When looking up at the night sky, a luminous ring sometimes appears to encircle the Moon. This atmospheric phenomenon, often mistaken for a simple ring, is actually a Lunar Corona. The presence of color, specifically the blue or bluish-white glow nearest the Moon, is a defining characteristic of this event. This corona is formed when light from the Moon passes through a veil of thin clouds.
Distinguishing the Lunar Corona from a Lunar Halo
The Moon can be surrounded by two distinct types of light rings, and the blue ring you observe is the smaller of the two, known as a corona. A lunar corona is characterized by its small angular size, appearing tightly fitted around the Moon, typically with a radius of only a few degrees. Its formation depends on light interacting with very small particles, primarily supercooled water droplets or tiny ice crystals found in mid-level clouds. The corona often presents a bright, whitish central disk, called the aureole, which is immediately surrounded by one or more concentric, pastel-colored rings.
This corona is different from the Lunar Halo. A halo is a significantly larger ring, always appearing at a fixed 22-degree radius from the Moon. Haloes are caused by refraction, where light bends as it passes through hexagonal ice crystals located in high-altitude cirrus clouds. Unlike the colorful corona, the halo usually appears white or faintly tinted, with the space between the Moon and the ring often appearing dark.
The Physics of Color Separation: Diffraction
The mechanism responsible for creating the lunar corona and separating the light into its distinct colors is called diffraction. Diffraction occurs when a light wave encounters and bends around the edge of an obstacle, or passes through a small opening. In the case of the lunar corona, the moonlight waves are bending around millions of individual, small water droplets or dust particles suspended in the cloud layer. The size of these atmospheric particles is incredibly small, often having diameters in the range of 10 to 15 micrometers, which is comparable to the wavelength of visible light.
This bending effect is not uniform across the entire spectrum of light; instead, it is dependent on the light’s wavelength. Shorter wavelengths of light, such as blue and violet, are diffracted at a smaller angle than the longer wavelengths, like red and orange. Because the blue light is deflected less, it forms the innermost portion of the concentric ring pattern. This results in the observed sequence where the blue ring is closest to the Moon, followed by green, yellow, and red on the outermost edge of the first ring.
If the cloud layer contains particles of a highly uniform size, the diffraction patterns from all the individual droplets constructively interfere, creating a sharp, brightly colored corona. The angular diameter of the corona is inversely related to the size of the droplets; smaller droplets produce a larger, more spread-out corona. The vibrant, multi-ringed spectacle is a direct indication of a cloud layer composed of uniform, small particles.
What the Blue Ring Reveals About the Atmosphere
Observing a lunar corona provides insights into the atmospheric conditions overhead. Since the corona is produced by light interacting with liquid water droplets or fine aerosols, its visibility indicates the presence of thin, mid-level clouds, most commonly altostratus or altocumulus. These clouds are generally located several kilometers above the surface, and the uniformity of the corona’s rings suggests the cloud’s constituent water droplets are all close to the same size.
The existence of these mid-level clouds, especially those that are slowly thickening, is often associated with approaching weather systems. Folklore has long connected a ring around the Moon with a coming change in weather, a connection that has a scientific basis. The conditions that create a widespread, uniform layer of altostratus clouds are frequently the forerunners of a warm front or a low-pressure system, which can bring precipitation within the next day or two.
Therefore, the blue ring around the Moon is more than just an aesthetic display; it functions as a natural indicator of the atmosphere’s state. It confirms the presence of uniform, liquid water particles in the middle layers of the troposphere. The visual phenomenon acts as a weather predictor, signaling a shift in atmospheric conditions that may lead to rain or snow in the near future.