Have you ever glanced up at the night sky and noticed a luminous circle embracing the moon? This captivating phenomenon, known as a moon halo or lunar halo, has intrigued sky watchers for centuries. But what exactly causes this ethereal ring to form around the moon? Let’s delve into the science behind moon halos and explore why they grace our night skies.
Decoding Moon Halo Formation: Ice Crystals and Light Refraction
A moon halo is an optical phenomenon, a visual trick of light, created by the refraction and reflection of moonlight. The key players in this celestial light show are tiny ice crystals suspended high in our atmosphere, typically within cirrus or cirrostratus clouds. These clouds reside at altitudes of 20,000 feet (6,000 meters) and higher, reaching up to 40,000 feet (12,000 meters).
Imagine these ice crystals as miniature prisms floating in the sky. As moonlight passes through these hexagonal-shaped crystals, it bends, or refracts. This refraction is similar to how a prism splits white light into a rainbow of colors. However, in the case of moon halos, the collective refraction from millions of ice crystals creates a ring of light around the moon.
The 22-Degree Halo: A Universal Measurement
One of the fascinating aspects of moon halos is their consistent size. Most lunar halos appear as 22-degree halos. This means that if you were to measure the angle from the moon to the inner edge of the halo, it would consistently be around 22 degrees. This uniformity arises from the specific hexagonal shape of the ice crystals and the refractive index of ice.
When light enters a hexagonal ice crystal and passes through its sides, it behaves like it’s entering a prism with a 60-degree apex angle. Due to the properties of ice, the angle of minimum deviation – the angle at which light bends the least – is approximately 21.84 degrees. This is why the radius of moon halos is almost always around 22 degrees, roughly the width of your outstretched hand at arm’s length when held up to the sky. The diameter of the halo, spanning across the moon, is thus around 44 degrees. Both solar and lunar halos of this size are commonly referred to as 22-degree halos due to this consistent measurement.
Subtle Colors and the “Hole in the Sky” Effect
Similar to how rainbows display colors, moon halos can also exhibit faint tints of color. As white moonlight refracts through the ice crystals, it can be separated into its component colors. Typically, if colors are visible, you might notice a reddish hue on the inner edge of the halo and a bluish tint on the outer edge. However, these colors are often very faint and difficult to discern with the naked eye, especially compared to the more vibrant colors sometimes seen in solar halos due to the sun’s greater brightness.
Another interesting effect associated with 22-degree halos is that the sky enclosed within the ring can often appear darker than the sky outside the halo. This occurs because the ice crystals preferentially refract light away from the center of the halo, creating the illusion of a “hole in the sky” surrounded by a bright ring.
Moon Halos and Their Atmospheric Companions
While the classic 22-degree moon halo is the most well-known, the atmosphere can conjure other related optical phenomena.
Coronas: Smaller, Colorful Rings
Sometimes, you might observe smaller, more colorful rings closer to the moon than a halo. These are called coronas. Unlike halos formed by ice crystals, coronas are created by the diffraction of moonlight by tiny water droplets in the atmosphere. Coronas are typically about half the width of halos, with a radius of around 10 degrees, and often display more vibrant colors.
Double Halos and Beyond
Under certain atmospheric conditions, more complex halo displays can occur. Refraction from ice crystals can sometimes create double halos, with two concentric rings surrounding the moon. In rare instances, these double halos might even exhibit spokes radiating outwards.
Sun Dogs and Solar Halos: Lunar Halo Relatives
Moon halos are closely related to solar halos, which are rings of light around the sun formed by the same ice crystal refraction process. Additionally, the same atmospheric conditions can produce “sun dogs” or “mock suns” – bright spots of light appearing 22 degrees to the left or right of the sun, often seen in pairs. These are also caused by ice crystals and are essentially fragments of a 22-degree halo.
Furthermore, larger 46-degree halos can sometimes accompany or appear independently of 22-degree halos. These fainter and more diffuse halos form when light enters randomly oriented hexagonal ice crystals through a face and exits through the base, causing light to be dispersed at a wider angle.
Rare Multiple Halos
In extremely rare cases, even more complex multiple halo displays have been observed. NASA documented an extraordinary quadruple lunar halo observed over Madrid, Spain, in 2012. This remarkable event involved a combination of 22-degree and 46-degree halos, along with rarer circumscribed halos and arcs, creating a breathtaking and exceptionally unusual celestial display.
When and Where to Witness Moon Halos
Moon halos are surprisingly common, meaning you have a reasonable chance of spotting one if you know when and where to look. While they can occur at any time of year, they are more frequent during winter months.
The key ingredient for moon halo formation is the presence of cirrus clouds. These thin, wispy clouds are often transparent and can cover vast areas of the sky. A bright, full or nearly full moon veiled by cirrus clouds provides the ideal conditions for halo visibility. So, unlike many astronomical observations that require clear skies, cloudy conditions, specifically cirrus clouds, can actually be advantageous when searching for moon halos. In fact, sometimes the presence of a lunar or solar halo is the only indication that these thin, high-altitude clouds are present.
Folklore, Weather Prediction, and Cultural Significance
Throughout history, moon halos haven’t just been objects of beauty; they’ve also been imbued with cultural significance and folklore, particularly related to weather prediction.
Farmers’ Almanac and other traditional sources note that lunar halos have long been associated with impending changes in the weather, especially the arrival of storms. This association has a basis in scientific observation. Cirrus clouds, which cause moon halos, often precede approaching warm fronts. Warm fronts, in turn, are linked to low-pressure systems, which can bring about unsettled weather conditions, including temperature drops, rain, and even storms.
This connection between cirrus clouds and approaching weather systems led to the development of weather lore around moon halos. Before modern meteorology, observing a moon halo became an empirical method for predicting weather changes. A well-known proverb illustrates this:
“If the moon show a silver shield,
Be not afraid to reap your field;
But if she rises haloed round,
Soon we’ll tread on deluged ground.”
While this folklore connection to weather prediction is intriguing, it’s important to remember that it’s not a foolproof method. Cirrus clouds don’t always indicate an imminent warm front or storm. Similarly, other folklore beliefs, such as counting stars within the halo to predict the time until bad weather arrives, are largely superstitious.
Conclusion: Appreciating the Science and Beauty
The next time you see a ring around the moon, take a moment to appreciate this captivating display of atmospheric optics. Moon halos are not just beautiful phenomena; they are a testament to the fascinating interplay of light and ice crystals in our atmosphere. They remind us that the sky above is full of subtle wonders, waiting to be observed and understood. By unraveling the science behind “Why Is There A Ring Around The Moon,” we gain a deeper appreciation for the natural world and the intricate processes that shape our visual experiences.
References
- Ring Around The Moon? Here’s What It Means, Farmers’ Alamac. [Accessed 11/19/22], [https://www.farmersalmanac.com/ring-around-the-moon-9657]
- 22° Halo around the moon, Atmospheric Optics. [Accessed 11/19/22], [https://atoptics.co.uk/halo/circmoon.htm]
- Moon Halo, Hyperphysics. [Accessed 11/19/22], [http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/moonhalo.html]
- Quadruple Lunar Halo Over Winter Road, NASA Science. [Accessed 11/19/22], [https://science.nasa.gov/quadruple-lunar-halo-over-winter-road]
- “Disk with a hole” in the sky, Atmospheric Optics. [Accessed 11/19/22], [https://atoptics.co.uk/halo/circ2.htm]
- Apperson. G. L., Dictionary of Proverbs, Wordsworth Editions Limited, 2006, ISBN 1 84022 311 1
