Anthelion Phenomenon Explained: How It Forms and Where to See It

The anthelion phenomenon is a rare atmospheric optical event that appears as a pale white spot opposite the sun. It forms when sunlight interacts with airborne hexagonal ice crystals through refraction and reflection. It often shows up with other halo display features, especially the parhelic circle.

Key takeaways

• An anthelion appears on the sky point directly opposite the sun.
• It is part of atmospheric optics and usually looks white, not rainbow coloured.
• It forms when ice crystal optics, especially refraction and reflection in hexagonal ice, send light to the anti-solar point.
• You are more likely to notice it when thin high cloud or ice crystal haze is present.
• It can appear alongside the parhelic circle, pillar of light, subanthelion and Wegener arcs.

Anthelion phenomenon: what it looks like

Look opposite the sun and you may see a faint white glow or spot. That is the anthelion. It is usually diffuse rather than sharp, so it can be easy to miss if the light is strong or the sky is busy with cloud.

The anthelion sits on the parhelic circle, a horizontal line of light that can stretch across the sky. When the right ice crystals are present, several halo display features can appear together, giving the sky a neat set of lines, spots and arcs.

The anthelion is not the same as a pillar of light or a standard halo ring. A pillar of light rises above or below a light source, while a halo wraps around the sun or moon. The anthelion is a spot opposite the sun, caused by a different set of ice crystal optics.

How the anthelion phenomenon forms

The anthelion forms when sunlight enters hexagonal ice crystals and is redirected by a mix of refraction and reflection. The exact light path depends on how the crystals are shaped and how they are orientated in the air.

The role of hexagonal ice

Most halo displays depend on hexagonal ice. In the atmosphere, ice crystals can grow as plate crystals or column crystals. Their flat faces and clean edges act like tiny optical surfaces, sending light in clear but sometimes surprising directions.

When many crystals share similar alignments, the light can gather at certain angles. That is why the anthelion can appear as a focused pale patch rather than a full ring.

Ice crystal optics and the sky opposite the sun

The anti-solar point is the spot in the sky directly opposite the sun. When the crystal geometry and viewing angle line up, light can be concentrated there. That is why the anthelion is often described as a target point within atmospheric optics.

Other features may appear at the same time. The parhelic circle can cross the sky through the sun, while subanthelion features may appear below the anthelion in some displays. Wegener arcs are another rare part of the same family of optical effects.

What sky conditions help you see it?

You are most likely to spot an anthelion when the sky contains thin cloud or suspended ice crystals and the sun is bright enough to light them. Thick cloud usually hides the effect.

Clear, crisp air can help, but the key ingredient is not cold ground air on its own. It is the presence of ice crystals at the right height and the right angle to the sun.

Thin cloud and ice crystal haze

High thin cloud can carry the crystals needed for a halo display. If the cloud is too thick, the light scatters too much and the anthelion becomes harder to see. If the cloud is too patchy, the optical path may not form at all.

For that reason, the best viewing usually comes when the sky is partly veiled by ice crystal cloud rather than fully clear or fully overcast.

Ground-level ice crystals

In some very cold conditions, ice crystals can form close to the ground as diamond dust. When that happens, halo displays can appear at eye level and the anthelion may stand out more clearly, often following specific ice crystal orientation rules.

Even then, it is still the crystal shape and orientation that matter most. Temperature helps set the scene, but the optics do the work.

Anthelion and other halo display features

The anthelion is easiest to understand when you compare it with other common and rare features in atmospheric optics.

Where to look in Australia

If you are in Australia, the best place to look is anywhere you have a wide view of the sky and the sun is low enough to make sky features easier to notice. The effect is subtle, so a clear view matters more than a famous location.

Cold inland areas and alpine districts can be useful when ice crystals are present. Coastal sites can also work if the cloud structure is right. The main requirement is the right mix of sunlight, ice crystals and thin cloud.

Simple viewing tips

1. Face away from the sun and scan the sky opposite it.
2. Look for a faint white spot, a horizontal band or a narrow arc.
3. Check thin cloud layers rather than thick rain cloud.
4. Give your eyes a minute to adjust to the brightness around the sun.