The ring of light often seen encircling the moon is a common atmospheric optical phenomenon known as a lunar halo, specifically the 22-degree halo. This luminous ring is caused by the moon’s light interacting with our planet’s atmosphere high above the ground. The halo appears consistently at an angular radius of approximately 22 degrees from the moon’s center. This effect is created by the bending, or refraction, of light.
The Critical Role of Ice Crystals
Lunar halos depend on the presence of microscopic ice crystals suspended high in the atmosphere. These crystals are typically found within thin Cirrus or Cirrostratus clouds, which float at high altitudes, usually above 20,000 feet (6,000 meters). At these frigid heights, water vapor freezes directly into countless ice particles.
The specific shape of these ice crystals determines the halo’s circular form and precise size. The crystals are almost always six-sided, structured as tiny hexagonal prisms. Light bending occurs as moonlight passes through these geometric shapes, which act like miniature prisms floating in the sky. The six faces and the 60-degree angle between alternating side faces are the physical basis for the 22-degree phenomenon.
The Physics Behind the 22-Degree Ring
The halo is created by the refraction of light as it passes through the hexagonal ice crystals. Moonlight, which is reflected sunlight, enters one face of the six-sided crystal and then exits through an alternate face, bending twice on its path. This double-bending of light is analogous to how a glass prism works.
The specific geometry of the ice crystal dictates the exact angle of light deviation. When light passes through the 60-degree angle formed by two non-adjacent faces of the hexagonal prism, the minimum angle it can be deflected is 21.84 degrees, which is rounded to 22 degrees. Light is concentrated at this minimum deviation angle, ensuring that only rays bent by this precise amount reach the observer’s eyes.
The collective light from millions of randomly oriented ice crystals refracting light at this fixed angle creates the impression of a ring with a 22-degree radius. No light is refracted toward the observer at angles smaller than this minimum, which is why the sky inside the circular halo often appears noticeably darker. This mechanism explains why the halo remains a consistent size regardless of the cloud’s altitude or the observer’s location.
Predicting When You Might See a Halo
The best time to witness a lunar halo is when the moon is bright and a thin veil of high-altitude clouds is present. These clouds are typically transparent sheets of Cirrostratus clouds, composed entirely of the necessary ice crystals. They float at altitudes ranging from 20,000 feet up to 40,000 feet, where temperatures maintain the ice particles.
Observing a lunar halo can offer a subtle hint about potential changes in the weather. The high, thin clouds that produce the halo often precede the arrival of a large, low-pressure storm system by 24 to 48 hours. This is the basis for the old adage, “Ring around the moon means rain soon,” though it is not a completely reliable prediction method.
To best view the phenomenon, look up on a clear night when the moon is visible, especially if the sky has a faint, milky quality. Moving away from light pollution will enhance the contrast between the faint halo and the night sky. While the halo often appears white, close observation might reveal a faint reddish tint on the inner edge and a bluish tint on the outer edge, caused by the ice crystals separating the colors of the moonlight.