What Causes Halos Around the Moon and Sun? The Science Behind Solar and Lunar Halos

Understanding Halos: A Simple Overview

A halo around the Sun or Moon is a ring of light that forms when light passes through ice crystals in high-altitude clouds.

The most common type is the 22-degree halo, named for the angle between the Sun or Moon and the ring.

These halos are not rare, but they are often overlooked because they require the right combination of light, clouds, and viewing conditions.

The key ingredients are:

• A bright light source (Sun or Moon)

• Thin, high-altitude cirrus or cirrostratus clouds

• Hexagonal ice crystals

When these conditions align, the atmosphere acts like a giant prism.

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The Role of Ice Crystals in the Atmosphere

Halos form in cirrus and cirrostratus clouds, which exist at altitudes above 20,000 feet (6,000 meters). At these heights, temperatures are extremely cold, allowing water vapor to freeze into tiny hexagonal ice crystals.

These crystals are not random shapes. They naturally form six-sided prisms due to the molecular structure of ice.

When sunlight or moonlight enters one side of a crystal and exits another, it bends—a process known as refraction.

This bending of light is the fundamental cause of halos.

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Refraction: The Physics Behind the Ring

Refraction occurs when light changes direction as it passes from one medium to another. In this case, light moves from air into ice and back into air.

Because ice has a different refractive index than air, light slows down slightly inside the crystal, causing it to bend.

The hexagonal geometry of the ice crystals ensures that light is bent at a minimum angle of about 22 degrees for the most common halo.

When millions of ice crystals are randomly oriented in the sky, they collectively refract light at this same angle, forming a circular ring around the Sun or Moon.

The ring appears at a consistent radius because the bending angle is consistent.

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Why 22 Degrees?

The 22-degree halo forms because of the specific angle at which light passes through the hexagonal crystals.

The geometry of a six-sided prism causes light to refract most strongly at approximately 22 degrees from its original path.

This angle is determined by:

• The shape of the crystal

• The refractive index of ice

• The wavelength of visible light

Although other types of halos exist (such as 46-degree halos), the 22-degree halo is by far the most common.

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Why Halos Appear as Perfect Circles

The circular shape of halos is a result of geometry.

Ice crystals are distributed in all directions around the Sun or Moon. Since the 22-degree bending angle occurs uniformly in every direction, the light forms a ring rather than a partial arc.

If you imagine drawing a circle with the Sun at the center and a radius of 22 degrees, you have essentially mapped the halo.

The symmetry of the atmosphere produces the symmetry of the ring.

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Differences Between Solar and Lunar Halos

Solar halos are often brighter and more colorful because sunlight is intense.

Lunar halos, on the other hand, appear pale or white because moonlight is reflected sunlight and is much dimmer.

The Moon reflects sunlight from its surface, which was famously explored during the Apollo missions led by NASA.

Because the Moon’s brightness is lower, the colors in lunar halos are usually faint or invisible to the human eye.

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The Colors in Halos

Like rainbows, halos can display subtle colors.

The inner edge of a 22-degree halo often appears reddish, while the outer edge may look bluish.

This occurs because different wavelengths of light refract at slightly different angles.

Red light bends less than blue light, creating a spread of colors.

However, halo colors are typically much fainter than those of rainbows because the refraction process differs and involves smaller angular separation.

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Historical Interpretations of Halos

Before atmospheric science explained halos, they were often viewed as omens.

In Ancient Rome, halos were sometimes interpreted as signs from the gods.

In medieval Europe, solar halos were considered warnings of storms or divine intervention.

Interestingly, these early weather associations were not entirely wrong.

Halos frequently precede storms.

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Halos and Weather Prediction

Halos are often linked to approaching weather systems.

Cirrostratus clouds, which create halos, commonly form ahead of warm fronts associated with low-pressure systems.

If you see a halo around the Sun or Moon, precipitation may arrive within 24 hours.

The presence of widespread ice-crystal clouds often indicates that a large-scale weather system is developing.

Thus, halos can serve as natural weather indicators.

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Other Atmospheric Halo Phenomena

The atmosphere can produce several related optical effects:

• Sun dogs (parhelia) – Bright spots on either side of the Sun

• Light pillars – Vertical columns of light

• Circumzenithal arcs – Rainbow-like arcs high in the sky

These phenomena also result from refraction or reflection in ice crystals but involve different crystal orientations.

Sun dogs often appear as bright patches 22 degrees to the left and right of the Sun.

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Why Halos Are More Common in Winter

Halos are more frequently observed in colder seasons because ice crystals are more abundant in high-altitude clouds during winter.

Additionally, colder surface temperatures allow ice crystals to remain stable in the atmosphere.

However, halos can occur year-round if the right cloud conditions are present.

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Viewing Safety: Never Look Directly at the Sun

If you notice a halo around the Sun, avoid staring directly at it without eye protection.

Direct solar viewing can damage your eyes.

Lunar halos are safe to observe since moonlight is much weaker.

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The Moon’s Role in Lunar Halos

Lunar halos require a bright Moon—typically near Full Moon.

The Moon’s surface reflects sunlight back toward Earth. As this light passes through ice crystals, it refracts just like direct sunlight.

The Moon’s phases—caused by the Earth-Moon-Sun alignment—determine how bright the lunar halo appears.

The orbital mechanics that create lunar phases were described centuries ago by astronomers such as Galileo Galilei and mathematically explained by Isaac Newton.

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Are Halos Rare?

Halos are not rare, but they require attentive observation.

Because cirrus clouds are thin and sometimes nearly invisible, many people fail to notice halos unless they actively look for them.

Urban light pollution can make lunar halos less noticeable, while bright daytime glare can obscure solar halos.

In rural or clear-sky environments, halos are easier to observe.

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The Science of Atmospheric Optics

Halos belong to the field of atmospheric optics, which studies how light interacts with particles in the air.

This branch of science explains:

• Rainbows

• Mirages

• Glories

• Coronae

Halos are among the most geometrically precise atmospheric phenomena due to the consistent refractive properties of ice crystals.

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Why Halos Inspire Awe

Even after understanding the physics, halos remain visually stunning.

They remind us that the atmosphere is not empty space but an active medium filled with microscopic structures capable of bending light into precise patterns.

A halo is the visible result of countless ice crystals working in collective harmony.

It is physics made visible.

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Conclusion: A Ring of Light Explained

So, what causes halos around the Moon and Sun?

They form when light passes through hexagonal ice crystals in high-altitude cirrus or cirrostratus clouds. The crystals refract light at a specific angle—most commonly 22 degrees—creating a circular ring around the light source.

Solar halos are bright and sometimes colorful.

Lunar halos are softer and pale.

Both are beautiful demonstrations of atmospheric optics.

What may appear mystical is actually a precise interplay of geometry, light, and ice suspended miles above Earth.

The next time you see a glowing ring encircling the Sun or Moon, you will know:

It is not magic.

It is sunlight or moonlight bending through frozen crystals in the sky.

And in that bending of light, science reveals beauty hidden in the air.