A luminous ring occasionally appears around the moon, a captivating sight known as a lunar halo or “moon ring.” This atmospheric phenomenon arises from specific interactions between moonlight and Earth’s atmosphere.
Understanding Lunar Halos
A lunar halo typically appears as a large, often faint, luminous ring encircling the moon. This ring maintains a consistent radius of approximately 22 degrees from the moon’s center. While sometimes appearing white or hazy, lunar halos can occasionally display subtle colors, with a faint red tint on the inside and a blue tint on the outside. The sky immediately inside the halo may also appear darker than the surrounding night sky.
The Science of Their Formation
Lunar halos form when moonlight interacts with ice crystals in high-altitude cirrus or cirrostratus clouds, typically above 20,000 feet (6,000 meters). Moonlight refracts, or bends, as it passes through these hexagonal ice crystals. Each crystal acts like a miniature prism, bending the light at a specific 22-degree angle from its original path. This consistent bending creates the circular halo effect.
The hexagonal shape of these ice crystals is essential for forming the 22-degree halo. While the prism effect can separate moonlight into its component colors, similar to a rainbow, these colors are often too subdued for the naked eye. This is because moonlight is significantly dimmer than sunlight, making it difficult to fully stimulate the eye’s color-detecting cells.
What a Moon Ring Can Predict
Observing a lunar halo is associated with impending weather changes. The high-altitude cirrus clouds that produce halos often precede a warm front. Warm fronts are frequently associated with low-pressure systems, which can bring precipitation like rain or snow within 24 to 48 hours. A moon ring can therefore indicate active weather is approaching.
While a link exists, a lunar halo does not guarantee specific weather or act as a mystical omen. The presence of these ice-laden clouds signifies atmospheric conditions that often precede a change in weather patterns, rather than directly causing precipitation. Folklore suggesting that counting stars within the halo predicts bad weather is not supported by scientific evidence.
Commonality and Observation
Lunar halos are common atmospheric phenomena. They can occur year-round but are frequently observed during colder months in the Northern Hemisphere due to a greater abundance of high-altitude ice crystals. The best conditions for viewing a moon ring involve a clear, dark night with a bright moon, veiled by a thin layer of cirrus clouds. Some cloud cover is necessary for the halo to form, unlike other astronomical observations.
Viewing is enhanced away from urban light pollution, which diminishes the halo’s visibility. The same optical principles create similar rings around the sun, known as solar halos, when sunlight interacts with ice crystals. However, directly observing solar halos is challenging due to the sun’s intense brightness.