Rare blue moon to light up sky over San Francisco Bay Area


A blue moon will brighten the Sunday night sky, and depending on your location in the San Francisco Bay Area, you might be able to get a clear view, and coastal areas are likely covered in fog.

The blue moon occurs once every 2.5 to three years when four full moons fall in a season. The third moon is known as the blue moon, according to NASA. Normally, there are only three full moons in a season.

“It has nothing to do with color,” said Gerald McKeegan, an astronomer at Chabot Space and Science Center in Oakland. “The moon will not appear blue. But if there is smoke in the sky, it may appear orange.”


The blue moon is an older definition of the term that dates back to 1528, NASA you say. “Speculation about the origin of the term includes an old English phrase meaning ‘betraying the Moon’ or a reference to rare events, such as when dust in the atmosphere makes the Moon actually appear blue,” according to NASA’s 2021 full moon guide.

Beginning in the late 1940s, a second definition of a blue moon came into use, McKeegan wrote in an email. This definition refers to the occurrence of full moons in the same calendar month. The second full moon of the month is considered a blue moon. This also happens in rare cases and is not expected to happen again until August 2023.

“That’s why you get the expression once in a blue moon,” McKeegan said.

The Halloween Blue Moon, or second full moon of the month, rose behind Coit Tower as seen in San Francisco on Saturday, October 31, 2020. The full moon was the first day of Halloween to be visible in most time zones on Earth since 1944, and they will not happen again until 2039.

The Halloween Blue Moon, or second full moon of the month, rose behind Coit Tower as seen in San Francisco on Saturday, October 31, 2020. The full moon was the first day of Halloween to be visible in most time zones on Earth since 1944, and they will not happen again until 2039.

Carlos Avila Gonzalez / The Chronicle

A full moon in August is also called a sturgeon moon, according to the The farmer’s calendar. Almanac says: “The full moon of August has traditionally been called the Sturgeon Moon because they are the largest sturgeon in the Great Lakes and Lake Champlain during this summer’s party.”

A marine layer is likely to darken the moon on the coast, but National Weather Service forecaster Jeff Lorber said it will likely see clear inland depths.

“If you’re in San Francisco you probably won’t get a good view, but San Jose, yes, and Oakland will probably be good for the first part of the night,” Lorber said.

The clouds are expected to push inside later in the evening, and Lorber recommends taking in the sky show earlier in the night.

“It looks like the clouds will get very far into the valleys of North Bay,” he said. “If you’re in Sonoma and Napa you’ll probably be able to see it before midnight. Unless you’re right around the coast you should get a view of the moon at the early part of the night.”

Fire smoke has covered the bay area in recent days, and in the evening, the soot air made the moon appear orange. While a change in wind is expected to push the smoke out of the region over the weekend, Lorber said there is likely to still be some persistent smoke.

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