What Causes a Ring to Appear Around the Moon?

Although many lunar phenomena are one-night-only events, one lunar sight is much less elusive: rings around the moon.

Also known as lunar halos, these bright white rings of light can appear anytime during the lunar calendar and any time of the year, especially in winter. But if you hope to see one, you'll want to disregard the number one rule of stargazing: not to stargaze in cloudy weather. Lunar halos are actually caused by thin, wispy, cirrus and cirrostratus clouds and the refraction and reflection of moonlight by their ice crystals.

Here, we explore this lunar spectacle and the best viewing conditions.

Ideal Sky Conditions for Ring Formation

Similar to rainbows, lunar halos form when light interacts with water suspended in mid-air. That water is frozen and found in cirrus and cirrostratus clouds—veil-like clouds that float 20,000 plus feet (6 km) above our heads where temperatures are too gelid to remain liquid water.

Ideally, sky conditions should be clear with only a thin layer of cirrus. If thicker clouds are present at lower levels, it'll obscure the halo effect from sight.

As moonlight shines through the cirrus clouds, it strikes the cloud's millions of tiny ice crystals and refracts, or bends and changes direction, as it enters each. The light then refracts again as it exits a crystal's other side. 

How much the moonlight bends depends on the size and shape of the crystal itself. In the case of lunar halos, the ice crystals are tiny pencil-shaped (hexagonal) columns measuring less than 20 microns across. And they all bend light at a 22-degree angle from its original path. (If you've ever heard lunar halos referred to as "22-degree halos," this is why.)

The fact that light is dispersed in this way in all directions (above, below, beside, and diagonal) to the moon is what creates the characteristic circular shape.

How and Why We See a Ring

Of course, in order to see the halo, the crystals have to be oriented and positioned with respect to your eye. The light reflected off the ice crystals and that coming directly from the moon should intersect at your eye at angles of 22 degrees.

That’s why, like rainbows, halos around the moon (or sun) are personal. Every observer sees their own particular halo made by their own particular ice crystals, which are different from the ice crystals creating the halo observed by the person standing next to you. The sight varies from person to person depending on factors such as personal height and the elevation at the spot where you stand.

Because the sun is 400,000 times brighter than a full moon, a lunar halo's colors tend to be dim. So dim, in fact, that its light is often too weak to be picked up by the color-detecting cells in our eyes. It's why lunar rings oftentimes appear milky white—white being the combination of all of light’s visible colors.

As for the sky between the ring and the moon, it usually remains dark. This is because none of the ice crystals reflect light at smaller angles than 22 degrees.

As long as cirrus clouds create a veil across the moon, the ring will remain visible.

Lunar Halo Look-Alikes

Lunar halos aren’t the only rings you’ll find encircling the moon. They're often confused with lunar coronas, but the latter are rainbow-colored discs that form when moonlight (or sunlight) interacts with water droplets in fog. Coronas also tend to form a tighter circle around the moon, forming a 10-degree rather than 22-degree radius.

Fogbows are white like lunar halos but form low to the ground. They, too, are created by water droplets, namely those tiny in size such as in a very fine fog or mist.

During winter 2020, the ring of all rings was sighted over Manitoba, Canada. Not only was the moon wreathed in white light, but corona, moon dogs, and tangent arcs occurred alongside the halo. Now, that's a sight that beats a macabre blood moon any day or night.