What is the meaning of a ring around the moon?

Sometimes, we see a halo in the night sky. Scientists referred to this phenomenon as the lunar halo. The visible ring around the moon means rain or snow is about to fall. Ancient people often used it to predict bad weather. 

In this article, we will explore how lunar halos form and common folklore about the halo around the moon. We will also discuss three other optical illusions similar to the lunar halo.

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What is the lunar halo, and how does it form?

The ring around the moon is known as the moon’s halo or lunar halo. People also call it the 22-degree halo or the moonbow, mimicking the rainbow, another optical illusion. It is an optical illusion of a bright ring around the moon.

The moon’s halo forms because of the moonlight’s refraction from ice crystals in the upper atmosphere, causing a beautiful, bright circle of light. You can see halos around the moon, mostly in cirrus clouds with tons of tiny ice crystals in the upper atmosphere. Cirrus clouds are at altitudes of 6,000 m to 20,000 ft.

Lunar halos don’t have a specific time of appearance. They appear anytime during the lunar calendar and the year, especially during winter. You can not see the lunar halo in cloudy weather. It appears in a bright sky with the moonlight’s refraction and reflection on the ice crystals in the thin cirrus clouds and cirrostratus clouds.  

Lunar halos form when moonlight hits water droplets suspended in the sky. Tiny ice crystals in cirrus clouds bend light and change direction, entering and leaving through the crystals’ sides. The size of the halo around the moon depends on the size and shape of the ice crystals in the cirrus clouds.

Ice crystals are tiny hexagonal columns that are not up to 20 microns in size. The light passing through these ice crystals bends at an angle of minimum deviation of 22 degrees from its initial direction. This constant moonlight refraction causes white light to scatter in all directions, forming the unique halo around the moon. 

Related Read: What Does A Red Sky At Night Mean?

Similar Optical Phenomena

Here, we will explore three other atmospheric optics; 

Solar halo  

Solar halos are similar to halos around the moon. The sun halo forms when sunlight reflects or refracts through the hexagonal ice crystals in thin cirrus and cirrostratus clouds. Solar halos are caused by refraction, which separates colors at the outer edge of the halo because the different colors of the incident light bend at slightly different angles.

However, sunlight reflects a bright white sun halo in the bright sky. Reflection doesn’t break incident light into component colors because the wavelengths reflect at the same angle, producing a large ring. 

The halo around the sun is often a 22-angle halo and a series of colored arcs, but it can also be a full circle with a 22-degree angular radius and the sun or moon at its center. Halos around the sun isn’t common, but it is a sight to behold when it does happen.

J.T. Brownell described his sighting of a halo around the sun on the morning of January 26, 1882, as surpassing brilliancy. He said the outer edge of the halo was a white light, followed by rings of pale lemon and orange, and the inner edge was grayish brown. The light ring around the sun started disappearing as the sun rose high and disappeared entirely by 10:30 a.m.¹

Rainbows  

Rainbows occur because of the refraction and internal reflection of light on water droplets in the sky. They can occur in fog, around a waterfall, during rainfall, or in sea spray. They are not confined to one spot in the sky; they appear in various places across the sky. Rainbows are a light spectrum, including red, orange, yellow, green, blue, and violet.

Whether the sun or moon is in the sky, rainbows appear as long as there’s light to interact with water droplets in clouds. Seeing a rainbow depends on the position of the viewer. The light source (sun or moonlight) is mainly behind the person standing. There are unique cases where a secondary rainbow appears above the primary one. 

The light entering the water droplet is refracted and then reflected by the back of the droplets. The reflected light still gets refracted multiple times at multiple angles. Water droplets’ refractive index determines the radius of every visible rainbow. The refractive index measures how much a ray of light bends through various mediums.

A water droplet with high refractive index will produce a rainbow with a small radius. Also, rainbows appear in full circles like the solar or lunar halo. However, people on earth can’t see the whole shape except people in an airplane. No one sees the same rainbow because everyone has a different horizon and anti-solar point. 

Glory

Another optical illusion is glory, a light phenomenon observable from an aircraft or hot air balloon. A mountain climber can also witness this phenomenon at high altitudes. It looks like an iconic saint’s halo around the viewer’s head. Glory mainly occurs through light backscattering when the sun shines in the sky.

The light interacts with ice crystals in the air, producing a series of centered circles of colored light. The rings of light surround the shadow of the person seeing it. Glory formation depends on the size of the droplets and ice crystals in high cirrus clouds. 

Glory can only occur when light interacts with crystals with diameters ranging from 10 to 30 micrometers. Droplets and crystals bigger or smaller than this range won’t manifest glory. The shadow of the observer is known as the Brocken Spectre.

It is often cast on the cloud where glory occurs, specifically in the center of the rings. The Brocken Spectre’s size and clarity depend on the distance between the observer and the cloud, the light angle, and the size of the ice crystals. Glory is more colorful and maintains the same size when the crystals are uniform.

We don’t know everything about glory yet, but we hope to make more discoveries soon. Some cultures and traditions view glory as a divine sign and manifestation. 

Folklore about the ring around the moon

The lunar halo is a unique occurrence that has inspired tales, myths, and folklore by Indigenous people worldwide. The halo around the moon is primarily a weather lore. They believe it is a warning sign of rain, snow, or sleet. People tried to use lunar halos to predict bad weather, especially during the winter. 

We can’t say their predictions were accurate, but they do have some element of truth because lunar halos only appear in cirrus clouds. Cirrus clouds sometimes show an approaching warm front associated with a low-pressure system. A low-pressure system is a storm that can carry a sudden drop in temperature, heavy rain, snow, hail, thunder, and lightning.   

Conclusion 

Lunar halos are a unique natural phenomenon that sometimes occurs via the interaction of ice crystals in thin cirrus clouds. They are a perfect sight during the full moon. Indigenous people often used them to predict how bad the weather would get, especially during winter. We hope you remember this when you next see a ring around the moon!