Why the moon above the Bay had a halo on Saturday night

Astronomers in the Bay noticed something unusual on Saturday night: a clear halo around the moon.

Twitter was surprised. “Moon ghost,” wrote one user. “Moon looks crazy tonight,” wrote another.

What causes it?

A local meteorologist, Jan Null, said on Twitter that, “The light of the near full moon breaks through tonight through ice crystals in the high thin cirrus clouds over the Bay to create a well-defined lunar radiance of 22 degrees.”

Null shared a photo of the phenomenon on Twitter showing the bright halo around the moon in a dark sky.

A halo is a ‘ring or light that forms around the sun or moon while the sun or moonlight breaks down ice crystals in a thin veil of circus clouds,’ according to national weather officials.

This is an optical effect that happens when the atmosphere’s ‘conditions are right’, weather officials said.

“In the atmosphere, water droplets and ice crystals can be a prism under certain conditions, enabling us to see the different colors that make up visible light,” said the weather official on his website, in which they radiate and other forms of optical effects explain. “Because of these properties, we get the different atmospheric optical effects.”

The halo was noticeable on Saturday night, when it was mostly clear and dry in the Bay, but rain is expected on Sunday and until Monday in the region.

Lauren Hernández is a staff writer for the San Francisco Chronicle. Email: [email protected] Twitter: @ByLHernandez