What Causes a Ring Around the Moon? People in the U.S. Spot 'Halo' in the Sky

Several people across the U.S. took to social media after spotting a "ring around the moon" on the night of March 21-22.

Some users posted images of the strange sight, while others simply commented on the phenomenon. Reports came from locations including northwestern Arkansas, northern Michigan and the Chicago area

But what causes a ring to appear around the moon?

This phenomenon is called a "moon halo." According to the National Weather Service, this ring of light, which is actually an optical illusion, forms around the moon when moonlight refracts off ice crystals in cirrus clouds, high up in the Earth's atmosphere.

Refraction is the change in direction of a wave—in this case, light—when it experiences a change in speed. This is most commonly observed when a wave passes from one medium to another at any angle other than 90° or 0°.

In the case of moon halos, the illusion is caused by tiny, hexagon-shaped ice crystals suspended within thin, wispy cirrus clouds that can be found at altitudes of around 20,000 feet or higher.

Anyone else notice the ring around the moon tonight? Chicagoland area. pic.twitter.com/dTAjxRLFq5

— April 1978 (@April19782) March 22, 2021
This image of last night's moon halo was captured by Kailan McKinney.

When rays of light emitted by the sun reflect off the moon and enter the Earth's atmosphere, they may encounter these cirrus clouds.

If this happens, the light rays can be refracted by hexagon-shaped ice crystals, bending 22° away from their original direction, according to WMC Action News 5 meteorologist Sagay Galindo.

The refraction of the light off the ice crystals creates a halo of light with an apparent radius of approximately 22° around the moon.

There's a ring around the moon
There's a ring around the moon
There's a ring around the moon
There's a ring around the moon @ElephantRevival #northrnmichigan pic.twitter.com/T3KtlrSvaQ

— craig napont (@C__na) March 22, 2021

The halos can appear in any season and are reported several times a year. The frequency of people reporting the illusion is usually determined by the amount of cirrus cloud coverage in any given area.

But for the illusion to be visible, the ice crystals within the cirrus clouds have to be positioned and oriented in just the right relation to your eyes. This means each individual will see the illusion differently whenever it is visible.

Moon halos are usually seen as a bright, white ring, although sometimes they can have color, appearing like faint rainbows.

Hey @TracyButlerABC7 why is there a ring around the moon tonight? pic.twitter.com/zXYu5HPtRd

— Mike Thomas (@mikethomasmt8) March 22, 2021

Similar halos are also sometimes seen around the sun. These illusions are caused by the exact same process as moon halos.

"Wait did y'all know lunar halos were a thing!? I'm driving with my roommate and there's this faint ring around the moon it's so cool," one social media user posted on Twitter after spotting the phenomenon last night.

A moon halo visible from New York
A halo appears around the moon in the sky above a home in the Bronx borough of New York City December 20, 2010. DON EMMERT/AFP via Getty Images