The colored rings observed around the moon are primarily known as lunar halos, majestic atmospheric optical phenomena. Less commonly, but also fitting the description of "colored rings," are lunar coronas, which are distinct in their formation and appearance.
Understanding Lunar Halos
Lunar halos are large, luminous rings that typically appear around the moon. They are a common yet captivating sight, often hinting at atmospheric conditions.
How Lunar Halos Form
A lunar halo is created when moonlight interacts with ice crystals suspended high in Earth's atmosphere. Specifically, these crystals are found within cirrus or cirrostratus clouds, which are thin, wispy clouds located at significant altitudes, typically ranging from 20,000 feet (6,000 meters) up to 40,000 feet (12,000 meters).
The formation process involves three key optical interactions:
- Refraction: As moonlight enters the hexagonal ice crystals, it bends, similar to how light bends when passing through a prism.
- Reflection: Some light reflects off the internal surfaces of the ice crystals.
- Dispersion: The light is separated into its constituent colors (like a rainbow) because different wavelengths (colors) of light bend at slightly different angles as they pass through the crystals.
This combination of refraction, reflection, and dispersion results in the characteristic circular ring. The most common and easily observed type is the 22-degree halo, which appears as a large ring with a radius of approximately 22 degrees from the moon's center. This specific angle is due to the way light passes through the specific angles of hexagonal ice crystals.
Appearance of Lunar Halos
Lunar halos often appear as a faint, large ring, sometimes with subtle colors. The inner edge may appear reddish, while the outer edge can have a bluish tint. However, due to the dimness of moonlight, the colors are often not as vibrant as those seen in a solar halo or a rainbow, and the ring may sometimes seem predominantly white.
Lunar Coronas: A Different Phenomenon
While also appearing as colored rings around the moon, lunar coronas are distinct from halos.
How Lunar Coronas Form
Coronas are formed by diffraction of moonlight through tiny water droplets or very small ice crystals present in lower-altitude clouds, fog, or mist. Diffraction causes light waves to bend around obstacles or through small apertures, creating a pattern of bright and dark fringes.
Appearance of Lunar Coronas
Unlike halos, coronas are much smaller and appear closer to the moon's disc, often directly touching it. They feature a central bluish-white disk (called the aureole) immediately surrounding the moon, which is then encircled by one or more concentric rings of vivid colors (blue, green, yellow, and red, with blue innermost). The size of a corona depends on the size of the water droplets or ice crystals; smaller droplets produce larger coronas, and vice-versa.
Key Distinctions Between Halos and Coronas
Understanding the differences between these two atmospheric optical phenomena helps in identifying them correctly:
| Feature | Lunar Halo | Lunar Corona |
|---|---|---|
| Formation | Refraction, reflection, and dispersion | Diffraction |
| Primary Cause | Hexagonal ice crystals in high cirrus clouds | Tiny water droplets or very small ice crystals in lower clouds/mist |
| Appearance | Large, faint ring (22-degree radius typical); often subtle colors or white | Smaller, closer to moon; central disk with vivid, concentric colored rings |
| Colors | Often faint red (inner) to blue (outer), or white | Distinct, vibrant colors (blue innermost) |
| Distance | Far from the moon's disk (22 degrees) | Directly surrounding the moon's disk |
When and Where to Spot Them
Both lunar halos and coronas require specific atmospheric conditions to form. Halos are more commonly observed because cirrus clouds are widespread. Coronas require very uniform droplet sizes, which makes them less frequent but often more striking in their color.
Observing these phenomena is best done on clear, moonlit nights when the appropriate cloud types are present. While they are beautiful to behold, folklore sometimes associates them with impending weather changes, particularly halos, which can indicate moisture advancing ahead of a weather front.
