Why is There a Light Ring Around the Moon?

Have you ever looked up at the night sky and noticed a mysterious ring surrounding the moon? This stunning phenomenon, known as a lunar halo or moon ring, has fascinated sky watchers for centuries. While it may appear magical, there is a scientific explanation behind this captivating spectacle. In this blog post, we will explore the reasons behind the creation of a light ring around the moon.

What is a Lunar Halo?

A lunar halo is a luminous circle, typically with a radius of about 22 degrees, that surrounds the moon. It forms when moonlight passes through ice crystals present in the Earth’s atmosphere. These tiny ice crystals act as natural prisms, bending and scattering the light, causing it to spread out and create a ring around the moon.

Ice Crystals and Refraction

The creation of a lunar halo hinges on the presence of ice crystals. These ice crystals are often found in high-altitude clouds, such as cirrus clouds, and they need to be oriented in a specific way for the halo to form.

When moonlight enters an ice crystal, it undergoes a process called refraction. Refraction occurs when light bends as it passes from one medium, in this case, air, to another, the ice crystal. The angle at which the light bends depends on the wavelength of the light and the refractive index of the medium. In the case of ice crystals, the light is refracted twice: once when it enters the crystal and again when it exits.

The three primary factors that determine the shape and appearance of a lunar halo are the shape of the ice crystals, the orientation of the crystals in the clouds, and the position of the moon relative to the observer.

Plate-like Ice Crystals

The formation of a lunar halo relies on the presence of plate-like ice crystals in the atmosphere. These crystals are typically flat, hexagonal shapes, resembling tiny dinner plates. Due to their specific shape, when moonlight enters a plate-like ice crystal, it refracts and bends at a 22-degree angle.

This 22-degree angle is crucial. It determines the radius of the halo and explains why lunar halos are typically seen as circular rings with a 22-degree radius. Each ice crystal acts as a prism, bending the incoming light and contributing to the overall ring formation.

The symmetry of the crystal shape is also important. The six-fold symmetry of the hexagonal crystals ensures that light is refracted evenly in all directions, giving rise to a perfect circular halo around the moon.

Orientation of Ice Crystals

In addition to their shape, the orientation of the ice crystals relative to the observer also plays a crucial role in the formation of a lunar halo. For a halo to form, the hexagonal faces of the ice crystals must be roughly parallel to the ground.

If the ice crystals are randomly aligned, or if their orientation differs significantly, the light will not be refracted in a way that creates a visible halo. However, when these crystals align with their flat faces parallel to the ground, the light passing through them bends at the 22-degree angle needed to form a halo.

Moon’s Position

The position of the moon relative to the observer is the final piece of the puzzle. Lunar halos are most commonly observed when the moon is high in the sky. This positioning ensures that the moonlight passes through a larger portion of the atmosphere, increasing the chances of encountering ice crystals capable of producing a halo.

When the moon is near the horizon, the path of moonlight through the atmosphere is much longer than when the moon is overhead. Consequently, the light is subject to more atmospheric scattering, resulting in a greater likelihood of being absorbed or deflected before reaching the observer.

Other Atmospheric Phenomena

Lunar halos are not the only atmospheric phenomena involving the moon. There are several other optical effects that can occur, adding to the beauty and intrigue of the night sky. Some of these phenomena include:

• Moon Dogs (Parhelia): These are bright spots, often colored, that appear on either side of the moon. They occur due to light interacting with hexagonal ice crystals in the atmosphere.
• Lunar Coronae: These are small, colorful rings that form around the moon. Unlike lunar halos, coronae result from light scattering off water droplets or small ice crystals.
• Moonbows: Similar to rainbows, moonbows are formed when moonlight refracts and reflects through rain or mist, producing a faintly colored circle.

In Conclusion

The presence of a light ring around the moon, known as a lunar halo, is a result of moonlight passing through ice crystals in the Earth’s atmosphere. The combination of plate-like ice crystals, their specific orientation, and the moon’s position relative to the observer determines the formation of the halo. From the symmetrical hexagonal ice crystals to the bending of light through refraction, this phenomenon offers a captivating display of natural beauty. Next time you notice a lunar halo in the night sky, take a moment to appreciate the marvelous interplay between light and ice crystals that creates this enchanting spectacle.