What Causes a Ring Around the Moon? Halo Spotted in Skies Around the U.S.

Social media users in the United States have been posting images of a strange ring that has been visible around the moon. But what is causing this spectacular "moon halo"?

According to the National Weather Service, the phenomenon is the result of moonlight being manipulated by ice crystals in a particular way, producing an optical illusion that looks like a halo.

The tiny, hexagon-shaped ice crystals responsible for the illusion are found at an altitude of around 20,000 feet or higher within thin, wispy cirrus clouds.

Rays of light emitted by the sun are reflected off the moon and enter the Earth's atmosphere. As the rays pass through the cirrus clouds, they are bent by millions of tiny ice crystals 22 degrees away from their original direction, according to WMC Action News 5 meteorologist Sagay Galindo.

The refraction and reflection of the moonlight off the ice crystals creates a halo with an apparent radius of approximately 22 degrees around the moon.

The halos can appear in any season—occurring several times a year—with the frequency of the optical illusion determined by the amount of cirrus cloud coverage in a given area.

The ice crystals have to positioned and oriented in just the right way in relation to your eyes for the illusion to be visible, EarthSky reported. This means that whenever the illusion is visible, each individual will see it slightly differently.

While these moon halos tend to be white in color, they can also appear like faint rainbows, with red light on the inside and blue light on the outside.

Halos like these are also sometimes visible around the sun. They are the result of the exact same process.

Social media users in California and other locations around the country reported seeing the moon halo on the night of November 29-30.

"This halo around the moon is insane," said one Twitter user Amber Hinojosa.

"In Los Angeles there's a mysterious giant ring around the full moon tonight! Very strange and beautiful!" actor Harland Williams tweeted.

The lunar halo is visible on the same night as a full moon. Technically, the moon is considered to be fully illuminated at 4:29 a.m. ET on November 30. But to most people it will appear full for around a day either side of this moment.

A partial penumbral eclipse is also currently visible in North America. These events occur when the moon passes through the diffuse part of the Earth's shadow, known as the penumbra.