A ring around the Moon, known as a lunar halo, is an optical phenomenon that appears as a large, bright circle encompassing the lunar orb. Lunar halos are a result of specific atmospheric conditions. These rings are illusions created within Earth’s atmosphere, not directly on the Moon itself.
How the Ring Forms
The formation of a lunar halo begins with high-altitude cirrus or cirrostratus clouds, composed of tiny ice crystals. These clouds typically float between 20,000 to 40,000 feet (6,000 to 12,000 meters) above the ground, where temperatures are well below freezing. The ice crystals within these clouds are hexagonal in shape, acting like prisms.
Moonlight travels through these ice crystals. As the light enters an ice crystal, it bends or “refracts” due to the change in medium. This double refraction causes the light to deviate from its original path at a specific angle.
The light is refracted at an angle of approximately 22 degrees. This consistent bending by millions of similarly oriented ice crystals creates the characteristic circular ring around the Moon, appearing at a constant 22-degree radius from the lunar disc. While typically white, a faint coloring (red on the inside, blue on the outside) can sometimes be observed due to light dispersion. However, the human eye often struggles to discern these colors in dim moonlight.
Specific conditions are necessary for these ice crystals to form at the correct altitude and density. Cirrus clouds, which are thin and wispy, can cover vast areas of the sky. The Moon needs to be bright, ideally full or nearly full, for the halo to be distinctly visible through these transparent clouds.
The Meaning of a Lunar Halo
A ring around the Moon has long held significance in folklore and for weather prediction. A common adage states that a halo around the Moon indicates approaching bad weather, such as rain or snow, a folk wisdom rooted in the atmospheric conditions that create these halos.
The cirrus clouds responsible for forming halos often precede a warm front. Warm fronts typically bring warmer, moister air masses that can lead to precipitation within 24 to 48 hours. Therefore, a lunar halo can suggest a change in weather is on the way.
While this prediction holds some truth, it is not always a guaranteed forecast. Cirrus clouds can form without an approaching weather system, meaning a halo does not invariably lead to rain or snow. Nevertheless, this observation served as an early method of weather forecasting before modern meteorology.
Culturally, lunar halos have been interpreted in various ways throughout history. Many cultures viewed them as omens or signs, sometimes associated with divine presence or impending change. Some ancient cultures considered them messages from gods or symbols of protection.