A captivating ring, known as a lunar halo, can sometimes appear around the Moon. This natural optical event is a mesmerizing spectacle.
What is a Lunar Halo?
A lunar halo is a large, often faint, luminous circle that surrounds the Moon. This atmospheric phenomenon is an optical illusion, appearing as a ring rather than a solid disc. The distinct circle typically has an apparent radius of approximately 22 degrees from the Moon. This measurement can be roughly gauged by holding an outstretched hand at arm’s length, with the Moon at the thumb and the halo at the pinky finger. While often appearing white or colorless, a lunar halo can sometimes display faint colors, with a reddish hue on the inside and a bluish tint on the outside. The dimness of moonlight often makes these colors difficult to discern with the unaided eye. The sky directly inside the halo may also appear noticeably darker than the surrounding sky. It is important to distinguish a lunar halo from a lunar corona, which is a much smaller, iridescent ring directly around the Moon caused by diffraction from water droplets, rather than the larger, refraction-based halo.
The Science Behind Its Formation
Lunar halos form due to the refraction of moonlight through millions of tiny ice crystals suspended high in Earth’s atmosphere. These ice crystals are typically hexagonal in shape. They are found within thin cirrus or cirrostratus clouds, which exist at altitudes ranging from about 20,000 to 40,000 feet (approximately 6,000 to 12,000 meters).
The process of refraction occurs as moonlight enters one face of a hexagonal ice crystal and exits another, bending the light. Each crystal acts like a tiny prism, redirecting the light. The specific geometry of the hexagonal ice crystal causes the light to bend at a precise minimum angle of approximately 22 degrees. This consistent bending angle creates the uniform 22-degree radius of the halo.
The circular appearance of the halo results from the random orientation of countless ice crystals, each refracting light at the same 22-degree angle relative to the observer’s line of sight. Because the crystals are randomly oriented, some are always positioned to bend light towards the observer, forming a complete circle. The atmospheric conditions necessary for cirrus clouds to form, such as cold temperatures at high altitudes, are a prerequisite for this phenomenon.
Understanding Its Significance
A ring around the Moon is often associated with changes in weather patterns. The presence of cirrus clouds, which contain the ice crystals responsible for halos, frequently indicates the approach of moisture-laden weather systems. These high-altitude clouds commonly precede warm fronts, which can bring rain or snow within a day or two. While lunar halos can serve as an indicator of specific atmospheric conditions, they are not a direct or guaranteed prediction of impending precipitation. The phenomenon is a natural optical event, demonstrating light interacting with ice crystals, not a sign of supernatural occurrences. Similar optical displays, known as solar halos, can also form around the Sun due to the same principles of light refraction through ice crystals. This reinforces that halos are widespread atmospheric occurrences, visible when conditions align.