This striking visual event occurs when moonlight interacts with the Earth’s atmosphere, creating a distinct, circular band of light. The ring often appears white or faintly colored and remains centered on the moon. It is an optical illusion dependent on specific atmospheric conditions high above the observer.
The 22-Degree Angle
The most distinguishing characteristic of this lunar ring is its consistent size, which is why scientists refer to it as the 22-degree halo. This measurement is an angular distance: the angle between the moon’s center and the inner edge of the halo as seen from the observer’s eye. This constant angular radius is a physical property determined by the refractive index of water ice, regardless of the observer’s location.
The consistent 22-degree radius means the halo’s size never changes, even if the moon is high or low in the sky. This predictable geometry is a direct result of how light bends through the microscopic ice crystals high in the atmosphere.
How Ice Crystals Create the Ring
The 22-degree halo is caused by the refraction of moonlight through tiny, six-sided ice crystals suspended in the upper atmosphere. These crystals are typically found in thin cirrus or cirrostratus clouds, which float at altitudes of 20,000 feet (about 6,000 meters) or higher. The light-bending process begins when moonlight enters one face of the hexagonal ice crystal and exits another, acting like a miniature prism.
The hexagonal crystal’s geometry forces the light to be refracted at a minimum deviation angle of almost 22 degrees. Since the crystals are randomly oriented, the light is scattered in a cone of 22 degrees around the moon, creating the full circle. Although the light is split into its component colors like a rainbow, the colors are usually too faint to be seen at night, resulting in the halo’s commonly observed white appearance.
Distinguishing Halos from Coronas and Other Phenomena
A lunar halo is often confused with a lunar corona, which is a much smaller, softer, and often multi-colored disk that hugs tightly around the moon. The halo is caused by refraction through high-altitude ice crystals, while the corona is caused by the diffraction of moonlight through tiny water droplets or small ice crystals in lower or mid-level clouds.
A lunar corona typically has an angular radius of only a few degrees, and its size changes depending on the size of the droplets. In contrast, the halo’s 22-degree radius is fixed. The color sequence in a corona is also different, usually featuring a bluish-white center that fades to reddish-brown rings at the edge.
Weather Significance
The appearance of a 22-degree lunar halo has long been associated with changes in weather, summarized by the folklore, “ring around the moon means rain soon.” The scientific basis lies in the type of cloud required to form the halo: Cirrostratus clouds frequently precede an approaching warm front.
A warm front is a weather system that pushes warmer, moist air over cooler air, often bringing precipitation within the next 24 to 48 hours. Therefore, seeing the high, thin ice clouds that create the halo is an early indicator of an impending low-pressure system and its associated rain or snow. The halo serves as a reliable visual marker of the atmospheric conditions preceding a storm.