A luminous ring encircling the Moon is a striking visual phenomenon caused by atmospheric optics. This appearance is not an inherent feature of the Moon but results from the interaction between moonlight and particles suspended in Earth’s atmosphere. The light acts as a celestial spotlight, revealing the presence and physical properties of high-altitude clouds. Understanding this spectacle requires looking closely at how light behaves when it encounters minute atmospheric components.
Defining the Phenomenon: Lunar Coronas and Halos
The rings seen around the Moon fall into two distinct categories: the lunar corona and the lunar halo. Although both involve light scattering through clouds, they differ significantly in size, appearance, and formation. The lunar corona is often responsible for the “blue ring” query, appearing as a relatively small, bright, multi-colored disk that hugs the Moon closely. Its inner region, the aureole, often features a distinct bluish-white tint.
The lunar halo, by contrast, is a much larger and more common ring that appears at a fixed distance from the Moon. This ring is typically a translucent white or faintly colored circle that spans a wide area of the sky. These two optical displays provide scientists with clues about the specific composition and altitude of the clouds present.
The Physics of Light Bending
The two different ring types are produced by two entirely different physical processes: diffraction and refraction. The lunar corona is created through diffraction, the slight bending and spreading of light waves as they pass around the edges of a small obstacle. These obstacles are incredibly small, uniform water droplets or supercooled liquid droplets found in thin, mid-level clouds. These droplets effectively scatter the moonlight, causing the light waves to interfere and produce the characteristic ring pattern. The size of the resulting corona is determined by the size of the droplets; smaller droplets produce a wider, larger corona.
Conversely, the lunar halo is a product of refraction, where light bends as it passes through a transparent material, much like light passing through a prism. Halos form when moonlight encounters millions of hexagonal ice crystals suspended high in the atmosphere, typically in wispy cirrus clouds. These six-sided crystals act as miniature prisms, bending the light at a specific minimum angle. The geometry of the hexagonal ice crystal dictates that light passing through it will be deviated by a minimum of 22 degrees. This consistent angle of deviation creates the large, fixed-radius ring known as the 22-degree halo.
Why the Ring Appears Blue
The striking blue color closest to the Moon is a signature feature of the lunar corona, resulting directly from the physics of diffraction. When moonlight diffracts around tiny particles, the degree to which a light wave bends depends on its wavelength. Shorter wavelengths of light, such as blue and violet, are diffracted at smaller angles than the longer wavelengths, like red. This difference causes the colors to separate into concentric rings, with blue light focused toward the interior, closest to the light source.
As the angle from the Moon increases, the colors transition through green and yellow, with red light appearing on the outermost edge. The intensity of the diffraction often means that only the innermost blue-white glow, called the aureole, is easily visible to the naked eye, leading to the perception of a simple blue ring. While the prismatic refraction of ice crystals in a halo also separates colors, they are usually too dispersed and faint for the human eye to perceive due to the dimness of moonlight.
How to Tell the Difference Between a Corona and a Halo
An observer can distinguish between a corona and a halo by focusing on three key characteristics: size, color, and cloud appearance. The most immediate difference is the size of the ring relative to the Moon. A corona is small, often only a few times the width of the Moon itself, appearing to hug its edges tightly.
A halo, conversely, is vast, always maintaining a radius of approximately 22 degrees, which is roughly the width of a hand span held at arm’s length. Coronas typically display bright, vivid colors, especially the blue or bluish-white closest to the Moon, followed by a reddish outer fringe. Halos, due to the dimness of refracted moonlight, are almost always seen as a pale, whitish ring, with any subtle color separation being extremely difficult to detect.
Finally, the type of cloud surrounding the Moon provides a definitive clue regarding the underlying physics. Coronas require thin, mid-level clouds composed of liquid water droplets. Halos require high-altitude cirrus clouds composed entirely of hexagonally shaped ice crystals.