What exactly was that ring around the moon Sunday night?

By Naomi DuBovis
Published: Monday, November 27, 2023 - 5:39pm

A lunar halo, which looks like a white ring around the moon
Naomi DuBovis / KJZZ
A lunar halo appeared Sunday Nov. 26 in Phoenix.

If you went stargazing Sunday night, you may have seen a white ring around the moon. It's called a lunar halo.

They appear when a certain type of cloud forms in the atmosphere. David Trilling, professor of astronomy and planetary science at Northern Arizona University, described the halo’s formation.

"There is a thin cloud of ice crystals, obviously between us and the moon, and so the moonlight shines through that cloud, and it creates this sort of fuzzy halo that we saw," he said. 

Trilling said the cloud is high up in the sky, which is "why it’s ice crystals, and not water droplets, like we have in these kind of low, monsoon storm clouds, you know, in the late summer."

The ice crystals refract the light, meaning the angle of the light changes as it passes through. Lunar halos are also more visible when there's a full moon.

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