SAN FRANCISCO (CBS SF) — A natural phenomenon known as the moon or winter halo lit up the San Francisco Bay Area skies Sunday evening, triggering an avalanche of social media posts and photos.

Researchers say the display is a ring with an apparent radius of approximately 22° around the moon and is formed as direct sunlight or moonlight is refracted in millions of hexagonal ice crystals suspended in the atmosphere.

It occurs several times a year, Sunday night’s moon was particularly stunning. It was not only seen in the Bay Area skies, but across the country.

While it’s unrelated, the National Weather Service tweeted Sunday — “Going to be up late tonight? If so, you may want to check out the partial lunar eclipse, which will peak at 1:42 a.m. early Monday morning. Thin high clouds overnight may obscure views of the moon to some extent, but the partial eclipse should still be visible.”